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Found 5 results

  1. Hey TT! You all know me as that one annoying 15 year old kid that rides dirt bikes on the street for YouTube content. Well I bought a 2002 YZ 125 that had no compression, and I got it for cheap. So, today I fixed the float and went for a ride, but felt like enough was enough. I called my dad to come and help me take off the top end, and so we did. Be aware that I am a 15 year old kid, that has no experience with top ends, and am basing everything off one 17 minute YouTube video that was watched two weeks ago. Anyways, here's how it looks. Now here's my question... What size will I need to purchase? It is a 53.96 currently. Also, will I have to bore it out or is it good? Posted pics of the cyinder too. OR, I can do the 144cc kit. Eric Gorr is charging $450-$500 which is too much, any cheaper alternatives? Gary, where are you?? Two Stroke YZ Doc I need your help man! Thanks!
  2. Rebuilding a top end is a task most two-stroke owners will run into at one point or another. Here, we go over critical steps and key tips to installing a new piston and ring(s) in your two-stroke. Periodically, if you own a two-stroke, there will come a point where you need to rebuild the top end of your engine. Hopefully, this won’t come as a surprise to you and will be part of your planned maintenance schedule versus experiencing an unplanned engine failure. While two-stroke engines are relatively simple mechanical devices, rebuilding them requires knowledge of how they work, attention to detail, and a systematic approach. We’re going to cover numerous tips pertinent to two-stroke top end rebuilds. These tips will be discussed chronologically and will encompass all phases of the build from pre-rebuild prep, to disassembly, through post build. The tips we’re going to share shouldn’t be considered inclusive of everything that has to be done, but are tips that focus on things that are either often overlooked or incredibly important. Let’s get started! Pre-Teardown Diagnosis - Before tearing the engine apart, are there any signs that a specific problem exists? If so, are there any diagnostic tests such as compression or crankcase leak down that are worth performing? Before tearing your engine down, asses the specific problem with you're engine if you're rebuilding due to a running problem. Clean Machine - Take time to thoroughly clean the machine before opening up the engine, especially if you will be servicing the top end without removing the engine from the machine. Service Manual - Performing engine maintenance without an OEM factory service manual is not recommended. Make sure you have a manual for your machine prior to starting work. The manual is the only place you’ll find service limits, torque specs, and other key data. Disassembly Limit Contaminants - Once the cylinder has been removed wrap a clean, lint-free rag around the top of the crankcase. Dirt is one of the leading causes of engine wear, and limiting the opportunity for dirt to enter the crankcase is very important. Keep a lint-free rag at the top of the crankcase at all times while it is open and exposed to potential contaminants. Piston Removal - Easy piston circlip removal can be accomplished by using a pick and needle nose pliers. Insert the pick into the dimple in the piston and behind the circlip. Then use it as a lever and pry the circlip out partially. Once out partially, grab the circlip with needle nose pliers. During this process, be careful not to scratch or mar the wrist pin bore as this will make removing the wrist pin much more difficult. Use tools as needed to aid in circlip removal, but be careful not to mar the pin bore so the wrist pin can be easily removed. The ease of pin removal will be largely dependent on the engine design and condition of the bore. If the pin can be removed by hand, great, if not, light tapping while supporting the rod is permissible. Otherwise, a pin puller should be utilized which can be bought or made. In its simplest form, this can consist of an appropriately sized bolt, nut, and socket. Once the wrist pin has been removed, the piston can be removed from the rod. Hopefully, the wrist pin can be removed by hand once the circlip is out. If not, an appropriately sized socket with some light tapping from the opposite end can help break it loose. Power Valve Disassembly - Prior to taking the power valve system apart, spend some time reviewing the procedure in your service manual. For additional insight into how the components interact, review the exploded views in the service manual and look at part microfiches, which can be found online. Online microfiches can be very helpful to double-check reassembly of the power valve. They can be found on many motorcycle dealer websites. When removing the power valve system, consider laying the components out on a clean rag in an orientation that correlates to how they are installed in the engine. This is a relatively simple thing to do that will help you remember how they are installed later. When it comes to cleaning the components, clean them one at a time or in small batches so that they don’t get mixed up. Lay out all the parts of your power valve assembly as you disassemble it. This will help you keep everything organized, and make sure you get it back together correctly. Inspection Reed Valve - Don’t forget to check the condition of the reed valve petals, cage, and any stopper plates. Most service manuals will detail the acceptable clearance between the petal tips and cage as well as the stopper plate height. Ensure any rubber coatings on the reed cage are in good condition. Inspect all reed valves components thoroughly before reassembling the top end. Any parts showing signs of excessive wear or damage should be replaced. Intake Manifold - Check the intake manifold for cracks. Cracks are more common on older engines, and propagation all the way through the manifold can lead to air leaks. Exhaust Flange - Check the condition of the exhaust flange and ensure that it is not excessively worn. An excessively worn flange will make exhaust gas sealing difficult, hamper performance, and leak the infamous spooge. Power Valve Components - Take a moment to review the condition of all the power valve components. Significant wear can occur over time and lead to performance losses. Rod Small End - Check the small end rod bore for surface defects such as pitting, scratches, and marring. Any severe defects in the bore will necessitate rod replacement. The rod small end is a critical point of inspection. Any damage to the inside surface could affect the small end bearing, leading to a chain of top end problems and potential failure. Sourcing New Components When freshening up the top end in your two-stroke, it’s important to reassemble with quality components. A deglazed and honed or bored and replated cylinder is a critical component to ensuring reliable performance from your new top end. Your local cylinder shop should be able to handle the bore and replate when necessary, and a simple deglazing can be accomplished with a Scotch-Brite pad. Be sure to retain the 45-degree honing mark angle. There are a lot of choices for new pistons from the aftermarket out there, but many people choose to stick to OEM. However, when ordering from the OEM, every individual part must be ordered separately, including the piston, ring, pin, clips, gaskets, etc. Dealing with all these part numbers and chancing forgetting a component can be a pain, and get expensive. ProX two-stroke pistons are manufactured by OEM suppliers, and come with the piston, pin, ring(s), and circlips all under one part number. ProX two-stroke pistons are manufactured by the same OEM-suppliers to exact OE specs. They are available in A, B, C, and D sizing for most applications. ProX pistons come with the piston, ring(s), pin, and clips all in one box. Complete top-end gasket kits can even be ordered under one part number. ProX pistons provide an OEM-replacement option with less hassle and less strain on your wallet. Find ProX pistons for your bike here. Even though ProX pistons are made by OE suppliers, the quality control difference is evident. On the left is a ProX piston for a Honda CR250, and on the right is a brand new piston out of the box from Honda. Which would you choose? Measurements The number of measurements that should be taken throughout the top end rebuild will be discretionary. At ProX, we strive for excellence and err on the side of caution when it comes to engine building, so our builds consist of numerous measurements and inspections prior to reassembly. For us, this ensures a high level of confidence and safeguards against external oversights. We recommend the same to anyone building an engine. Below is a list of measurements that we routinely make when rebuilding a two-stroke top end: Piston ring end gaps Piston-to-cylinder clearance Rod small end diameter Out of these measurements, confirming or adjusting the ring end gaps is by far the most important, followed closely by ensuring the cylinder bore is within spec with respect to diameter, straightness, and roundness. Understandably, some measurements may be difficult for the average home builder to execute, usually due to not having the right equipment, however, a competent shop should be able to assist. Ring end gaps can be checked by installing the ring in the bore without the piston, and using a feeler gauge to find the measurement. Correct ring end gap is listed in the installation instructions that come with a new ProX piston. ProX rings often do not need to be filed as they are pre-gapped, but it's always a good idea to make sure your end gap is within the provided spec. Piston-to-cylinder is another measurement that should be checked before final assembly. For this, use a bore guage and a set of calipers to measure the bore size. Next, grab a set of micrometers and measure the piston. ProX pistons should be measured perpendicular to the wrist pin, a quarter of the way up the piston skirt from the bottom. Subtract your piston size measurement from your bore size, and you have your piston-to-cylinder clearance. ProX pistons come with a chart on the instruction sheet that shows the range your clearance should be in. Measuring piston-to-cylinder clearance is a smart precaution to help ensure you won't run into any unexpected issues with your new top end. A final measurement we recommend taking is the rod small end diameter. This is important because sometimes these can get worn out and create free play for the small end bearing, resulting in damage to the bearing and most likely the entire top end. It can be done using the same method as the bore diameter. Compare your measurement to the acceptable range in your owner's manual. Making sure the diameter of the small end of the rod is within spec is often overlooked, but can prevent a serious top end failure. Prep Work Cylinder Cleaning - Once the cylinder has been deglazed or has come back from replating, it should be cleaned one final time. There is almost always leftover honing grit that will need to be removed. To effectively clean the cylinder, use warm soapy water and a bristle brush, followed by automatic transmission fluid or a similar cleaning solution and a brush or lint-free rag. To check the cleanliness of the cylinder, rub a cotton swab around the bore and look for contaminants. Clean the bore until no contaminants are visible on the cotton swab. Any honing grit that remains in the cylinder will facilitate premature wear of the piston rings. A clean, de-glazed, and properly honed cylinder is key to piston and ring function and longevity. Power Valve Function - Cylinders that have been exchanged or replated should have the power valve system reinstalled ahead of final installation. Often times, excess plating can inhibit power valve movement. To correct this, the excess plating must be carefully removed. On cylinders utilizing blade style power valves, the blade position with respect to the cylinder bore should be checked to ensure the blade does not protrude into the bore. Assemble the power valve before installing the new piston and reinstalling the cylinder. Be sure to check that the power valve is moving as it should, and not protruding into the bore. Piston - It is usually easiest to prepare the new piston as much as possible by installing one of the circlips and the ring pack ahead of joining it to the connecting rod. Unless your service manual dictates which circlip must be installed first, choose the easiest installation orientation. Typically, your dominant hand and preferred work orientation will dictate which side you choose to install the circlip on. It's easier to install one clip and the piston ring(s) before fixing the new piston to the connecting rod. Reference your service manual to determine the correct orientation of the circlip. Usually, the open end of the circlip should be oriented to the 12 or 6 o’clock position. Temporarily install the wrist pin and use it as a backstop so that the circlip is forced to move into its groove. Installing the circlip should be done by hand to limit the chance of deformation. Orient the circlip to the desired position, then push the open ends of the circlip into position first. Be careful not to scratch or mar the wrist pin bore in the process! Once installed, use a pick or screwdriver to confirm the circlip is fully seated and does not rotate. Any circlips that can be rotated must be replaced because they have been compromised and deformed during installation. Make sure to note the orientation of each clip after installation. Some manuals may recommend specific positions depending on the piston, but always be sure the gap is not lined up with or near the dimple(s). Rings - The compression ring(s) will be directional, and the top of the ring is typically denoted by markings near the end gaps. Apply a thin coat of oil to the ring, then carefully work the ring into position, making sure to line up the ring end gaps with the locating pin in each ring groove. Install the ring(s) with the marking(s) facing up, and make sure the ring end gap is lined up with the locating pin in the ring groove. Installation Piston - On the top of the piston, an arrow will be imprinted, which typically denotes the exhaust side of the piston. Consult your service manual to confirm the proper orientation of the arrow and piston. Apply a light amount of assembly lube to the small end bearing and wrist pin bore on the piston, then install the bearing. Align the piston with the small end of the rod, and slide the wrist pin into place. Once again, use the wrist pin as a backstop, then install the remaining circlip into position. Use a pick or screwdriver to confirm it is fully seated and does not rotate. Don't forget to apply some assembly lube to the ring and piston skirts before assembly! Cylinder to Piston - In most applications, a ring compressor is not required to compress the rings and install the piston into the cylinder. Lightly oil the cylinder bore with assembly lube or engine oil, then lube the piston skirt and ring faces. Prior to installing the piston and rings, confirm one final time that the piston ring ends are oriented correctly to their respective locating pins. Once the new piston is installed on the connecting rod, apply some assembly lube to the cylinder wall, and carefully slide the cylinder over the piston. Squeeze the ring with your hand as you slide the cylinder on, simultaneously making sure the ring end gap remains aligned with the locating pin. Position the piston at or near TDC then carefully lower the cylinder bore down onto the piston. Use your fingers to compress the ring(s) and ensure the cylinder bore is square to the piston. Feel how easily the cylinder slides over the piston and rings. The installation of the cylinder should be smooth and offer little resistance. If resistance is felt, stop immediately and assess the ring pack. Occasionally one of the rings may come out of position in its groove and snag the cylinder bore. This typically happens as the ring transitions out of your fingers and into the cylinder bore. Once the cylinder is safely over the ring, slide it all the way on keeping the piston at top dead center (TDC). Don't forget to torque your cylinder and head nuts to the specification listed in your manual. Post Build Torquing - Your cylinder and head nuts should always be torqued to the specifications outlined in your service manual. Double check all the nuts are set at their corresponding specs. Spark Plug - Don’t forget to install a new spark plug and if necessary gap it appropriately. Air Filter - Be sure to install a clean air filter prior to start up. Crankcase Leak Down Test - As one final precautionary measure perform a crankcase leak down test. A crankcase leak down test will help confirm all the seals, gaskets, and joints are sealing as they should. Break-In - When running your new top end for the first time, keep the engine slightly above idle, with slow and mild revs until the engine starts to get too hot to touch. Then, shut the engine off and let it cool until it is warm to the touch. Repeat this process, revving slightly higher and letting the engine get partially hotter each time. After 3 cycles like this, let the engine completely cool, then check all your fluids and re-check the torque on your cylinder and head bolts. Once that is squared away, you can begin break-in runs riding the bike. Make sure to keep the RPMs varied while riding for the first time, not letting the engine lug or sit at idle. A safe bet would be to ride the bike like this for 5 minutes, then 10 minutes, and finally 15 minutes, with adequate cooling in between. This will ensure your piston ring(s) are evenly and properly broken in. It’s never a bad idea to double check your fluids and torque one more time after complete cool down.
  3. We know this is technically 'Thumper'Talk, but we also know a lot of you have 2-strokes in your garage. We want to share some tips we put together that will help you prevent and diagnose potential problems with your 2-stroke engine. Bookmark this maintenance guide to help keep your bike on the track or trail and off the bench! Article by Paul Olesen. Two-stroke engines have a storied history of being finicky beasts. If you’ve been around two-strokes for any length of time, you’ve probably heard stories that start and end to this effect: “It was running amazing...then the next thing you know - it blew”. We find these stories interesting, and empathize with the unfortunate owners or riders who tell them. At the same time, however, it is often wondered if there were any signs that could have predicted the fateful date with destruction. Photo: Steve Cox We're going to discuss and share a number of observations and diagnostic tests that can be performed to help identify whether or not your engine is going to leave you in the unfortunate role of the broken engine storyteller. While many operators are insistent that their engine gave up without warning, this is often not the case. We’ll start by going over observations that can be made with the engine running, and progress into diagnostic tests that can be routinely made to assess engine health. Recognizing Symptoms Startability Does the engine struggle to start when kicked, but is more prone to coming to life when the electric start is used or when the machine is bump started? Poor starting under normal conditions is not an inclusive sign that the engine is doomed to a spectacular failure, but it is a sign that something is amiss. Carburetion or injection issues are possible, but the bigger potential issue to be aware of resides within the cylinder. Worn piston rings can cause incomplete sealing, resulting in lower compression and more difficulty in starting. Worn piston rings or reed valves that are no longer sealing properly may be the cause of the poor startability characteristic. When the piston rings don’t seal properly, the engine doesn’t build good compression, so when kicked, the engine struggles to come to life. Similarly, if the reed petals are damaged or broken, less air will be trapped in the cylinder. Push the machine or use electric start, and the compression event is shortened via faster rotational speeds, which may be just enough to bring the engine to life. Damaged or worn out reed petals will allow air to leak out, creating less cylinder pressure and starting/running difficulties. Inconsistent Performance Does the engine struggle to hold a tune, or seem like the jetting constantly needs attention, despite relatively stable atmospheric conditions? Sporadic running is not always a death sentence, but should be investigated further. A dirty carburetor or worn spark plug can contribute to this behavior, but the problems that can lead to catastrophe are worn engine seals or gaskets. Stator side crank seals, leaking base gaskets, or intake manifold gaskets are all examples of seals that will result in air leaks which can lean out the air fuel ratio. Lean air/fuel ratios when running at full power can result in excessive combustion temperatures, which can melt a hole in the piston or seize them in the cylinder bore. Ignoring the possibility of a bad gasket or seal isn't worth the potential damage, especially with the affordability of OEM quality gasket kits from ProX. Gearbox Oil Consumption Loss of gearbox oil is abnormal, and in all cases should be able to be traced back to leaking seals or gaskets. In the unlikely event the bike tipped over or cartwheeled, gearbox oil can occasionally exit via the gearbox/crankcase breather. If the gearbox is losing oil but the leak path cannot be identified externally, there is a good chance the drive side crankshaft seal is leaking and allowing the gearbox oil to migrate into the crankcase. During the scavenging process the oil is transferred up into the combustion chamber and burned. Tracking down a leak like this and finding you need new crankshaft seals will commonly turn into a bottom end rebuild job. If you're going to tear into the bottom end to replace seals, that same amount of wear the seal experienced could be evident in other components as well, including your crankshaft and bearings. Not only does ProX make it easier to tackle a bottom end rebuild with our rebuild guide (10 Tips for a Dirt Bike Bottom End Rebuild), but one of the most recent additions to the OEM-quality replacements parts lineup are complete crankshafts. Dropping in a complete ProX crankshaft paired with a main bearing and seal kit is an affordable option for reliable performance. ProX crankshafts are assembled with double-forged, Japanese steel connecting rods, as well as Japanese big-end bearings, crank pins, and thrust washers, all manufactured by OEM suppliers. Excessive Smoke After Warm Up Since the engine is burning pre-mix oil we have to be careful here, because blueish-white smoke is a normal occurrence of two-stroke engine operation. However, excessive smoke after warm up can be an indicator of a couple problems. Blue smoke exiting the exhaust pipe after the engine has warmed may be a sign that gearbox oil is burning in the combustion chamber. While I would never encouraging sniffing your exhaust, combusted gearbox oil will have a different odor than the normal pre-mix oil the engine is using. White smoke exiting the exhaust pipe after the engine has warmed may be a sign that coolant is burning in the combustion chamber. The root of this problem is typically a leaking cylinder head gasket or o-rings. Excessive Coolant Exiting the Overflow Tube While it is common for coolant to exit the overflow tube when the bike has been tipped over or when it has overheated, it should not occur regularly. Coolant blowing out the overflow tube is another good indicator of a leaking head gasket. Note where your coolant overflow line runs, so you can keep an eye out for overheating issues. Coolant Weepage Dribbles of coolant exiting the engine around the coolant pump are indicative of a faulty water pump seal. If left unattended, the entire cooling system will eventually empty, causing overheating and an incredible amount of damage. Excessive Top End Noise Isolating top end noise in a two-stroke is easy since the only moving component is the piston assembly. Discerning what is normal takes a trained ear and familiarity with the particular engine in question. However, audible cues often present themselves when components wear or clearances loosen up. The most common noise associated with a two-stroke top end is a “metallic slap”. This is commonly referred to as piston slap, and is a result of the piston rocking back and forth in the cylinder bore as it reciprocates. This phenomena is normal, but the intensity of the slap will increase as the piston skirt and cylinder bore wear. Left unattended, excessive piston slap can result in failure of the piston skirt. Check out our complete 2-stroke top end rebuild guide here. Excessive piston slap can cause damage to the piston and weaken the skirts. It's important to check piston-to-wall clearance when installing a new piston to ensure a long operating life. 2-Stroke pistons fitted with skirt coatings also help reduce friction and operating noise. Diagnostic Checks & Tests Engine Coolant Coolant contaminated with black specks can often be traced back to a leaking head gasket or o-rings. Combustion byproducts are forced into the coolant system due to the high pressures in the combustion chamber during the combustion event. These black specks will often float and show themselves as soon as the radiator cap is removed. Gearbox Oil The composition of the gearbox oil can provide a lot of clues as to what is happening within the engine. For starters, what color is it and what is in it? Oil that appears milky is a good indicator that moisture is finding its way into the gearbox oil. The most common culprit is a faulty oil side water pump seal. A keen eye can spot various metallic particles within the oil itself. Aluminum will appear silvery gray. Bronze particles will have a gold shine. Ferrous particles will be dull and are often more discernable by dragging a magnet through the oil. Accumulation of all of these aforementioned particles will be normal in small quantities, but excessive amounts of any of them could be cause for concern. Fortunately, since gearbox and power cylinder lubrication are separate the number of causes for problems is limited and more easily pinpointed. Other than changing your gearbox oil regularly, keep an eye out for metallic particles, as those can be a sign of accelerated wear on internal parts. Cylinder Leak Down Testing While less commonly prescribed on two-stroke engines, performing a cylinder leak down test is by far one of the most definitive diagnostic procedures that can be performed to determine the health of the piston rings, cylinder bore, and cylinder head seal, whether gasket or o-rings. If any of the previously mentioned symptoms are observed, a leak down test is almost always a great next step. A leak down test pressurizes the engine’s combustion chamber and compares the amount of pressure going into the combustion chamber to the pressure that is retained. Pressurized air is administered via the spark plug hole and two pressure gauges are used to make the comparison. The piston is positioned at top dead center. Air exiting the combustion chamber can then be traced back to the piston rings or cylinder head seal. Compression Testing A compression test can be an tell-tale indicator of the health of your top end components. Be sure to compare your reading the manufacturer's recommended compression measurment. A compression test aims to quantify how much pressure builds during the compression event. A compression tester which is connected to the spark plug hole consists of a pressure gauge and a one way check valve. The engine is kicked repeatedly or turned over a number of times using the starter. The resulting pressure that is recorded can then be used to assess the health of the cylinder bore. Low pressure readings can then be attributed to problematic piston rings or leaking cylinder head seals. Crankcase Leak Down Testing A crankcase leak down test is utilized in order to assess the sealing integrity of the crankcase and cylinder. Personally, this is one of my favorite tests to perform because of its ability to isolate a number of potentially problematic seals and gaskets all at once. Components such as crank seals, base gasket, and power valve seals can all be checked to determine if they’re leaking. In summary, a crankcase leak down test is performed by sealing the intake manifold, exhaust outlet, and any power valve breathers. Then the crankcase is pressurized under low pressure. Typically, the goal is to retain the pressure in the crankcase over a set length of time. Loss of pressure is indicative of leaks, which can then be traced to their cause. Preemptively replacing components before the engine suffers a major failure is both safer and more affordable than dealing with the problem after the engine has stopped working entirely. Most problems that can occur within the two-stroke engine can be mitigated by servicing components such as pistons, rods, rings, bearings, seals, and crankshafts. Many riders dread the thought of having to service these items due to the excessively high costs associated with OEM or premium aftermarket parts. Fortunately, brands such as ProX offer a comprehensive lineup of OEM-quality components at reasonable prices, many of which are produced by OEM suppliers. Depending on what you need to service, components such as piston kits, connecting rods, crankshafts, bearings, gaskets, and seals can all be found in the ProX catalog. Replacing components as part of preventative maintenance can save time and money, especially with the availability of affordable, OEM quality parts. Find ProX parts for your machine here. Discussing specific time intervals in regards to when things should be replaced is futile. The reason is simple: different engines, maintenance practices, and applications will all have different intervals. Installing an hour meter on your engine so that you can log the number of hours the engine has run can be one of the most insightful ways to establish maintenance and replacement intervals specific to your engine, riding, and maintenance habits.
  4. Hi Everyone, I have an XR75 that I am rebuilding, I have finished the bottom end and am working on the top end. When I try to line up the TDC mark on the flywheel and put the timing chain on the cam sprocket, the 0 mark is slightly offset to the right, so then I turn the sprocket and move it around one link to the left and then the mark is slightly offset to the left. How do I know what is right and what is wrong because there surely is no possible way to get it to line up perfect in my situation. Thanks
  5. I have recently purchased a 2007 YZ450F that had thrown the chain and cracked the case. I purchased a replacement case and I am rebuilding it. In the gasket set there is one green o-ring. Can anybody tell me what that is for? The service manual is close to worthless. It doesn't even show the Oil system check valve (ball and spring) that keeps oil in the top end when you shut down. I purchased a complete gasket set from the shop when I purchased the new cases. I also got a complete gasket set with a bottom end kit i purchased. Both have the same green o-ring but I have no idea what it is for. Insert other media
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