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About this blog

Moto Mind is a technical blog written by Paul Olesen who is a powertrain engineer working in the motorcycle industry. The blog covers a wide variety of topics relating to two and four stroke engine performance, design, and optimization.

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Paul Olesen

This week I want to talk about two-strokes. To kick off this post I have some awesome news. The Two Stroke Dirt Bike Engine Building Handbook is off to the printers and will be available for pre-sale very soon! Getting the book off the ground has been no cake walk. It's been two years coming and we are so thankful our riders and fans have been patient with us! At the end of this post I'll give you instructions on how you can stay updated on the launch. With that said, let's get started. Today's post aims to provide an overview of the important aspects of the two-stroke cylinder and answers a couple commonly asked questions relating to cylinder modifications.

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The ports found within a two-stroke cylinder in combination with the exhaust system have the greatest influence on power, torque, and the RPM at which maximum power is created out of the various engine subsystems found within a two-stroke engine. Typically when a new engine is designed the port characteristics are one of the first parameters to optimize. With this being the case they are also one of the first things anyone planning on altering an existing engine should consider improving or tailoring to their specific application. A two-stroke cylinder consists of exhaust, transfer, and occasionally inlet ports (true inlet ports are only found on piston or rotary valve controlled engines). The port heights, widths, areas, directions they flow, and relationships to one another all have a significant influence on how the engine will behave. The cutaway of the cylinder shown details the port arrangement and common nomenclature.

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The inlet port/passage delivers air into the engine’s crankcase, most commonly through a reed valve, on a dirt bike engine. On older engines, a rotary valve or the piston may also be used to control the opening and closing of the inlet port. On modern machinery, the inlet simply connects the reed valve to the cylinder or crankcase. In this case, the primary restriction in the inlet port is the reed valve and as such the valve’s geometry and flow capabilities often dictate the inlet port's performance.

The transfer ports are responsible for moving fresh air and fuel up from the crankcase into the cylinder. This occurs as the piston travels downward after the cylinder has fired. Once the piston uncovers the tops of the transfer ports the blowdown phase is complete, at which point much of the exhaust gas has been expelled from the cylinder. As the transfer ports begin to open, the exhaust pipe sucks fresh mixture up through the transfer ports into the cylinder. To a lesser extent, the downward motion of the piston also aids in creating a pressure differential between the crankcase and cylinder. The shapes and flow capabilities of the transfer ports play a big part in how effectively the cylinder can be scavenged of exhaust gases and filled with fresh air and fuel. The transfer ports also help cool the piston.

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The exhaust ports dictate how much and how well exhaust gases depart the cylinder. Similar to the transfer ports, the duct shape, angle, length and volume have a large influence on how well gases can flow through the port. Typically, dirt bike engines commonly feature bridge port or triple port designs.

General insights into a cylinder’s performance can be made by characterizing attributes such as the timing of the exhaust and transfer ports, the port widths, and the directional flow angles, but a deeper analysis is required to truly optimize a cylinder. Today, tuners and designers rely on computer software which computes a port’s specific time area (STA). As defined in the EngMod 2T software suite, “STA provides an indication of the effective port window area that has to be open for a certain length of time to allow enough gas to flow through the port to achieve the target power at the target RPM for the given engine capacity”. STA values are used to quantify the exhaust, transfer, and inlet port geometry as well as the blowdown phase of the two-stroke cycle. The blowdown phase occurs between exhaust port opening and transfer port opening and is one of the most important parameters in predicting engine performance.

By manipulating STA values and subsequently the height, shape, and size of the exhaust, transfer, and intake ports, an engine’s power characteristics can drastically be altered. Port modifications can be made which allow more air to move through the cylinder, ultimately increasing the power of the engine. Conversely, ports can be filled or welded and reshaped which tame the engine and provide less peak power but a broader spread of power. Simple modifications to the ports can also be carried out which improves the air or exhaust gas flow through the port yielding better cylinder scavenging.

Can I modify my own cylinders?
Unless you have a deep passion for two-stroke tuning, are willing to spend money on software and porting equipment, and are comfortable throwing away botched cylinders, I would recommend having a reputable professional carry out any desired port modifications. Experienced tuners have developed a number of porting combinations that will work well for various makes/models and riding applications which will take the guesswork out of the situation and provide you with a good performing cylinder.

Who should consider two-stroke porting modifications?
For the sake of simplicity, I will lump porting modifications into two categories: major and minor.

  • Major port modifications would include tasks such as significantly changing the port timings (by either removing or adding material), altering the shapes of the ports, or changing the directions the ports flow. Anyone drastically altering their engine, such as turning an MX engine into a road racing engine, should consider major porting modifications. Other examples of applications that may require or benefit from major port modifications include drag racing, hare scrambles, ice racing, or desert racing.
     
  • Minor port modifications would include basic tasks such as removing casting flash, slightly altering the ports to achieve the stock port timing, and correcting areas that result in minor flow deficiencies. Just about everyone could benefit from these types of corrective actions; however, if the engine is already performing or producing adequate power, they often aren't considered. 

I hope you enjoyed this writeup on key features affecting the performance of two-stroke cylinders. To stay officially updated on The Two Stroke Dirt Bike Engine Building Handbook we created an email sign up for our readers. Click this link to see the new cover, the Table of Contents, and some sneak peek pages right from the book.

Thanks for reading and have a great rest of your week!

-Paul

The Two Stroke Dirt Bike Engine Building Handbook

 

Paul Olesen

Today I want to share some pointers on preparing new or re-plated cylinders that will help ensure your engines run stronger and last longer. Plus, I've got an update on the two-stroke book I've been working on that I'd like to share. Let's get started!
 

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A Universal Concern
First, both new and re-plated cylinders must be cleaned prior to assembling. Normally the cylinders will arrive looking clean, but looks can be deceiving. I have no doubt that the factories and re-plating services clean the cylinders as part of their processes, but I highly recommend cleaning the bores a final time prior to use. Shown below is a new Yamaha cylinder that I extracted quite a bit of honing grit out of.

Dirty cylinder


If left in place, the honing grit will ensure that the piston rings will wear out faster than they need to, so be sure to take the time to properly clean new cylinders prior to assembly.

What’s the best way to clean the cylinder bore?
Start by using warm soapy water and a brush to clean the cylinder. Take your time and be thorough.

Cleaning sequence

After the majority of the honing grit has been removed switch to automatic transmission fluid and a lint free rag for one final cleaning.  

As a test to check cleanliness, rub a cotton swab against the cylinder bore. If the swab picks up any debris and changes color, your cleaning duties are not over. The swab should be able to be rubbed against the bore and remain perfectly clean.
 

Two-Stroke Port Dressing
For two-stroke owners, the second item I want to bring to your attention is port dressing. Port dressing is a term used to describe the process of deburring/breaking the edge at the intersection of the cylinder plating and the ports in the cylinder. During the plating process, plating usually builds up excessively at the edge of the port and must be removed after honing. Proper removal is critical to ensure acceptable piston ring life.

port dressing

Manufacturers and plating services will break the edge in different ways and to different magnitudes, which ends up being a whole other topic. The important thing is to ensure that any new or re-plated cylinder you use shows visible signs that the port edges have been dressed. A dressed port edge will be easy to spot because it will feature a different surface finish than the cross-hatch created from honing. This is easily visible in the image shown above. Many port dressing operations are done manually so some irregularity in the geometry will usually be present. If there is no visible edge break on the port edges, I would be highly suspicious and contact the service that plated the cylinder or sold the cylinder and confirm with them if a step was missed. Typically a chamfer or radius in the .020 - .040” (0.5 - 1mm) range is used.

Two-Stroke Power Valves
Lastly, it is possible that some of the power valve components, such as blades or drums, will not fit correctly on cylinders that have been replated. This is because the plating can occasionally build up in the slots or bores where the power valve parts reside. Prior to final assembly, be sure to check the function of the power valve blade and/or drums to ensure they move freely in their respective locations within the cylinder.

If plating has built up in a power valve slot or bore, it will need to be carefully removed. To do this, appropriately sized burs for die grinders or Dremel tools can be used. If one is not careful, irreversible damage to the slot or bore can result. When performing this work proceed cautiously or leave it to a seasoned professional. Burs for the job can be difficult to track down in stores, but are readily available online from places like McMaster-Carr. When purchasing burs, be sure to pick up a few variants, such as rounded and square edged, designed for removing hard materials.

The Two-Stroke Book
From February to March we photographed the entire book. From April onward we have been formatting and proofreading. Needless to say, we are in the final stretch! If you want to stay updated on the moment the Two-Stroke Dirt Bike Engine Building Handbook is ready for pre-order, sign up at the link below. We can't wait to get this book out the door and into your garage.
 

Sign Up for Updates on the Two-Stroke Book

Thanks for reading and have a great rest of your week!

-Paul

Paul Olesen

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Today I'm going to cover how to check and set cam timing, which is something you can do if you have adjustable cam gears in your engine. This is a procedure often performed by race engine builders to ensure the valvetrain performs just as they intend, and ultimately so that they extract the desired performance out of the engine. Adjustable cam gears typically aren't a stock option but are abundantly available in the aftermarket. The following text is exerted from my book, The Four Stroke Dirt Bike Engine Building Handbook, so if you find this info valuable please take a look at the entire book.  

Degreeing the camshafts is the process of checking, and if necessary altering, the cam timing so that the timing is set perfectly to specified timing values. On stock and performance engines, cam timing can be off slightly due to manufacturing variations in parts such as the camshafts, cam gears, cam chain, cylinder, cylinder head, crankshaft, crankcase, and gaskets. With so many parts having an influence on cam timing, it is necessary to adjust and correct the timing so it coincides precisely with the desired timing values. 

The biggest factor determining how the camshafts must be timed is whether the cam lobes are symmetrical or asymmetrical. Camshaft lobes that are symmetrical have opening and closing ramps that share the same profile. Asymmetrical cam lobes have opening and closing ramps with different profiles. Symmetric and asymmetric camshafts are timed differently. First we will focus on the timing of symmetrical camshafts.

Symmetric camshafts are timed most accurately by determining the position of the camshaft’s lobe center in relation to crankshaft position. A camshaft’s lobe center is where peak lift occurs, which is the most important timing event of the camshaft. Since the tip of the camshaft is rounded, it would be difficult to determine the lobe center by taking a direct measurement of peak valve lift. The opening and closing points of the camshaft are also of little use because the cam opens and closes gradually. This makes it difficult to determine the precise position in which the camshaft opens or closes the valves.

The lobe center position is a calculated value based on the position of the camshaft at two specific points of valve lift, typically with valve clearances set to zero. Normally the position of the camshaft is recorded at 0.050” (1.27mm) of lift as the valve opens and 0.050” (1.27mm) of lift when the valve closes. By recording the position of the camshaft at a specific valve lifts, the cam lobe is on a predictable portion of the opening and closing ramps. The center of the cam lobe is exactly in the middle of these two measurements.

To calculate the lobe center of a symmetrical cam lobe you will need to do the following: 
1. Add the measured opening and closing timings together
2. Add 180 degrees to the sum
3. Divide the answer by 2
4. Subtract the smaller value of the two opening and closing numbers from the answer to reach the lobe center value.

Once the actual lobe center value has been determined on the engine, it can be compared to the specified lobe center timing presented by the manufacturer, aftermarket cam supplier, or the engine tuner. If the measured lobe center position coincides with the targeted position, all the work is done. If not, the cam gear will need to be adjusted so the timing is corrected. 

If you are checking the timing on stock cams and lobe center information isn't presented, you will need to determine the lobe centers the manufacturer recommends. To do this, the opening and closing timing information supplied in the service manual can be used. Aftermarket camshafts should come with a timing card full of useful information to set the cams correctly if they are adjustable, otherwise the lobe centerline can be calculated if the opening and closing timings are known. If you don’t like math, there are plenty of lobe center calculators available on the internet you can use. 

For the Kawasaki KX250F engine with the stock camshafts, the timing information is as follows: 

Intake Opens 40° BTDC (Before Top Dead Center)
Intake Closes 72° ATDC (After Top Dead Center) 
Intake Lobe Center = ((40 + 72 + 180) ÷ 2) - 40 = 106° 

My calculated lobe center timing is 106°. When I check the cam timing, this will be the value the real engine hopefully yields. The lobe center for the exhaust cam can be found the same way. For the KX250F exhaust cam: 

Exhaust Opens 69° BBDC (Before Bottom Dead Center)
Exhaust Closes 49° ATDC (After Top Dead Center) 
Exhaust Lobe Center = ((69 + 49 + 180) ÷ 2) - 49 = 100° 

Something not obvious I want to touch on is that if the intake opens after top dead center, a negative value for the opening should be used. If the exhaust closes before top dead center, a negative value should be used here as well.

To start the process of checking the timing the valve clearances should be set to zero. Thicker shims can be used and zero clearance can be confirmed with a lash gauge. A degree wheel and pointer will need to be installed on the engine. There are many ways of attaching these items and each engine will provide its own challenges.

Here I’ve left the flywheel on and installed a couple washers behind the degree wheel to space the degree wheel from the flywheel. Then the flywheel nut is used to secure the degree wheel. The pointer can be made from welding rod, a coat hanger, or anything else you can find. I’ll be finding TDC with the cylinder head installed, so I used one of the exterior head bolts to secure the pointer. If you will be finding TDC with the head off, choose another location. 

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Before the cams can be timed, TDC must be found. This can be done with the cylinder head on or off depending on the process you use. The piston dwells a few degrees at TDC so more accuracy than zeroing the degree wheel to the piston’s highest position is necessary. Similar to finding the cam lobe center, TDC can be found by measuring equal distances on the piston’s up and down stroke and then confirming that the degree wheel timing is equal on both sides at the measured distances. Dial indicators or piston stoppers are commonly used to do this. 

HOT TIP: Piston stoppers can easily be made by removing the center section of a spark plug and then tapping a suitably sized threaded hole in the remaining part of the plug so a bolt and lock nut can be installed. The stopper can then be easily threaded into the spark plug hole. 

Whichever method of finding TDC you decide to use, start by moving the crankshaft to the approximate TDC position. Then without rotating the crankshaft move the degree wheel so that TDC on the wheel coincides with the pointer. Next, set up your piston stops or measure piston travel on both sides of TDC. In this example I’m using a dial indicator which extends through the spark plug hole down into the cylinder. I’ve decided to take measurements at 0.050” (1.27mm) of piston travel before and after TDC. At each measurement point the number of degrees indicated on the degree wheel before and after TDC should be the same if I have found true TDC. 

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If the degree wheel values don’t read the same before and after TDC determine which way the wheel must be rotated so that the values become equal. Then carefully rotate the degree wheel without rotating the crankshaft to alter the degree wheel’s position. Once altered, recheck and confirm that true TDC has been found. This can be a tedious process but is extremely important for checking cam timing accurately. Repeat the procedure for checking TDC 3 - 5 times to ensure repeatability and accuracy.

After true TDC has been found, be extremely careful not to inadvertently move the degree wheel or pointer. Do not rotate the crankshaft using the nut securing the degree wheel to the crankshaft. Instead, use the primary drive gear nut or bolt to rotate the engine over. 

Next, set up a dial indicator on the intake or exhaust lifter bucket, depending on which camshaft you are checking. You’ll have to use some ingenuity here in determining the best way to secure the dial indicator to the engine. I’ve used a flat piece of steel and secured it to the cam cap using the cylinder head cover holes. Make sure the indicator travels as parallel to the path of valve travel as possible for accurate readings. Also makes sure at least 0.060” (1.52mm) of travel from the indicator’s resting position is possible so adequate valve lift can be measured. 

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Once the indicator has been set up, the cam timing can be checked. Whenever checking timing only rotate the engine over in the direction of engine rotation. Reversing engine rotation will result in inaccurate measurements due to the reversal of gear meshes and chain slack. If you miss a measurement point, rotate the engine over until you get back to the previous position. 

Slowly rotate the engine over until 0.050” (1.27mm) of valve lift has occurred. Then record the position of the degree wheel. Next, rotate the engine until the cam begins to close the valve. Once only 0.050” of indicated valve lift remains record the position of the degree wheel. Repeat this process of checking opening and closing positions 3 - 5 times to check for repeatability before calculating the cam lobe center. 

Once you are confident in your measurements proceed to calculate the cam lobe center. On the KX250F engine my intake lobe center is as follows: 

Measured Intake Open (0.050” Lift) 39 ° BTDC
Measured Intake Closure (0.050” Lift) 74 ° ABDC 
Intake Lobe Center = (( 39 + 74 + 180 ) ÷ 2 ) - 39 = 107.5° 

On my stock KX250F engine the actual lobe center is 107.5°. At this point if I had adjustable cam gears, I could rotate the gear slightly so that the lobe center corresponded to the specified lobe center value. The same procedure is followed for checking and adjusting the exhaust cam timing. Remember if mistakes are made when setting cam timing big problems can result, so it is best to be very patient and focused when performing this task. Always check your work 3 - 5 times to make sure the timing is repeatable and making sense. When tightening adjustable cam sprockets, use a locking agent and be sure to torque the bolts to their specified values. 

When working with single camshafts that have both the intake and exhaust lobes ground on them, focus your efforts on achieving correct intake timing. Correctly setting intake timing is more important since it has a larger effect on power. The intake valves also have higher lift than the exhaust valves, potentially creating clearance troubles between the piston and valve if the intake valves are mistimed.  

With your new fangled ability to adjust cam timing, you may be wondering what happens if you advance or retard the intake and exhaust cams from their standard positions? The lobe separation angle refers to the number of degrees which separate the lobe center of the intake lobe from the lobe center of the exhaust camshaft. The lobe separation angle can be calculated using the following formula:

LSA = (Intake Centerline + Exhaust Centerline) ÷ 2 

As a rule of thumb, reducing the lobe separation angle by advancing the intake and retarding the exhaust camshaft will increase valve overlap, move power further up the power curve, increase cylinder pressure, increase the chance of detonation, and reduce the piston to valve clearances. On the contrary, increasing the lobe separation angle by retarding the intake cam and advancing the exhaust cam will have somewhat of the opposite effect. There will be less valve overlap, power will move to a lower RPM, chances of detonation will be reduced, and the valve to piston clearances will increase.

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The likelihood of finding more or better power by advancing or retarding the camshafts is not all that likely because manufacturers, tuners, and aftermarket companies already test specific combinations of cam timings to death. In addition, if the lobe separation angle is reduced, the piston to valve clearances should be checked to ensure they are adequate. My advice is to run the prescribed cam timings to reduce the chance of problems occurring.

Asymmetric camshaft timing can be set in a similar fashion to symmetric camshafts, however instead of focusing on the lobe center position, the specific opening and closing points will need to be measured. Timing cards supplied with asymmetric cams should have specific instructions for setting timing, but normally valve clearance is set to zero and cam positions are recorded at specific lift heights. Based on the measured opening and closing positions, adjustments are made to the timing until the timing matches the specified values.

I hope you enjoyed this exert on checking and adjusting cam timing. As always feedback is appreciated so please leave comments below. 

If you're interested in more engine building info check out my book The Four Stroke Dirt Bike Engine Building Handbook. Right now we are having a 4th of July Sale where everything on our site is 20% off with the discount code fourthofjuly2017. Just be sure to enter the code upon checkout so you receive your 20% off!  So if you've had your eye on our Four Stroke Dirt Bike Engine Building Handbook or even our Value Pack, but haven't pulled the trigger yet - go for it!
 

Availabe at:

- Paul

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Paul Olesen

It's time to open up a can of worms and talk about a hotly debated topic in the powersport community - four stroke cylinder head reconditioning best practices. I've perused the forums and had discussions with people about reconditioning four stroke cylinder heads and there appears to be a lot of mixed opinion and beliefs on what is right or wrong. I'm certainly not going to say my take on the subject is the only way, but I do want to share my opinion, explain the technical details, as well as touch on the machining process. The text below is out of my book, The Four Stroke Dirt Bike Engine Building Handbook, and details why cylinder heads should be reconditioned a certain way. 

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Whenever new valves are installed in a cylinder head, it is best practice to recut the valve seats since the valves and seats are mated parts, otherwise the new valves are very susceptible to premature wear when run in the old seats. If a major overhaul is being performed, there is a good chance that enough seat wear will have occurred during the engine’s life that the valve seats will need to be recut before new valves are installed. This may be news to you, so I want to provide an explanation of why this is necessary. 

The term concentricity is used to describe the relationship between the axis of two circular objects. When two objects are perfectly concentric, their axis match up precisely with one another.

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In the case of the cylinder head, the valve guide axis and the valve seat axis must be as close to perfectly concentric as possible and parallel to one another. Usually, guide to seat concentricity is kept around 0.001” (0.025mm) or even less for racing applications. This is achieved by the factory by using a manufacturing process where the valve guides are reamed first. Then the freshly reamed valve guide bore is used to center the valve seat cutter. Once centered, the valve seat is cut. This process is then repeated for all the valves and results in very good concentricity between the valve guides and valve seats.

As the engine is run, the valve guides, valve seats, and valve faces will wear. The valve guides will wear from front to back in an oval shape at the top and bottom of the guides. In a cross sectioned view the valve guide will take on an hourglass shape. The guide will become oval as a result of thrust forces stemming from the way the camshaft contacts the lifter bucket or rocker arm. These forces are transmitted to the valves and cause the valves to thrust against the sides of the guides, eventually causing the guides to become oval shaped.

Once the guides start to become oval shaped, the valve faces will no longer be as concentric to the valve seats as they originally were. When this happens the valves will start to slide against the seats, causing the seats and valve faces to wear. The valve seats will eventually become out of round and the sealing between the valve face and seat will suffer. Installing new valves into oval shaped guides and out of round seats will ensure that the new valves wear out very quickly!

To ensure the new valves being installed last as long as possible, the cylinder head’s seats and guides must be reconditioned once they are worn out. Complete cylinder head replacement is always an option, but I want to focus on freshening up the original head which is usually a more economical option, but comes with many more variables surrounding the quality of the job.

There are numerous services offered in the marketplace for valve seat cutting, however, not all valve seat cutting methods are equal in terms of quality. There are hand operated seat cutters, dedicated seat cutting machines, and a few other options to choose from. Selecting the correct seat cutting process and entrusting the work to a competent engine builder is very important. The valve seat cutting process should mimic the OEM process as closely as possible. A concentric valve seat will never be able to be cut without first servicing the valve guides. If the valve guides are out of round then they will either be reamed to a slightly larger size if they are not too oval in shape or they will be replaced. Once any issues with the valve guides are addressed and they are perfectly round from top to bottom, it will be possible to cut the valve seat. Ensuring the valve guide is perfectly round is extremely important since the valve seat cutter is centered off of the valve guide bore.

Cutting the valve seat concentrically to the guide requires a combination of skill and using modern machinery. The best valve seat cutting equipment in the world is worthless without a good experienced operator running it. There are two main factors which make cutting a seat concentric to the valve guide difficult. To start with, the valve seat cutter uses a pilot which locates in the valve guide. Since the valve stems are very small in diameter the pilots used to guide the seat cutters are also very small in diameter. A small diameter pilot shaft that centers the cutting tool can flex easily, which presents a real problem when cutting the seats. The next issue that arises when reconditioning seats is that often times the cutting tool will try to follow the path of the old valve seat which can make it hard to cut a concentric seat. Couple these factors together with slop within the machine, setup error, and operator error and you can see how quickly things can come out of alignment and you can end up with a poorly cut seat.

In addition to seat concentricity, the depth the seat is cut to will influence valve spring pressure, shim sizes, and the compression ratio of the engine. All three of these variables will be reduced the deeper the seat is cut, which is not a good thing. The surface finish of the seat itself will influence how well the valve seals. A seat with chatter marks or other machining blemishes will not seal as effectively as a smooth seat. The valve seat width and the contact point between the seat and the valve face are also very important.

Due to the complexities involved with cutting valve seats on modern four-stroke dirt bike engines, the job should not be left up to just anybody. There are numerous businesses which specialize in valve seat cutting which have both the right equipment and expertise to do the job correctly. I highly recommend spending some time researching and finding a reputable cylinder head machining company when it comes time to recondition your head. If the cylinder head must be shipped off in order to do business with a reputable company, the additional wait will be worthwhile.

If you found this information helpful and would like more technical info on maintaining your four stroke engine, check out my book, The Four Stroke Dirt Bike Engine Building Handbook.

Thanks for reading and happy wrenching!

As always if you have comments or want to share your thoughts please leave a note below.

-Paul

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Available at: 
- Amazon
- Moto Fix Website

Paul Olesen

I hope you all have been out riding and enjoying spring. I got back into the hare scramble racing scene over the weekend after a three year hiatus and had a blast. Today, I just want to share a quick tip and start a discussion on preparatory things that help shorten the time it takes to do complex maintenance tasks, such as rebuilding an engine.

Quick Tip
Prior to turning a wrench carefully look over the service manual scanning through all the applicable procedures and subsystems. If I’m working on an unfamiliar model, I find it is helpful to jot down a rough outline of the disassembly sequence. This saves me time in the long run as I don’t have to rely as heavily on the service manual or continually flip through various sections. Another option is to use post-it notes to bookmark each relevant section in the manual. Mark the post-it notes with numbers or headings so you know where to turn to next. Earmarking or bookmarking the torque tables is also a huge time saver no matter the task. 

Be sure to scan through the manual as well to identify any specialty tools that are required that you may not have.

Discussion Points

  • What other preparatory things can be done to help speed up the major maintenance process?
  • Is there a method to your madness or do you dive right in?

Thanks for reading!

Paul
https://www.diymotofix.com/

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Paul Olesen

Three easy ways to improve <a href='https://thumpertalk.com/link/click/1133/' rel='nofollow' data-ipsHover-target='https://thumpertalk.com/index.php?app=autolink&module=links&controller=content&id=1133' data-ipsHover target='_blank' data-autoLink>cooling.</a>png

This month I want to discuss three easy ways to improve engine cooling for your dirt bike or ATV and explain why they are effective.

As improvements are made to an engine that increase its power, the amount of heat the engine will create will also increase. Effectively removing heat from the engine and cooling it is very important as the power output of the engine goes up. The cooler an engine runs, the more power it can produce. There are three ways that the aftermarket attempts to improve the cooling system of a particular engine.

1. Increase flow through the cooling system.

2. Increase the cooling capacity of the radiators.

3. Increase the pressure of the cooling system.

Let's dive in.

1. Increase flow through the cooling system
The flow through the cooling system can be increased by installing a water pump impeller designed to increase the flow rate of the coolant. The reason increasing the flow rate of coolant works is because the rate of heat transfer from the engine to the cooling system is directly proportional to the mass flow rate of coolant. This is thermodynamics jargon, but there are two key parts to consider. First, how much coolant is flowing, and second, at what speed the coolant is flowing. The more coolant that flows and the faster it flows will reduce the temperature difference between the point where the coolant enters into the engine and where it exits. This next part is not quite as intuitive. When the temperature difference between the inlet and outlet is reduced, the average coolant temperature is lowered. When the average coolant temperature is lowered the engine will run cooler. This is why fitting a water pump, which increases the flow of coolant through the engine, improves cooling.

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2. Increase the cooling capacity of the radiators
Radiators consist of a series of tubes and fins which run from the top to the bottom of the radiator. These are often referred to as the radiator’s cores. As coolant enters the radiator it moves through the series of tubes and heat is transferred from the coolant to the fins. Air passes over the fins and heat is transferred from the fins to the air. This transfer of heat from coolant to air is how radiators reduce the temperature of the coolant.

Coolant temperatures can be reduced by upgrading radiators in three ways, by increasing the frontal area of the radiators, by making the radiators thicker, or by using materials with better heat transfer properties for the cores. For all practical purposes, increasing the radiators’ frontal area and improving the core materials is rarely a viable option for dirt bike applications. This is because there is little room for the radiators to begin with and they are susceptible to damage, making the use of expensive core materials a risky affair. Unfortunately, both of these options are better improvements to make before resorting to increasing the thickness of the radiators.

Increasing the thickness of a radiator is not as efficient of an improvement as increasing the frontal area of the radiator. In order for thicker radiators to cool more effectively than their stock counterparts, airflow past the radiators is key. When the thickness of a radiator is increased, air must travel a greater distance through the radiator before exiting. The speed the air is traveling plays a big role in determining how quickly the air heats up as it moves through the radiator. If the air is not traveling fast enough through the radiator, the air temperature will rise and equal the coolant temperature before reaching the end of the radiator. Once this happens, heat transfer stops and whatever portion of the radiator remains will not help with cooling. In order for a thicker radiator to be effective, air must flow quickly enough through it so that the exiting air temperature is at, or better yet, below the coolant temperature. In conclusion, benefits from adding thicker radiators will be more prominent in applications where speeds are relatively high. Whereas in applications where the bike is hardly moving, improved cooling may not be noticeable.

3. Increase the pressure of the cooling system
The last alteration to the cooling system that can be made is to install a high pressure radiator cap. As coolant temperature increases, pressure increases inside the cooling system. The radiator cap is designed to be the pressure release point in the cooling system in the event that too much pressure builds up. This can occur as a result of overheating or a blown head gasket for example. By designing the radiator cap to be the weak link in the system, other parts of the system, such as seals, don’t end up getting damaged from being over pressurized. The radiator cap features a plug and spring on its underside. The spring is designed to compress once a certain pressure is reached, at which point the plug will move upwards and uncover a pressure release hole where excess pressure will be vented.

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The coolant’s boiling point and ability to conduct heat are necessary factors in understanding why a high pressure radiator cap can help improve engine cooling. Water alone boils at 212°F (100°C) while a 50/50 mix of water and antifreeze boils at 223°F (106.1C). Radiator cap pressure designations are usually advertised in bar, with most stock radiator caps designed to withstand pressures up to 1.1 bar (16psi). The more pressure a fluid is under, the more difficult it becomes for the fluid to vaporize, and the higher its boiling point becomes. When water is under 1.1 bar of pressure, the temperature water will boil at is 260°F (127°C) while a 50/50 antifreeze mix will boil at 271°F (133°C). By installing a radiator cap designed to withstand higher pressures, an additional increase in the coolant’s boiling point will be seen. High pressure caps are usually designed to withstand 1.3 bar (19psi) of pressure. This 0.2 bar (3psi) increase in pressure over the stock system will increase the boiling point of water or antifreeze by 8.7°F (4.83°C). This will then bring the boiling point of pure water or a 50/50 antifreeze mix to approximately 269°F (132°C) and 280°F (138°C) respectively.

While this small temperature increase alone won’t do a lot for your engine, coupling a high pressure cap and using coolants with better heat transfer properties can do wonders. Antifreeze (ethylene glycol) alone is not an inherently good conductor of heat. In fact, pure antifreeze conducts heat about half as well as water, while a 50/50 mix of antifreeze and water conducts heat approximately three quarters as efficiently as pure water. This means a cooling system using a 50/50 mix of antifreeze would have to flow faster than a cooling system filled with pure distilled water in order to achieve the same cooling efficiency. What this means for you is significant cooling gains can be made by using distilled water and an additive called “Water Wetter” in place of an antifreeze-water mix. Water Wetter is an additive that improves water’s “wetting” abilities (another whole subject), adds corrosion resistance, and slightly increases the boiling point of water. A high pressure radiator cap in conjunction with distilled water and Water Wetter as the coolant is by far the best route to go for high performance applications where freezing is not an issue. For applications which must still be resistant to freezing, the antifreeze-water ratio can be altered in favor of mixtures incorporating more water than antifreeze so that the cooling efficiency of the mixture is improved. Just bear in mind the freezing point of the mixture as it is thinned with water will be reduced, so you will need to pay close attention to the environment you are operating in so that the coolant is never susceptible to freezing. A frozen coolant system can ruin an engine and makes for a very bad day!

I hope you enjoyed this post on three easy ways to improve your engine’s cooling. 

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One more thing before I wrap up! April is Autism Awareness month, and here at DIY Moto Fix we couldn't be more excited to announce that we will be donating 15% of all profits made in April to AutismMX.

If you haven't heard of AutismMX, this amazing non-profit brings Autism awareness to the motorcross community. Founder, Matthew Dalton, created this non-profit after finding that motorcross was an amazing way to connect with his autistic son.

At DIY Moto Fix this non-profit also touches a chord with us. Our filmmaker and photographer, Kelsey Jorissen, loved dirt biking with her autistic brother throughout their childhood.

The Autism MX Project focuses on four areas:

  • Autism MX Day Camps are days for ASD kids and families to have the chance to ride AMX’s little dirt bikes and quads and enjoy the sport of motocross.
  • Team Autism MX Sponsoring amateur MX racers, riders as well as sponsoring AMA pro racers. Through doing so, they are getting out the word on Autism Awareness to millions.
  • AMX Puzzle Piece Apparel from shirts, graphics, goggles, to help stand out and support Autism Awareness.
  • AMX Ride Days for Autism Awareness AMX celebrates Autism Awareness and is a fundraiser for The Autism MX Project.

So for the entire month of April - if you buy a booka video, even a poster - 15% of that purchase will go towards AutismMX and their amazing cause.

Thanks for reading and have a great rest of your week!

Paul Olesen

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With warmer weather and the riding season around the corner for many of us, I wanted to cover a topic that can either make or break an event. Whether you’re competing in a racing series or traveling to the track or trail, let's talk about event preparedness. More specifically, what spare parts should you keep on hand? Plus, what methods do you use to keep your spares organized?

Honestly, I struggled with organization until I started working on this post. I had no method to my madness. Every time an event came up I’d do the same thing; throw a bunch of stuff in a box or the back of my van and head to the event. The sad part is I now realize this was a weakness of mine for quite some time, but didn’t do anything about it! Maybe you can relate?

I finally said enough is enough. I don’t throw my tools in a cardboard box when I go to a race, leaving what I bring to the fate of my memory. So why would I do that with the spare parts I bring?

I started solving this problem by compiling a spreadsheet detailing what spare parts I keep on hand for ice racing and hare scrambles. I realize that each discipline will differ and may have niche parts that should be kept. The goal here is not to definitively define what spares one should keep on hand, but to have a conversation and provide a resource that can be used to help people get set up based on their own needs.

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Once I took inventory of everything I felt I wanted to bring to a race, I went to Menards and went hunting for the perfect organized storage bin/toolbox. Here’s what I ended up with:

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Naturally, once I returned with the toolbox, my list grew and I probably need to go back for a bigger one. I intend to store a copy of the spreadsheet in the tote so I can keep tabs on inventory and know exactly what I have available.

Should I get another bike, this system is easily replicable and my plan is to get another organized toolbox that goes with it.

This system is how I went from being an unorganized “throw it in the van at the last minute” rider to a more relaxed well prepared rider. I’d love to hear how you handle event readiness, what you bring, and how you keep track of it. My hope is that by sharing our strategies we’ll save someone the misfortune of having a bad day at the track or trail. Perhaps I'll even end up with more things I need to add to my list.

-Paul

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Paul Olesen

 

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Today I want to share a quick tip with those of you who are working on your own engines but just can’t justify buying a set of piston ring compressors. It’s entirely possible to make a perfectly good ring compressor from materials you can get at the hardware store. All you need is some plumber’s pipe hanging tape and a hose clamp that is sized according to your cylinder bore.

To construct a DIY ring compressor from plumber's pipe hanger tape you will need to determine the length of tape required. This is easily done using the following equation for calculating the circumference of a circle.

Length of Tape Required = Piston Diameter x π (Pi)

When the tape is wrapped around the piston tightly, the final length may need to be reduced slightly so that the ends don’t butt together. Once the tape has been cut to length, make sure whichever side of the tape will be contacting the rings is smooth and free of little plastic burrs that could catch the rings.

Simply lube up the tape, tighten down the hose clamp, and you are in business.

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Do you have a tip that makes compressing rings easier or cheaper? If so, leave a comment below!

- Paul

If you enjoyed this tip and want access to more like it, check out my book, The Four Stroke Dirt Bike Engine Building Handbook. On the fence about the book? Check out what other riders are saying: Thumper Talk Review

Available at:

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Paul Olesen

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Today I want to talk about a situation I hear all too often. Someone’s bike, whether it be a two-stroke or four-stroke, only starts when it is pushed.

Before I discuss potential causes for this scenario, take a moment to think through the situation yourself.

What mechanical factors would result in either a two-stroke or four-stroke only starting when it is bump started?

In either case, the reason the engine is able to start when it is push started is because it is able to build more compression than it otherwise could when it is kicked or the electric starter is engaged. More compression is achievable because the cranking RPM is higher than what’s possible with the aforementioned starting methods. With a higher cranking RPM for a four-stroke, more air will fill the cylinder on the intake stroke, and for a two-stroke the scavenging process will be improved. With this being the case we must look at reasons why the engine is struggling to build compression in the first place.

Starting problems specific to four-strokes:
1. Valve seat recession - When a valve seat wears out and recedes, the valve moves up towards the camshaft. This leads to diminished valve clearances and if left to run its course, the valve and shim will bottom on the camshaft’s base circle. This can prevent the valve from seating and make the engine hard to start.

2. The valve is bent - A valve with a serious bow to it may get jammed up inside the guide and not return all the way back to its seat. Bent valves typically result from an over-revved engine where the valves contact the piston. Valves can also bend to a lesser extent if they were mated to valve seats that were not cut concentrically to the guides, or they were paired with worn seats.

3. The valve stuck in the guide - This is usually due to the engine overheating. When the engine overheated the clearance between the valve and guide diminished which caused metal to transfer from one part to the other, ultimately ruining the surface finish on one or both parts. Once this happens the valve may be prone to sticking in the guide until the engine warms up.

4. The valves and seats do not seal well - Worn valves and valve seats can compromise the seal between them. Valve and seat wear is a natural part of running an engine but can also be accelerated by ingesting dirty air.

Starting problems specific to two-strokes:

1. The reed valve is worn - Reed petals that don’t close all the way, are chipped, or bent will not allow sealing of the crankcase and efficient gas flow up from the crankcase into the cylinder.

2. An engine seal or gasket has failed - A two-stroke engine requires a well sealed crankcase and cylinder in order for it to scavenge gases efficiently. A worn crank seal, leaky base gasket, or problematic power valve seal can all make starting more difficult.

Two and four-stroke problems:

1. The piston rings are worn - Worn piston rings will allow compressed gases to escape past them.

2. The head gasket or o-rings are leaking - Usually a leaking cylinder head will be accompanied by white smoke if coolant is being pushed into the combustion chamber, by coolant being blown out the radiator, or both.

I hope you found this rundown of potential problems useful for diagnosing bikes that like bump starting over a kick or the push of a button. Can you think of any other problems that would lead to lack of compression? If so, leave a comment and share them.

If you liked this post and want more technical info, check out my book, The Four Stroke Dirt Bike Engine Building Handbook. In it you will find over 300 pages of technical knowledge to help you get off on the right foot when rebuilding!

- Paul
 

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Paul Olesen
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I hope you all had a good holiday season and are excited for 2017. The ice bike has been kicking my butt so far, but I’m thankful I’ve been able to get out and ride. I’m definitely excited for the new year and today I wanted to discuss my upcoming blogs and share a quick tech tip with you.

 

My blog post pool is low right now and I’d like your help! Looking ahead to 2017 I want to deliver informative posts tailored to what you need to know and want learn. In the comments section below be sure to share your thoughts on what you’d like to learn about this year. Whether it’s maintenance, tuning, suspension, two-stroke, four-stroke, or anything else-- I want to hear what you have to say. :prof:

 

Depending on post length it takes me anywhere from 2 - 5 hours to write a piece I feel comfortable publishing for you, so I want to make sure I’m spending my time covering topics that are truly beneficial and relevant.

 

Moto Mind Quick Tech Tip
Anytime you drain fluids from something you’re working on and halt progress (think waiting for parts) tag the throttle or any other visible location and note that there’s no fluid in the machine.

 

This isn’t always applicable but here’s an instance of when it was. I’ve moved a few times in the last couple years and during that transitional period my bikes were five hours away in northwestern Wisconsin. One weekend I was set to make the trip back home to go riding, certain that my bike was ready to go. I got there and found that I’d left myself a tag noting that I was only part way through the oil change I so vividly remember completing. While I usually check my sight glass anyway, I can’t say I always do, especially if I’m in a hurry. Whether or not I would have caught the lack of oil without the tag I will never know, but I’m just glad I left myself a visual reminder.

 

If there’s something you’d like to learn about, please leave a comment below and I’ll see what I can do to work it into this year’s blogging schedule!

 

Plus, if you got socks or other stuff you didn't want over the holidays, be sure to check out my book and treat yourself! ;)

 

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- Paul

 

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Paul Olesen
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I hope you’re all enjoying the fall weather. For those of you in northern states, I hope that you’re getting in some end of season riding. This month I want to touch on bolted joints and the importance of adhering to tightening techniques outlined in your model’s service manual.

 

How A Clamped Joint Works
I’m going to discuss the importance of criss-cross patterns, tightening sequences, incremental steps, and joint lubrication but first I want to explain how a bolted joint works. As a bolt is tightened to secure a pair of parts, the bolt will stretch a very small amount. The stretch in the bolt creates tension or preload in the joint which is the force that keeps the joint together. The amount of preload created is dependent on bolt size, bolt material, the torque applied, and the friction between the threads. There are additional variables, however a discussion on bolt engineering would be very long and not all that exciting! As long as you understand the basics for engine building you can begin to appreciate the importance of correctly tightening fasteners.

 

As you are well aware, an engine consists of many parts fastened together. What you may not consider as much is that the majority of these parts are fastened by more than one fastener. This means that how much you tighten/preload one fastener will have an effect on the surrounding fasteners. This interaction between the fasteners begins to shed light on why tightening sequences are so important.

 

The evenness of the preload across the bolts securing a part can affect part life. Warpage can occur in parts which are improperly tightened, ultimately rendering the part useless. A prime example of a part that can warp is a four-stroke cylinder head. If the bolts are unevenly tightened over time, the cylinder head can become permanently distorted. Gasket sealing problems can also occur from improper preloading of bolts across a part. In order for a gasket to seal it must be evenly compressed. If one area of a gasket is highly compressed and tensioned while another area is not, the gasket can easily leak through the low tensioned area. In the case of plain bearing bores, such as the cam cap, uneven preloading may cause the bearing bore to distort. As a result the cam may become difficult to turn. Or if run, the cam bearing bore will wear unevenly and in severe cases the cam could seize.

 

While ensuring bolt preload is even can be a problem there are three tightening techniques that virtually eliminate the issue. If you’ve been building engines for any length of time you’ve probably already been utilizing these techniques. Hopefully now you may have a better understanding of why the service manual instructs you to tighten parts a specific way.

 

Criss-Cross Patterns
Criss-cross patterns are called out when tightening or loosening parts with a simple square pattern or circular bolt pattern. These basic patterns have been around for a very long time and are a proven method for evenly distributing clamping load across a part. Most cylinder heads will utilize this type of clamping pattern.
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Tightening Sequences
For more complex bolt patterns, such as those found on cam carriers and crankcases, the manufacturer will usually identify a specific sequence for tightening and loosening the fasteners. This sequence is based on testing and the past experiences of the manufacturer.

 

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Incremental Steps
Highly torqued bolts, such as those found on the cylinder head, are almost always tightened and loosened in incremental steps. An incremental tightening sequence consists of torquing all the fasteners to a specific torque value, then increasing the torque and tightening again, and finally arriving at the final torque value. This sequence is typically performed in two to three steps.

 

Here’s something important to keep in mind regarding incremental steps! When torquing bolts in steps the change in torque between the steps must be large enough to induce bolt movement. For example if a bolt was torqued to 35Nm at the first step and the second step was 38Nm this would not be enough of a change to make the bolt move at the second step. The torque wrench would not overcome the friction of the stationary bolt and would hit 38Nm before the bolt even moves. As a rule of thumb incremental changes should be no less than 5Nm and if possible should be greater.

 

Lubrication
For highly torqued fasteners often times the service manual will specify that the threads of the fastener must be lubricated. The lubricant can be as simple as fresh engine oil or a specifically formulated thread lubricant product.

 

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Adhering to any lubrication guidelines is of utmost importance. Since we most commonly measure torque to determine whether a bolt has been tightened/preloaded enough any change in the amount of force required to turn the bolt will influence the resulting bolt preload for a given torque value. The force required to turn a bolt is partially dependent on the amount of friction in the joint. If we had two identical fasteners where one was lubricated and the other was not, and we set the torque wrench to the same value for each, then both were tightened, the resulting bolt preload would be different between the two. Due to the reduced friction in the lubricated joint the bolt would stretch more and the preload in the joint would be higher at the specified torque wrench setting than the unlubricated joint. Depending on the criticality of the joint this can be a really big deal! It also shows why in some applications (think two piece conrods) directly measuring bolt stretch is a more accurate means of determining bolt preload.

 

I hope you enjoyed this quick summary of tightening techniques and their importance! If you have tips of your own you’d like to share or other pearls of wisdom please leave a comment.

 

For those of you interested in more engine building knowledge check out my book, The Four Stroke Dirt Bike Engine Building Handbook. You’ll find more detailed and comprehensive info on engine building there. Simply follow the links below. Thanks for reading and have a great week!

 

-Paul

 

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The Four Stroke Dirt Bike Engine Building Handbook

 

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Paul Olesen
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Having a clutch that works correctly is key to being able transfer all the power your engine produces to the rear wheel (or wheels if you're a quad guy/gal). In this post I want to share some key clutch inspection techniques I use and recommend to help ensure your clutch works as it should. These tips are presented in a step by step format and are taken right from my book, The Four Stroke Dirt Bike Engine Building Handbook.

 

Basket Inspection
Inspect the driven gear which is secured to the basket. Look for damaged gear teeth and other imperfections. Grasp the gear and basket firmly, then try to twist the gear. The gear is secured to the basket either with rivets or fasteners. With use, the rivets or fasteners can loosen causing the gear to become loose. Most baskets use round rubber dampers to locate the gear to the basket, which are sandwiched behind the backing plate. The dampers can wear out and break, which will create excessive play between the gear and basket. Any looseness may have been accompanied by excessive gear noise or rattling sounds when the engine was previously running.

 

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On baskets with loose gears and riveted backing plates the corrective action which will need to be taken is to either replace the basket or drill the rivets out. The idle gear may need to be pressed off in order to remove the backing plate. Once this is done, holes can be tapped and bolts installed which will secure the gear in place. Any rubber dampers that have worn can be replaced with aftermarket options. Check out this article for more details on clutch basket damper replacement: How to repair your clutch basket dampers for less than $30.

 

Inspect the needle bearing bore surface on the basket next. Run your fingernail across the bore feeling for signs of wear. The bearing surface should be smooth and free of imperfections. If the surface is grooved or worn the basket will need to be replaced.

 

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Inspect the area inside the basket where the large thrust washer resides. Wear should be minimal in this area. If any grooving is present, the needle bearing and spacer the basket rides on may have worn causing the basket to wobble or the pressed in steel insert has backed out, ultimately causing the face of the basket to rub on the edges of the washer.

 

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Check for bent clutch basket fingers on the basket. Then look for grooving on the basket fingers where the clutch discs come in contact with the fingers. Grooving is caused by the clutch discs slamming into the clutch basket fingers. Normally grooving will be more pronounced on the drive side fingers. Grooving is not abnormal and occurs through usage of the clutch.

 

If any grooving is present, use the end of a pick to evaluate how deep the grooves are. Any grooving that can catch the end of the pick is also likely to be able to catch the edge of the clutch discs. When this happens, the clutch will have difficulty engaging and disengaging. If your bike had clutch disengagement/engagement problems prior to disassembly, basket grooving is the most probable cause.

 

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A file can be used to smooth the grooves so the discs no longer catch, however deep grooving is an indication that the basket is near the end of its life. When filing clutch basket fingers, attempt to remove as little material as possible and remove material evenly from all the fingers.

 

Some manufacturers provide a specification for the clearance between the clutch disc tang and the basket fingers. This clearance can be measured by temporarily installing a clutch disc into the basket and using a set of lash gauges to check the clearance between the two parts. Both the clutch disc tangs and basket fingers will wear so if the clearance is outside the service limit it may be possible to prolong the life of the basket by installing new clutch discs. This is a short term fix however, and replacing both components at once is advisable.

 

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Bearing/Spacer Inspection
Inspect the clutch hub needle bearing and spacer for signs of wear. The needle bearing will be replaced with a new bearing, but if the spacer is in good condition it will be reused. Check for grooving or concavity along the surface of the spacer where the bearing rotates. While the needle bearing won’t be reused, it can be inspected as well to help confirm any problems associated with the clutch basket or spacer.

 

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Hub Inspection
There are two main areas on the clutch hub which will wear. First, grooving can occur on the splines which locate the clutch plates to the hub. The grooves are a result of normal clutch use and occur when the steel clutch plates rotate back and forth in the spline grooves. Any grooving which catches the end of a pick should be considered problematic. Careful filing to smooth the grooves or hub replacement are the two options available for remedying the issue. The clutch plates must be able to easily slide back and forth along the hub, otherwise clutch disengagement/engagement problems will occur.

 

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The second area susceptible to wear on the clutch hub is at the back face of the hub. This is where the outer clutch disc contacts the hub. When the clutch is engaged, the clutch disc and hub will rotate in unison. However, when the clutch is partially engaged or disengaged, the clutch disc will rub against the face of the hub causing both the hub and disc to wear. Look for uneven wear patterns and indications of how deep the clutch disc has worn into the clutch hub.

 

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If the face of the clutch hub has worn excessively or unevenly, the hub should be replaced.

 

Pressure Plate Inspection
The interaction between the pressure plate and clutch disc is identical to the situation previously described between the clutch disc and clutch hub. Wear will occur on the face of the pressure plate which contacts the outside clutch disc. Determine the condition of the pressure plate by looking for signs of excessive or uneven wear on the face of the pressure plate.

 

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Disc and Plate Inspection
Both the clutch discs and clutch plates are designed to be wear items which will need replacement from time to time. Thickness and straightness are the primary inspection criteria used to determine if either component requires replacement. If there are any problems with any of the discs or plates replacing them as a set is best.

 

Clutch Disc and Clutch Plate Inspection
Clutch discs are made out of various compositions of fibrous materials which wear at different rates, while clutch plates are made from steel. Service manuals will specify a minimum thickness that the clutch discs and plates can be. This thickness can easily be measured by using a caliper. Take measurements at three to four locations around the clutch disc or plate to confirm either has not worn unevenly.

 

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Once all the disc and plate thicknesses have been measured, both should be inspected for warpage. This can be done by laying the disc or plate on a surface plate or other flat surface. A set of lash gauges are used to determine any warpage. The service manual should specify a maximum warpage value which is usually around 0.006” (0.15mm). Attempt to insert the 0.006” lash gauge underneath the clutch disc or plate at multiple points. If the feeler gauge slides beneath either of the parts, those parts are warped and should be replaced.

 

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Clutch discs which have been overheated due to excessive clutch fanning by the rider, not only may warp, but also emit an unpleasant stinky burnt smell. If a noticeable smell is present, the discs have overheated and should be replaced. Likewise, clutch plates that have overheated will likely be warped and exhibit discoloration. The discoloration is a sign of excessive heat build up. Once the clutch plates have overheated, the material properties of the plate change, the hardness is reduced, and the plate becomes less wear resistant. This means discolored plates should be replaced.

 

Lastly, inspect the clutch disc tangs for wear, chipping, or damage. If any tangs are damaged the disc should be replaced.

 

Clutch Spring Inspection
Over time and due to normal clutch use, the clutch springs will shorten. Clutch spring minimum free length specifications are provided by manufactures and can easily be measured using a caliper.

 

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Clutch springs that are shorter than the minimum spec provided by the manufacturer will not have sufficient spring pressure to keep the clutch from slipping under heavy loads. Any springs at or past their service limits require the replacement of all springs as a set. This way when the new springs are installed, even pressure is applied to the pressure plate.

 

I hope you enjoyed this passage from my book detailing clutch inspection. If you have additional tips you'd like to share please leave a comment!

 

If you want more technical DIY dirt bike engine information, learn more about the book on our website or on Amazon. Simply follow the links below!

 

The Four Stroke Dirt Bike Engine Building Handbook

 

Amazon Store

 

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Thanks for reading!

 

-Paul

Paul Olesen
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This month I wanted to share an exert from the Race and Performance Engine Building chapter in my book, The Four Stroke Dirt Bike Engine Building Handbook. If you've been wondering how high compression pistons work and if they are right for your application, read on!

 

Piston upgrades are normally considered when changing the compression ratio is desired or larger valves are installed. In both instances the shape of the piston is altered either to reduce the volume in the combustion chamber or to allocate additional room for larger valve pockets.

 

The compression ratio defines how much the original air/fuel mixture which was sucked into the engine is compressed. The following equation shows how an engine’s compression ratio can be calculated.

 

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The swept volume is the volume that the piston displaces as it moves through its stroke. Mathematically this volume can be determined using the following equation.

 

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The clearance volume is the volume of the combustion chamber when the piston is at top dead center (TDC). While manufacturers specify what the compression ratio should be, due to subtleties in manufacturing, parts vary slightly from engine to engine so finding the exact clearance volume of your engine actually requires measuring the clearance volume.

 

Undoubtedly you have probably heard that raising the compression ratio will increase the power of an engine. This is definitely true, however you should be aware of the other consequences that come along with this.

 

The more the air/fuel mixture can be compressed before it is combusted, the more energy which can be extracted from it. The reason for this is due to thermodynamic laws. In summary, the temperature difference between the combusted mixture when it is hottest and coolest determines the power and efficiency of the engine. The hottest point of the mixture will arrive shortly after the mixture has been ignited and the coolest point will occur around the point where the exhaust valves open. Since the temperature of a gas increases as its volume decreases, it is easy to see how increasing the compression ratio increases the overall combustion temperature. Something less obvious is that because the gases are compressed more, they will expand more and actually be cooler by the time the exhaust valves open.
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If increasing the temperature of the compressed mixture is good, you might be wondering what keeps us from raising it higher and higher. Detonation, which is a by product of the additional heat and pressure in the combustion chamber, is the main reason the compression ratio can’t be increased beyond a certain point. Detonation occurs after the spark plug has ignited the air/fuel mixture. Normally once the spark has ignited the mixture, the flame will propagate outwards from the spark plug evenly in all directions. When detonation occurs some of the remaining air/fuel mixture situated towards the edges of the combustion chamber spontaneously combusts before the flame reaches it. When this happens a large spike in combustion pressure occurs. If severe enough detonation can cause engine damage in the form of pitting on the piston crown, broken ring lands, and scuffing of the piston from overheating.

 

To combat detonation there are a few different parameters which can be tweaked to help alleviate the problem. The air/fuel ratio can be altered along with the engine’s ignition timing to change the peak combustion temperatures, a fuel with a higher octane rating can be used which will be more resistant to detonation, and upgrades to the cooling system can be carried out to help keep the combustion chamber cooler.

 

Along with increasing the likelihood of detonation as a result of increasing the compression ratio, the engine will also produce more heat. The cooling system must absorb this additional heat and be able to adequately cool the engine, otherwise overheating and detonation may be problematic. Radiator size, thickness, and the speeds at which you ride at all play a big role in how efficiently the cooling system operates.

 

Now that you have an understanding of how high compression pistons affect performance, you can consider if this will be a good modification for you. Aftermarket pistons are usually offered in a few different compression ratio increases. You will want to look closely to see if any high octane fuels will be required to use in conjunction with the piston and if any cooling system improvements are necessary.

 

For racers looking to extract all the power from their bike, adding a high compression piston is one of the things that will be necessary. If you do a lot of tight woods riding, hare scramble racing, or enduros where low speeds are the norm, you may want to shy away from raising the compression ratio as the cooling system will have difficulty dealing with the increased heat at low speeds where airflow is limited.

 

I hope you enjoyed this excerpt on piston modifications and how they affect an engine. If you liked this write up and are interested in learning more about performance options and four stroke engine building, pick up a copy of my book. Right now the book is on sale at 20% off our list price when you order within the next two weeks.

 

You can grab your discounted copy off our site here: The Four Stroke Dirt Bike Engine Building Handbook Or on Amazon: Amazon Book Store

 

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Thanks for reading and have a great week!

 

-Paul

Paul Olesen
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Air Filter Maintenance - What You Need To Know
In my last post I shared an account of what happens when dirt gets past the air filter and into an engine. This was a telling tale, however I want to go further and discuss key components of what can be done in terms of maintenance to limit the chances of sucking in dirt. Whether you ride a two-stroke or four-stroke, it makes no difference, the importance of keeping dirt out cannot be overstated.

 

I want to start off by thanking those that left constructive comments in my previous post. Your insights into filter maintenance are much appreciated and help reinforce what I’m about to share.

 

How often should I change my air filter?
This depends entirely on the conditions you ride in. Dusty dry conditions will warrant more frequent filter changes than a damp riding environment where dust is non-existent. The amount of dirt accumulation that is acceptable is subjective, but I always err on the safe side. As an example, my filters are blue when freshly oiled and as soon as they start to become blotchy and start to turn color I change them.

 

Can I change my air filter too often?
Yes and no. I say yes only because every time the filter is removed there is a chance for dirt to enter the engine. A sensible changing regimen decreases the odds of dirt getting into the engine as the filter is removed/installed.

 

What to Use
I’ve personally been using FFT filter oil, however, there are many great options out there. No Toil’s water based oil system is something I’ve heard good things about and would like to try too. Asking other riders or doing a quick search will certainly turn up more great options as well.

 

Removing the filter
The main point I want to mention here is to be careful when removing the filter from the airbox so that dirt does not come off the filter or surrounding areas and find its way into the intake. On most bikes, fitting the filter between the subframe is a tight fit and dirt can occasionally come off as the filter is pulled up.

 

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To help prevent this, clean the subframe or any areas the filter is likely to contact prior to removing it. Also watch for dirt accumulation at the top of the filter between the sealing flange and airbox.

 

Airbox Cleaning
Prior to any cleaning efforts be sure to use an air box cover or stick a clean rag in the intake tract which will help ensure any dirt that is dislodged won’t make its way into the engine.

 

Filter Cleaning
The correct way to clean a filter depends entirely on the type of oil used. Petroleum based oils will require a two step cleaning process. First a solvent must be used which removes the majority of the dirt. Second, the filter must be cleaned in soapy water and rinsed.

 

Water based oils only require a one step cleaning process using soapy water or a water based filter cleaner.

 

Selecting Solvents for Cleaning Away Petroleum Oils
Air filters consist of multiple foam elements which are bonded together chemically with adhesives. Depending on the adhesives used in the filter, certain solvents may or may not react. If a reaction occurs, the joint can break down and the filter can be ruined.

 

When selecting a solvent, it is always a safe bet to follow the recommendations provided by the filter manufacturers. However, as many will point out through their own experiences, there are several potential solvents that can work in place of the manufacturer’s.

 

A quick forum search will surely result in an overwhelming number of hits on filter cleaning and potential solvent solutions. I personally use parts washing fluid which I've downgraded from the washer to a bucket.

 

Cleaning Technique
The biggest tip I can share here is to make sure you only squeeze the filter when cleaning, don’t twist it. Squeezing lessens the likelihood of the glued joints getting damaged.

 

Filter Oiling
The goal is to get complete uniform saturation without over oiling. This can be done a number of ways and is largely dependent on the method of application (rubbing in by hand, dunking in oil, spray on, etc.).

 

My preferred method is to dispense oil from a bottle and work it in by hand. I believe this process keeps the amount of excess oil at bay, isn’t too messy, and it’s relatively easy to get good uniform saturation.

 

Many filters have two stages, a coarse foam filter element good for trapping large particles and a fine element suited for trapping smaller contaminants. Be sure to work oil into both elements.

 

Remember when working the oil in to be gentle with the filter. Rub and squeeze but don’t twist.

 

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Once the filter is saturated with oil remove any excess by carefully squeezing the filter. Ideally, very little excess oil should get squeezed out, but remember, this is entirely dependent of how generous oil was applied. After excess oil has vacated the filter a nice even thin layer of oil should be visible.

 

Batch
Filter oiling is a dirty job. No matter how hard I try, oil always seems to end up where I don’t want it. To make things messy less frequently, batch the filter cleaning and oiling process. Buy a few filters, oil them, use them, clean them, and then repeat the process all over again so the task isn’t done as regularly.

 

Keep the pre-oiled filters in Ziploc bags so that they’re ready to go when you need them.

 

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Greasing the Flange
Is it necessary? I believe the directive to grease the flange of the filter may have originated long ago when the sealing flanges of filters were not predominantly foam. Nowadays whether grease is necessary or not is mostly personal preference accompanied by whether or not the filter cage and airbox seal flat to one another, and how tacky the oil is that is being used. Personally, I still use a waterproof grease on my filter rims, however I’m aware it is probably not necessary in all circumstances.

 

Installation
Keeping dirt off the freshly oiled filter during installation is the main challenge. There are a few helpful tips I can share for doing this.

 

First, make sure the bolt is installed in the filter cage! It’s frustrating when you forget it.

 

Once you’re ready to install the filter I find that rotating the filter 90 degrees to its normal direction so that it can more easily be slipped past the subframe makes things much easier. Once down in the airbox the filter can be rotated into position.

 

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The other option is to use a plastic bag as a shield effectively covering the filter while it is being lowered down. Once in position the bag can be removed.

 

Wrap Up
I hope you’ve enjoyed my post on air filter maintenance. If you have any questions or comments please share them below!

 

For those of you interested in all things engine related check out my book, The Four Stroke Dirt Bike Engine Building Handbook for more awesome information. In honor of Independence Day we’re having a two week sale where you can save 15% by entering offer code july4th at checkout if you order before July 17th.

 

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Thanks for reading!

 

-Paul

Paul Olesen
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I thought this week it would be a good idea to share with you an example of what can happen when dirt gets passed an engine's air filter. This will be a short post, but a picture is worth a thousand words. In my next post I’ll go into detail on how to properly care for your air filter to help ensure that this never happens to you.

 

The series of photos below shows a sad case where dirt has found its way into the engine and wreaked havoc. The photos are all from the KX250F I bought on the cheap with the sole intention of rebuilding the engine and documenting the process for my book, The Four Stroke Dirt Bike Engine Building Handbook. Honestly, I couldn’t have bought a better bike for the project, nearly everything on the bike was worn out or screwed up from the previous owner.

 

Here is how the air filter and airbox looked prior to disassembly.
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Here is the back side of the air filter. The filter was completely dry. There was no grease on the sealing face of the filter or the airbox flange. In this particular case, dirt could have got into the engine through the filter or between the filter and sealing flange. The amount of dried mud in the airbox and on the bike also makes me suspicious that muddy water got into the engine instead of just dirt. I honestly can’t say for certain.

 

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The airbox itself was also extremely dirty.

 

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Once the engine was disassembled I carefully examined the piston assembly and cylinder bore. At first, I could not get any of the rings to move freely. Only after I had pounded a pick between the ring ends of the compression ring was I able to get the compression ring off. As I removed the compression ring, a load of sand came with it.

 

This photo of the compression ring doesn’t do the situation justice. Some of the dirt was actually removed from the ring as I handled it.
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Here is a close up of the compression ring. Note all the grit!

 

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The oil rings didn’t fair any better, were just as stuck, and had a lot of dirt on them.

 

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Here you can see dirt inside the ring grooves and at the edges.

 

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Here is dirt I rubbed off the oil rings.

 

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Miraculously (and fortunately for me) whether the engine sucked in dirty air or water, it happened quickly and stuck the rings to the piston so they could no longer seal correctly, and the engine subsequently lost compression and power in a hurry. This speculation is based on the fact that the cylinder bore showed no signs of excessive wear or damage and it measured well within the service limits. This is an outcome I never though possible and is hard to believe.

 

I hope you enjoyed this brief write up on the damage that can result from ingesting dirt, whether from abnormal circumstances such as dropping a running engine into a mud hole or simply neglecting to take care of the air filter when running the engine in dusty conditions. In my next post I’ll show you how to care for and install your filters so these problems don’t happen to you! Questions or comments are always welcome and I enjoy hearing from you all!

 

-Paul
https://www.diymotofix.com/

 

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Paul Olesen
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Alright guys, this week I just want to share a short and simple tip with you on how to stay more organized during an engine build.

 

When it comes to major engine maintenance or repairs, usually the engine covers have to come off or the crankcases must be split. The covers and cases are almost always retained using different length bolts. The repercussions of installing the bolts in the wrong order upon reassembly can be very damaging. This is especially true if you install a bolt that is too short for its location and only a couple of threads engage, ultimately stripping the threads when you tighten the bolt.

 

So what’s an easy way to keep track of cover or case bolts that are arranged in a pattern of different lengths?

 

My favorite way to organize these bolts is to take a thin piece of cardboard (think cereal box thickness) and then slit the approximate bolt pattern into the cardboard so that the bolts cannot get mixed up. A picture is worth a thousand words so check out the one below. You need not be an artist to apply this tip, simply slit the pattern, add a couple reference points and you’re done!

 

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Do you have any organizational tips you’d like to share? Leave a comment below because I'd love to hear about them!

 

If you are looking for more helpful tips and engine building info, feel free to check out my book, The Four Stroke Dirt Bike Engine Building Handbook. You’ll find 301 pages filled with crucial and down-to-earth four-stroke engine building knowledge. You've got one more week left to use the offer code tt2016 and receive 15% off your order!

 

Paul

Paul Olesen
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I hope you enjoyed my last post on ice tire studding! The season in my neck of the woods has been a bit short this year and I may be getting back to the dirt sooner rather than later. Nonetheless, Part II, which covers mounting ice tires is now up on my blog. You can view it here: Ice tire mounting.

 

In today's post I'm going to shift focus back to the engine and talk a little about valve technology. Valve technology and manufacturing techniques have changed substantially from the earlier days of engine development and I want to share with you some information about the current valve technology being implemented in your engines. I also want to discuss one way you can get a feel for how much life is left in your valves. Let’s get started.

 

The following excerpt is copied directly from my book, The Four Stroke Dirt Bike Engine Building Handbook. If you want to learn more helpful tips, which will bring your maintenance knowledge and engine building skills to the next level, I’d like to invite you to pick up a copy of my book by clicking here. Be sure to use the offer code tt2016 to get 15% off when ordering!

 

Alright, on to valves shim sizes.

 

The cylinder head assembly of most engines will wear out before it resorts to telling you it has had enough by catastrophically failing. Diagnosing these wear signs and knowing when it is time to replace components is the key to keeping the cylinder head assembly from failing. Due to the aggressive camshaft profiles, high compression ratios, and high RPMs required to make a lot of power, the valves and seats typically are the first parts to wear out within the cylinder head. Worn valves and seats will cause the engine to become difficult to start, have low compression, and have reduced power.

 

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Modern valves found in dirt bike engines are made from either titanium or stainless steel alloys. Regardless of valve material, modern valve faces are either coated in a variety of anti-wear materials or hardened using various hardening processes. Common examples of trade names you might be familiar with include diamond like coatings (DLC) and black diamond coatings. These coatings are typically harder than the base material of the valve and help the valve resist wear, which occurs from ingesting dirty air and repeatedly contacting the valve seat. Coating and hardening processes are only present at the surface of the valve face. Depending on the type of valve and process used to harden it, the coating thickness can range from as little as 0.0001” (0.003mm) to around 0.003” (0.076mm). An easy way to visualize the thickness of the coating is to pluck a hair from your head and either measure it or feel it between your fingers. Most human hairs are around 0.002” (0.05mm) which should give you a good idea of how thick the coatings used on the valves are.

 

The important takeaway here is that if the coating is only a few thousandths of an inch thick, the valve can only be adjusted a few thousandths of an inch before it will have worn through the coating. Monitoring the starting valve shim size once the engine has been broken in (or new valves installed) and comparing that size to the shims required the next time the clearances are adjusted is a great way to assess valve health. Normally within the first 3-5 hours of breaking in a new engine the valve shim sizes may change slightly. This is due to the mating of the new valves to the seats and any valve seat creep which may occur. After this occurs and the valves have been shimmed to compensate, usually an adjustment up to around 0.004” (0.10mm) is all that can be done before a valve has worn through its hardened surface. Once this happens the valve face will wear much more quickly and start to wear out the seat as well. This will result in more frequent valve shim intervals and necessitate the need for having the valve seats cut. By paying attention to changes in shim sizes you will be able to approximate when the valves have worn through their hardened surfaces and must be replaced.

 

Thanks for reading and please leave questions or comments below. I enjoy hearing from you!

 

Remember you can get 300 pages worth of in-depth dirt bike engine information with The Four Stroke Dirt Bike Engine Building Handbook. Be sure to use discount code tt2016 at checkout to receive 15% off your order!

 

-Paul

 

DIYMotoFix.com

Paul Olesen
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Happy belated New Year's! I hope the holidays were good to you and that you're looking forward to a new season of two wheeled excitement. I know I am and I'm excited to get back to work on this blog.

 

In today’s post I’m going to get into the details related to tire studding with the help of an industry expert. To help bring you the best information I can on studding ice tires, I’ve enlisted Jarrett King of Two Wheel Endeavors to help with this article. For those of you that don’t know, Two Wheel Endeavours is heavily involved in supporting Canadian ice racing efforts and offers studded tires, ice racing accessories, and custom ice solutions. Jarrett was involved in the development of the Mitas Ice King tires, knows his craft, and brings a lot of knowledge to the table.

 

Many people are under the impression that there isn’t much to studding a pair of tires, just screw some screws in and you’re done right? There is actually a hefty amount of skill involved with studding tires. These skills come down to knowledge of screw angle, head position, and screw length. Of course there are many parameters which all affect how well the tire will perform, but today we are going to talk mostly about studding. This attention to detail is a huge reason that guys who have perfected the art of tire studding can make a living at it. I’m not saying this to scare you off from trying to stud your own tires, just that if you’re going to go for it, it will take some practice and advice from an expert.

 

Now I’m going to turn it over to Jarrett who will go into detail on the aspects of tire studding.

 

Key Factors Affecting Tire Performance by Jarrett King

 

Tire Choice: Selecting the right tires to stud is critical in terms of traction and tire life. Lug height, tread pattern, carcass thickness, and rubber composition all have a huge influence on how well a tire will work. Unfortunately, there is not a lot of data supplied by tire manufacturers available to help guide a person in the right direction, but there is plenty of empirical data floating around among the ranks of ice riding enthusiasts. To help get you started I put together a list of the most common ice tire choices.

 

Front Tires
Mitas Ice King - (Top Left)
Bridgestone ED11 - (Bottom Right)

 

Rear Tires
Mitas Ice King - (Top Right)
Kenda K335 - (Bottom Left)
Motoz X-Circuit - (Not Shown)

 

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Tire Liners: Depending on the tire chosen, a liner can be used that will provide protection for the tube and allows for the use of longer screws. The liner is usually a cut up street tire which fits inside the chosen tire.

 

Pattern: The pattern in which the screws are laid out on the tire has a huge influence on the traction and grip characteristics of the tire. Specific patterns may be tailored to provide more grip or slip depending on the rider and how the tire is used.

 

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Consistency: Care must be taken to ensure the screw pattern is consistent from one lug to the next. Any deviation in screw location and angle can cause the tire to wander as it moves over the ice.

 

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Rim Trueness: The trueness of the rim can have a big effect on how the tire performs. A wonky rim can cause inconsistency in screw alignment. This can lead to similar handling problems because the screw pattern is not aligned accurately.

 

Screw Type: AMA or Canadian style screws are the primary options for competitive ice racing. The two screw types are defined below:

 

AMA screws - 3/16” head height, sizes #8 or #10, and range in length from ⅜” to 1 ½”

 

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Canadian screws - ¼” head height, size #12, and range in length from 1” to 1 ½”

 

Along with the screw requirements for the different racing classes, keep in mind purpose made ice screws go through a different hardening process than normal hardware store screws, allowing them to stay sharp longer. If you’re going to stud a pair of tires and want longevity, be sure to use a good quality screw such as those offered by Kold Kutter.

 

Screw Threads: Fine thread screws are preferred because they do less damage to the rubber during installation. They are also easier to set to the correct height when fine tuning the screws.

 

Screw Angle: The angle the screw is driven into the tire dictates how the screw contacts the ice. The screw angle can be broken down into two parts, the fore/aft angle, and the side angle.

  • Sweep: Tire builders refer to the fore/aft angle of the screw as the sweep angle. Ideally only the leading edge of the screw should make contact with the ice. This can be achieved by angling the screw anywhere from 10 to 30 degrees upon installation.

  • Side Angle: Screws used to grip the ice when the bike is leaned over will be installed at an angle which complements the contour of the tire.


Head Alignment: The alignment of the slot in the screw head can be tuned to provide better grip in a given direction. For screws used for braking (front) and drive (rear) the screw slot is aligned perpendicular to the direction of travel. For cornering grip the slots of screws on the side of the tire are aligned parallel to the direction of travel.

 


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Paul: Now that Jarrett has provided a great framework of what goes into studding a tire, we’re going to get into the specifics. It was mentioned previously that ice screws have three primary functions: braking, accelerating, and cornering. Next, we’ll get into the details of what makes each of these three types of screws functional for their specific purpose.

 

Braking Screws: Braking screws are at the rear of the lug on top, but when they are on the ground they are on the leading edge (biting edge) when under braking, thus the name “braking screw”. Sweep is used to prevent the screws from chattering on the ice under braking because of the fact that the crown would strike the ice at two points if installed flat. The magic sweep angle is the shallowest possible angle without the “rear” part of the screw crown biting in. With Canadian screws, this angle is much more straight up and down but still usually has 10 degrees or so. If you leaned them too far forward it will damage the knobs because the screw isn’t sunk into the liner enough, if you went straight in they will function but it makes the tire feel a bit strange under heavy braking.

 

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Acceleration Screws: Again, there are differences between AMA and Canadian screws. The Canadian screws can go virtually straight in, AMAs need that biting edge so they don’t deflect or lose traction because of two different contact points. Picture a skate blade. The more sharp and precise the edge, the more ground pressure is focused on that area. Same with screw tips, if two parts of it hit the ice it will start to “float”. Optimal angle is shallowest possible (as close to straight up and down as possible) before the back edge of the screw starts touching the ice surface.
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Side Grip Screws: Cornering screws are typically run in at one angle, there is no sweep to them. Some builders have tried adding some sweep, however, never with too much angle. If you run an ice tire over a piece of cardboard under lean you will see that the top edge of the screw is contacting the ice at an angle that prevents the front tire from low-siding. In essence these screws do the exact reverse thing that the rear tire does under acceleration.

 

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Paul: The last thing Jarrett is going to talk about is the screw pattern and some of the compromises that are made while studding.

 

Jarrett: When it comes to general screw pattern and arrangement, there are a couple things to consider. First, is that on the rear tire the inverse “V” pattern is there for a reason… what it does is each screw passes the load onto the next screw while under lean (picture them passing sandbags to each other). To prove this, reverse a V-pattern tire, it will be all kinds of squirrely under acceleration and then the rear will try to jump out from under the bike when you hit the brakes, it’s truly scary.

 

My second point, ideal screw pattern is a balance between a few different factors. Knob spacing, contact pressure and knob count/pattern. On a tire like a Kenda, so many screws are striking the ice at once that the tire is floating on the surface of the ice. Traction is being gained by getting the maximum number of screw heads to hit the ice at the same time. This is great until the moment that there is a hint of snow on the lake and the tire begins to act like a crazy carpet under the bike. It floats because it can’t maintain ground pressure.

 

The old Pirelli Lagunacross tire became amazing the moment that Marcel Fournier came out with the modern Canadian Ice screw. The knob pattern was ideal (V shaped paddle) for the application and the knob spacing was super wide, which meant great ground pressure on each screw. Unfortunately that also meant a much larger radial load on the screws and knobs which often lead to premature knob or screw failure. The Mitas Ice King does not generate the same ground pressure as the Pirelli because the knobs are quite a bit closer but the tire’s compound and knob pattern allow for a much better balance of ground pressure (traction) to durability ratio. Using AMA screws, a Mitas Ice King does not benefit from additional screw rows the way that a Kenda will because it will float much quicker, but without as many screws contacting the ice.

 

Ice tire building is a compromise. The perfect ice tire doesn’t exist in the same way that there is no perfect Intermediate MX tire… but there are some that are MUCH more effective than others.

 

My third and final point, ice tires have been built in north America since the early 30s. The angle of screws is something that has been tried in multiple arrangements hundreds of times over. For someone getting into the sport their enthusiasm may make them believe that changing things will create a magic setup, but the reality is that a true set of wheels (no dings dents, warps), with consistent screw angles and heights, proper air pressures, and properly balanced is the most effective way to kick ass out on the ice racing track. Oh and don’t forget to duct-tape that face (frostbite sucks!).

 

Paul: Whether you're new to the sport or have a few seasons under your belt I hope you found Jarrett’s info on ice tires beneficial . Check the DIY Moto Fix website for part two in a week and here again for part three. For those of you in warm states I encourage you to take a trip to a cold destination and give ice riding a try! Be sure to check out Two Wheel Endeavors if you're in need of tires or anything else ice racing related. If you have any comments or want to share some info please leave a comment below.

 

-Paul
DIYMotoFix.com

Paul Olesen
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I hope you’re all having a good fall and are getting excited for the holidays. It snowed for the first time this year here in Wisconsin and I’m getting eager for the lakes to freeze over so I can get out and ride the ice. I need to set aside a good 7 hours to stud my tires and set up my bike before that can happen though!

 

Today I want to talk about six characteristics that are necessary to have when one sets out to build an engine. I’ve detailed how to tackle many different jobs, but honestly that is only half the battle. If you’re in a rush or lack the desire to understand the reasons behind what you’re doing, you will make mistakes and miss out on important things. Listed below are the traits that I believe can help you take your build to the next level.

 

1. Being Detail Oriented
What’s worse than getting started on a build only to realize you didn’t buy an important replacement part? Focussing on the details of a project can feel tedious at times but can pay off in the grand scheme of things. Before I get started on a project I spend a hefty amount of time researching what parts I’m going to replace and where the best prices are. Also, I will have a solid idea of the sequences I’ll use for disassembly and assembly. Another good habit for the detail oriented is to take notes throughout the build, which you can use at a later date should the need arise. When you have an appreciation for all the small details that go into a build, it will make for a much smoother project.

 

2. Having Patience
Have you ever been in a rush to do something and after you’re done you realize if you had spent just a bit more time the project could have turned out much better? I was this way with so many of the things I did when I was younger, but have learned to slow down and be patient as I work. Engines don’t go together instantaneously and being patient throughout the process, especially when things aren’t going as planned, is very important. There is nothing worse than making a huge mistake because you’re in a rush. Imagine finishing a build and realizing you left an important part on the table, depending on where the part came from, you just bought yourself another few hours of work. Trying to skimp on time more often than not costs you more time in the long run. Have patience and enjoy the process.

 

3. Being Observant
Just about every mechanical thing is gleaming with a story, and that story only reveals itself if you know what to look for. An engine is no different. From the parting lines on a component left by the casting tooling used to create it to wear patterns on a piston, there are hundreds of observations that can be made while working on an engine. As you work, keep an eye out for subtle anomalies that may tell you why something failed or broke. For example, things like snail tracks across a gasket, raised edges on gasket surfaces, or covers that don’t sit flat on a table - these are all good indicators of why a particular part was leaking.

 

4. Being Curious
Perhaps more appropriately titled, “a desire to understand mechanical workings”. It is incredible how much can be learned about the engine just by studying how specific parts interact within it. An engine is composed of many different subsystems and they must all work in order for the engine to function. By looking at the various interactions of the parts within an engine, the condition of the parts and reasons for any failures can be more easily understood. The next time you build an engine, challenge yourself to learn how all the different subsystems of the engine work. Once you learn this, diagnosing problems and identifying all the faulty parts becomes much easier.

 

5. Being Meticulous
The necessity to be thorough and meticulous throughout a build cannot be overstated. Whether it be taking extra steps to inspect components, measuring new parts, or taking extra time to ensure the condition of surrounding subsystems are okay, having meticulous tendencies can pay off. As an example, on more than one occasion I’ve purchased new parts that have been mispackaged or out of spec. Had I not made the choice to carefully measure the problematic new parts, I could have ended up with an engine that was destined to fail. While it may take more time to be meticulous throughout a build, there is a lot at stake, both in terms of time and money, making it all the more important to ensure everything is done correctly.

 

6. Having Ambition
Building an engine can be hard, things can go south unexpectedly, and projects can easily stall. Being ambitious and having a can-do attitude is important to ensure the engine doesn’t sit half torn apart in the garage never to be completed. Until you tear into the engine, you never know what you might find. I’ve disassembled engines many times in the past only to find I need to replace a lot more parts than I had planned (this seems to be my luck when I shop for bikes on Craigslist as of late). This can be a huge downer, but keeping the end goal of getting back out and riding in mind and having the desire to push through any and all obstacles is a must.

 

Do you have any engine building characteristics you want to share? Leave a comment below and tell everyone what you think it takes to build a great engine!

 

For those of you that believe you possess the characteristics of a good engine builder, be sure to check out my book, The Four Stroke Dirt Bike Engine Building Handbook, to learn more about the how and why behind engine building. Whether you want to be taught about the relationships between all the various parts within an engine, you are in need of pointers on picking the right performance parts, or you would like to see examples of wear patterns found on engine components, my book is here to guide and help you throughout your build.

 

With the holidays coming up, I want to extend a special four day offer to you for the handbook and all the other products at DIY Moto Fix. Between November 27th and November 30th if you purchase anything from DIY Moto Fix you will save 30% on your order. If you’ve got a significant other trying to do some holiday shopping for you, be sure to send the site their way before Monday the 30th ;)

 


Save 30% and check out the book and other products by clicking this link: DIY Moto Fix

Paul Olesen
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Octane Ratings
In my last post I shared a tip I like to use when filling up fuel cans at the gas station. I also presented an example detailing how mixing two different fuels with different octane ratings affects the blended fuel's octane rating. Out of that post some comments were left regarding the necessity of running specific octane fuels. Today, I want to discuss the octane attribute of fuels in more detail.

 

What is the Octane Rating and How Is It Determined?
The octane rating associated with a fuel is a measurement of the fuel's resistance to detonation. To measure a fuel's octane rating it is either tested in accordance with test procedures established for the Motor Octane Number (MON) or the Research Octane Number (RON). In summary, a single cylinder engine with a variable compression ratio is used for testing. MON test procedures require the engine be run at 900RPM, inlet air temp be 38C, and ignition timing is set between 14 and 26 degrees BTDC (specific timing depending on the compression ratio). RON procedures are slightly less stringent with the engine operating at 600RPM, inlet temp between 20 and 52C (barometric pressure dependent), and the ignition timing fixed at 13 degrees BTDC.

 

The test fuel is run in the engine, the compression ratio is increased, and the point where the engine starts to detonate is observed. Once the test fuel has been run, its octane rating is determined by blending two base fuels together. Iso-octane with a RON of 100 and heptane with a RON of 0 are mixed together until the the engine detonates under the same conditions as the test fuel. By blending, the octane rating of the test fuel can be established. For example, if the blend of iso-octane and heptane resulted in a mixture of 13% heptane and 87% iso-octane then the test fuel would have an octane rating of 87.

 

What Rating System is Shown on Gas Pumps?
This depends where you live. In the United States the octane number on gas pumps is the average of the MON and RON numbers established for the fuel. In Europe the RON number is used. This is why if you ever go to a gas station in Europe it looks like they have higher grade octane fuels than we do in the U.S.

 

How Does the Octane Rating Affect Power?
The higher the octane rating of a fuel, the more resistance it has to detonation and pre-ignition. The more resistance a fuel has to detonation, the higher the engine's compression ratio can be set (within limits) or boost pressure if we're talking turbos.

 

When an engine is designed one of the driving parameters for development of the design is the type of fuel(s) that will be compatible with the engine. For example, if an engine is designed to work with fuels that have a minimum octane rating of 87, then the compression ratio of the engine must be limited to a value which ensures the engine never detonates when 87 octane fuel is used. For this example, say the compression ratio is 12:1. With a 12:1 compression ratio when fully optimized, the engine will produce a finite amount of power. Say 50hp.

 

Now take the same engine, raise the compression ratio to 13:1, and require a fuel with a 93 minimum octane rating be used in the engine. Due to the increase in compression ratio, the engine's power will increase. Perhaps to 52hp. This power increase is only possible by raising the compression ratio and increasing the octane rating (detonation resistance) of the fuel being used.

 

The takeaway here is that you will not see a power increase in an engine designed to run on 87 octane by putting in fuels with higher octane ratings without changing another parameter, such as the compression ratio. You would however increase the engine's detonation resistance by using a higher octane fuel. This may be beneficial if the engine is being operated under heavy loads or in harsh conditions where heat loads on the engine are abnormally high and the engine is more prone to detonation.

 

Wrap Up
I hope my brief explanation on octane ratings has shed some light on how they affect engine performance. As always, if you have questions or want to leave a comment please do so below. I enjoy hearing from you.

 

At this point, I want to thank everyone who has purchased a copy of my book. I appreciate the support and warm response to the release of the printed version. For those of you interested in the handbook please check it out by following this link: The 4t Handbook

Paul Olesen

Filling Up At The Pump

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How Residual Pump Fuel Affects Your Fill Up
This week I have a quick tip I want to share with you regarding buying fuel and filling up gas cans for your bikes. I know many of you, myself included, rely on premium grade gasoline dispensed from local gas station pumps to put endless grins on your faces. One of the downfalls of gas station pumps is that fuel from the previous sale is left in the hose. According to the American Petroleum Institute, the amount of fuel left in a gas pump's hose is around 1/3 of a gallon.

 

Generally speaking, when two fuels are blended the octane rating of the resulting fuel is approximately the average of the two fuels. So if you had a gallon of 87 octane and a gallon of 93 the resulting blend would have an octane rating of 90. I'll be the first to admit that 1/3 of a gallon of fuel added to a two gallon gas can won't have much effect on the octane rating. For those of you that like numbers, 0.33 of a gallon of 87 added to 1.67 gallons of 93 will yield the following octane rating:

 

0.33 gallon of 87 / 2 gallons = 16.5% of the total mixture
1.67 gallons of 93 /2 gallons = 83.5% of the total mixture

 

(0.165 x 87) + (0.835 x 93) = 92 octane blended fuel

 

So in a two gallon can, the octane rating of the fuel has dropped a point due to the 1/3 gallon of 87 in the pump hose. Unless you have a very well developed performance engine, this isn't anything to lose sleep over. I think a bigger reasons to want to keep that 1/3 of a gallon out of your can is due to the possibility of ethanol being in the hose from the previous sale. Many articles can be found outlining why ethanol should be avoided, but the main reasons include part corrosion due to the exposure to alcohol, rubber seals and o-rings may not be compatible with ethanol resulting in swelling and failure, and some plastics deteriorate when exposed to ethanol. Not to mention ethanol contains less energy than gasoline. Again, we're not talking about a large percentage of ethanol in the overall scheme of things but I prefer to stay away from the stuff when I can.

 

Fueling Tip
I'm very careful about what I run through my powersport engines. To safeguard against filling up a fuel can with residual fuel from the previous sale, I like to donate the first gallon of "premium" to my vehicle before filling my gas cans. This ensures whatever fuel was in the hose and pump is flushed out and that I'm filling up my cans with premium. If you are borderline OCD about what goes in your engines like I am, you may consider adopting this practice.

 

I suspect many of you have other tips and tricks regarding fueling. Leave a comment below and share your thoughts and experiences so other motorheads can benefit!

 

Book News
I also wanted to invite you to check out my book on how to build four-stroke engines, which is now officially available in print form. It took a ton of work to bring the print book together and get the right help on board. The project hasn't been easy, but I'm proud to offer this book to you and can assure you it will make a great addition to your workshop. You can learn more about the book by following this link: The Four Stroke Handbook

 

To celebrate the arrival of the print book, I'm running a sale until the 27th of September offering all versions of the book at a 20% discount. After the 27th the sale will end and the price will go up. If you've got a build coming up now or in the future and are interested in the book, now is a great time to pick up a copy.

 

Thanks for reading and have a great week!
-Paul

 

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Paul Olesen
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Hey everyone, this week we're going to switch it up and talk shock absorbers! Over the last few months I've gotten many requests to broaden the topic spectrum and cover other dirt bike topics. So today, we'll do just that by discussing and providing some resources to get you familiar with servicing shocks.

 

I suspect many of you currently take your shock to someone to have it serviced when it needs to be freshened up. I also bet that it is usually a pain to be without a bike for perhaps a week and that it probably costs around $100 each time? I know I always dreaded having suspension work done on my bike because it seemed to take forever, plus I always had to drive over an hour and half to the nearest shop. For me, those days are long gone. Now do all my suspension work myself.

 

I believe the majority of you are completely capable of servicing your shocks yourself, but just don't quite have all the pieces of the puzzle you need. Maybe you're not quite sure what tools you need; or once you get the shock apart, you don't know what parts you will have to replace? To help clarify what's needed to service a shock and answer some of the common questions about shock building, I created a detailed guide for you. The guide will help you decide if outsourcing your shock maintenance is the way to go or if you are in fact ready to take the job on yourself.

 

Before I discuss the details of the guide, I want to provide you with a little background on shock absorbers. For major motorcycle brands, shocks are sourced from the following companies: Showa, KYB, and Works Performance (WP). These three brands are primarily the companies responsible for equipping OEM bikes. Companies, such as Ohlins, cater more towards the aftermarket. Out of the three common OEM shock brand options, Showa and KYB are the go-to's for the Japanese manufacturers, while European brands, such as KTM, gravitate toward the WP brand. So if there is any question as to what brand of shock you have, you can keep this in mind. Out of the three common OEM brands, Showa and KYB shocks are very similar, while WPs feature a slightly different design.

 

The guide I created is geared towards those of you with either Showa or KYB shocks. Those of you with WP shocks may still find the guide useful, but there are a couple tools missing. Within the eight page guide, you'll be provided information on all the tools you need to service a Showa or KYB shock. These tools include any specialty tools and discuss shock pressurization options. Plus there are some pointers on how to make your own specialty tools if you are on a budget.

 

Once you get through the tools section you'll be presented with a detailed outline on replacement parts. Knowing what to replace within the shock when it is due for servicing is extremely important and the replacement parts section will walk you right through what you may need. It will also provide you with different options for buying replacement parts.

 

To receive the eight page guide and learn more about shock servicing, click the following link: Shock Building Tools and Replacement Parts Guide

 

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Thanks for reading and feel free to comment below with any questions or concerns. Bringing high quality DIY advice is what Moto Mind is all about, and I enjoy hearing from all of you and your DIY experiences.

 

-Paul Olesen
DIY Moto Fix

Paul Olesen
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Hey everybody, hope you are having a great summer thus far! I've been able to get on the bike a lot this summer and I hope you've been getting your fair share of riding in too. A couple weeks ago I covered what a leak down test is, in this blog post here, and how it can be used to determine engine problems. Today I’ll go through and detail how to do a leak down test on your dirt bike engine step-by-step. Now that we're past the halfway point of the summer riding season it may not be a bad idea to check in and see how your engine is doing by performing a leak down test.

 

How to Perform a Leak Down Test

 

To perform a leak down test you will need an air source capable of at least 115psi output pressure. Most leak down tests are performed at a regulated pressure of 100psi. This makes testing simple and the correlation of leakage a breeze since you’re working on a scale from 0-100. Lower test pressures such as 90psi can be used in the event that the air system isn’t capable of anything over 100psi or the specific leak down tester you have doesn’t work on a 100psi scale. Just remember if you test at a value other than 100psi, you will need to mathematically determine the leakage percentage since it is no longer a direct correlation.

 

Leakage testing can be tricky when working by yourself, so if you’re able to round up a friend to help you out it makes things a lot easier. With 100psi of pressure being applied to the piston it is a full time job making sure the piston and crankshaft don’t move. Even with a long breaker bar, holding the crank in place can be quite a task, couple that with having to operate the pressure regulator and obtain a good reading. Again, I highly recommend having two sets of hands on deck for this procedure.

 

How To Calibrate Your Leak Down Tester

 

A leak down tester consists of an air inlet, a pressure regulator, two pressure gauges, and an air outlet which pressurizes the cylinder. It is important to check to see if both pressure gauges read the same prior to any testing. If they do this is great, however occasionally the gauges won’t read exactly the same so a baseline will need to be established.

 

1. Start by setting your air source so that its output pressure is 115 psi. By setting the air source higher than the test pressure this will ensure it does not interfere with the pressure regulation during the test.

 

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2. Slowly increase the leak down tester pressure by adjusting the regulator. Set the incoming air pressure to 100psi.

 

3. Once the incoming air pressure is set at 100psi read the outlet pressure gauge. If the outlet gauge reads 100psi you are good to go. If the outlet gauge is adjustable set the gauge so it also reads 100psi. If the outlet gauge is non-adjustable (most common) write down the outlet gauge pressure reading. Here you can see the outlet gauge reads 97psi. Since no pressure is being lost between gauges the only explanation for the difference in reading is gauge deviation. The 97psi value will be equivalent to 100psi.

 

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4. After you have recorded the outlet gauge pressure that corresponds to the inlet gauge pressure you are ready to start leakage testing. Set the initial regulated pressure back to 0psi so that the tester doesn’t rapidly and unexpectedly pressurize the cylinder when you reconnect the air line for the test.

 

Leak Down Test Steps
1. Make sure the petcock is turned off and remove the fuel line from the carburetor or throttle body. Use a rag to catch any fuel draining from the line.

 

2. Remove the seat and fuel tank from the bike.

 

3 . Remove the radiator cap and pull out the crankcase breather tube.

 

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4. Remove the spark plug cap . Prior to removing the plug blow compressed air into the plug cavity to rid it of dust and debris so that it can’t get into the engine. Then remove the spark plug.

 

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5. Remove the crankshaft hole cap cover so that you can gain access to the crankshaft.

 

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6. If working from the clutch side of the engine, rotate the engine over so that the piston is just shy of TDC (approximately the width of a punch mark off) on the compression stroke. There are usually alignment marks which can be used depending on the manufacturer to help gauge how close to TDC you are. On the Honda pictured there are punch marks on the crank and balances shaft gears denoting TDC. As you rotate through to just shy of TDC feel for compression building up in the cylinder. As you turn your wrench, resistance should build up as you approach TDC. If you do not feel resistance, rotate the engine over once more and realign to just shy of the TDC markings. This will take you from the exhaust stroke to the compression stroke where you should feel the compression start to build.

 

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If working from the flywheel side of the engine, rotate the engine over so that the piston passes TDC by around a ¼ of a rotation of the crankshaft. Reverse direction and set the crank so that it is just past TDC using the applicable alignment marks. On the Kawasaki pictured you can see the alignment slots. As you came up on the compression stroke you should have felt a little bit of compression build up. If you do not feel resistance, rotate the engine over once more and realign to just shy of the TDC markings. This will take you from the exhaust stroke to the compression stroke where you should feel the compression start to build.

 

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7. Install the leak down tester in the spark plug hole with the regulated pressure set at 0psi initially.

 

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8. It usually takes a breaker bar and two hands to lock the crankshaft from moving once the cylinder is pressurized. This is where having an extra set of hands helps immensely. One person should focus on keeping the crankshaft in position while the other operates the leak down tester. Remember to set the piston just shy of TDC (whichever side is applicable for your application) and to lock the crankshaft in place while the piston is still traveling upwards so the rings sit in the bottom of the ring grooves.

 

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9. Once the crankshaft is locked in place pressurize the cylinder. Slowly turn the pressure regulator up to 100psi.

 

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10. Note the reading of the outlet pressure gauge. This corresponds to the amount of air the combustion chamber is retaining. If both pressure gauges read 100psi when they were calibrated the difference in pressure on the outlet gauge is the cylinder leakage. For example if the outlet gauge reads 95psi the cylinder leakage would be 5%.

 

If however during calibration both gauges did not read 100psi then the leakage will need to be calculated. For example if the outlet pressure gauge read 97psi during calibration and 92psi when the combustion chamber was pressurized, then the leakage could be found by dividing 92 by 97.

 

92 ÷ 97 = 0.948

 

0.948 x 100 = 94.8%
100 - 94.8 = 5.2% leakage

 

11. Open the throttle and listen for an audible hissing sound coming from the intake, exhaust, or crankcase breather. Also look in the radiator for air bubbles.

 

Here are the four problems that can result:

 

1. Air passing through the intake indicates leaking intake valves.
2. Air passing through the exhaust indicates leaking exhaust valves.
3. Air passing through the crankcase breather indicates worn rings.
4. Air bubbles forming in the radiator indicates a leaking head gasket.

 

12. Depressurize the combustion chamber by turning the regulator back so it is at 0psi and no air is entering the combustion chamber. Unhook the air source and disconnect the leak down tester.

 

13. Reinstall the spark plug and plug cap.

 

14. Reinstall the radiator cap and crankshaft hole cap.

 

15. Reinstall the fuel tank and seat.

 

That's all there is too it! I hope you have enjoyed my posts detailing how to perform a leak down test. With a little practice you will quickly be able to determine what is going on inside your engine and be better suited to detect major problems before they turn into engine failures. If you have questions, a technique you want to share, or want to leave a comment please do so below.

 

If you found this post helpful and want more information on how to diagnose engine problems or how to perform high quality engine builds in your own garage, check out my book, The Four Stroke Dirt Bike Engine Building Handbook. You can pick up the eBook immediately and the print book will be out soon!

 

Thanks for reading everyone!

 

Paul
DIY Moto Fix - Empowering and educating riders

Paul Olesen

Save The Salt

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I want to switch gears with this post and bring your attention to an issue that is near and dear to myself and many land speed racers. Over the past decade salt mining has caused track conditions at the Bonneville Salt Flats to become worse and worse. Years ago, the salt layer was up to 5 feet thick, today it is less than an inch in some places. Salt mining is partly to blame for the depletion of salt at Bonneville and holding racing events there is becoming extremely hard!

 

I know not many of you here on TT may participate at, or have ties to Bonneville, but this is great opportunity to band together as powersport enthusiasts and help out those that do. Please help your fellow motorheads by signing the following petition which asks for salt mining to stop at Bonneville. I've signed and hope you do too!

 

Save the Salt Petition

 

The salt flats at Bonneville are a special place and it would be unfortunate to not preserve them for future generations. Thanks everyone!

 

Paul

 

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Paul Olesen
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Determining how healthy an engine is can be tricky business. I’ve previously covered compression testing (here and here), but now I want to discuss what a leak down test is and how to perform one on a four-stroke dirt bike engine. This will be a two part series, with the first part emphasising the details of a leakdown test, and the second part explaining in detail how to correctly perform a leak down test. Let’s get started! Leak down testing is a more definitive way to assess the health of an engine compared to compression testing because a leak down test allows the mechanic to pinpoint the problematic area within the engine. Whether the valves are no longer sealing, the rings are worn, or the head gasket is leaking - a leak down test can be used to find and diagnose all these potential issues.

 

The way a leak down test works is fairly simple. With the piston just shy of TDC and the valves closed (compression stroke), air pressurizes the cylinder to a defined pressure which is recorded by a pressure gauge. A second pressure gauge is used to monitor the amount of air escaping the combustion chamber. A comparison is made between the air going into the cylinder and the air escaping. The percentage of air escaping is used to determine the overall health of the engine.

 

The amount of air escaping can roughly be quantified to assess the condition of the engine. When race engines are built, the accuracy and precision that goes into the build results in the lowest leakage values. Most race engines will have a pressure loss of between 0% and 5%. Standard builds resulting in good running engines typically lose up to 15%. Any engine that is close to or past being ready for service will leak from 16% to 30%. These engines will most likely be running poorly, if at all. Engines beyond 30% leakage more often than not are broken and will not run. The more the engine leaks, the worse the engine’s health. Keep in mind these values are provided as a reference point and each engine can be a little different.

 

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It is possible to pinpoint where leaks are coming from once the cylinder is pressurized. When performing the test, the throttle should be fully opened and the radiator cap should be removed. Air can exit the combustion chamber at four points: past the intake valves, past the exhaust valves, past the rings, or past the head gasket. Each of these four points will exhibit a unique tell-tale sign if air is leaking.

 

Intake valve leaks can be diagnosed by listening for air escaping out the carburetor or throttle body. Exhaust valve leaks can be found by listening for air escaping out the exhaust system. A leaking head gasket will result in air bubbles showing up at the radiator fill cap neck. Excessive leakage past the piston rings will result in pressurizing the crankcase and the resulting air can be traced out the crankcase or cylinder head breather hose. Air escaping past the rings may also be heard or felt passing through the access hole in the engine side cover, where the wrench has been inserted to position the crankshaft. The location at which the air exits when it leaks past the piston rings will depend if the engine has a separate crankcase cavity or a joint cavity. Usually on engines with a separate cavity, the air will be routed through a one way valve, which then directs the air up into the cylinder head and out the cylinder head breather hose.

 

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I want to address one concern you may have at this point. You have probably heard before that the piston should be at TDC when performing a leak down test. In reality there are a couple issues with this that I am going to cover. First, with the short stroke engines of today, keeping the piston precisely at TDC with 100 psi of pressure pushing down on it is next to impossible. The piston and crank will want to rock to either side of TDC. Controlling which side of TDC the piston rests is important. Depending on which side of the engine is used to lock the crankshaft in place and the direction of rotation of the engine, the nut or bolt used to lock the crank in place may either try to tighten or loosen itself from the air pressure pushing against the piston. Even though these are highly torqued fasteners, the air pressure can still occasionally loosen the nut or bolt. This creates a serious problem, because now you’ve got to figure out how to lock out the crankshaft and retorque the nut or bolt.

 

The second issue is not so much a problem as it is a minor detail. To best simulate ring sealing conditions the rings should sit in the bottom of the ring grooves. This is how they would sit on a running engine and how the test should be performed. By ensuring the rings always sit in the bottom of their grooves, another level of repeatability is added to the test. Simply make sure when setting piston position that the piston is always traveling up just before you hold it in position. If you are working from the left side on a forward rotating engine, it will be necessary to rotate the piston past TDC then reverse direction so the rings sit in the bottom of their grooves and the flywheel nut will not try to loosen itself from the air pressure.

 

I hope you enjoyed my write up detailing leak down testing. Stay tuned for my next post where I’ll show you exactly how to perform a leak down test yourself. In my eBook, The Four Stroke Dirt Bike Engine Building Handbook, I cover leak down testing in further detail and invite you to pick up a copy if you want to learn more about how to diagnose engine troubles and build four-stroke engines.

 

Click Here To Learn More About The 4T Engine Building Handbook

 

If you have questions or thoughts, as always, I enjoy hearing them! Leave your comments below. Don't forget to follow my blog by clicking the button in the upper right hand corner of this page! Thanks everyone.

 

-Paul Olesen
DIY Moto Fix - Empowering And Educating Riders From Garage To Trail

 

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