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10 Tips for a Dirt Bike Bottom End Rebuild


Many riders associate the need for a bottom end rebuild with a costly trip to the shop. However, saving on labor and parts isn't out of the question with the help of ProX. We put together a list of 10 tips to help those who want to tackle the rebuild themselves.

Rebuilding the bottom end of your engine, whether two or four-stroke, can be a fun and rewarding job. Additionally, a considerable amount of money can be saved by taking on the work yourself versus tasking a shop to perform the work. These statements are only true, however, assuming the bottom end rebuild is performed correctly. This is a huge caveat, and for the average weekend warrior who doesn’t perform this task often, unfamiliarity with technique and componentry can lead to errors.

To help ensure your next bottom end rebuild is executed to a professional level, here’s 10  tips which will elevate your confidence and understanding as an amateur engine builder. The tips will be presented in chronological order. Let’s get started!


Rebuilding the bottom end yourself can be a daunting, yet rewarding, task. Just make sure you take the engine out of the frame first!

Correct Tools, Correct Diagnostics, Correct Expectations, Correct Replacement Components

Successfully rebuilding your bottom end starts with planning and preparation. Starting with tools, you’ll need a few specialty tools in conjunction with your standard sockets, wrenches, etc. Namely, the correct flywheel puller for your specific engine, a flywheel holding tool, a crankcase splitting tool, a blind bearing puller, and a crankshaft puller. Using heat to assist in removal/installation of the bearings and crankshaft is an effective method, so an oven and freezer are also noteworthy items.

You need to have a copy of the factory service manual or equivalent for your particular vehicle. I highly recommend reviewing the sequence of events and procedure in advance of executing the work.


If you are rebuilding the bottom end because of a failure, be sure to inspect all components to identify the cause of the failure and determine what was damaged. This will ensure your rebuilt engine will not encounter the same problem.

Bottom ends are taken apart for many different reasons. If a major failure occurred, the scrutiny of the rebuild will be at a much higher level than a bottom end that is merely being reconditioned. If any problems were persistent when the machine was operated, such as a poorly shifting gearbox, or leaks between the crankcases, the causation of these issues must be identified prior to reassembly.

Replacement components are a major factor to consider, both in terms of cost and engine performance, when diving into a bottom end rebuild. Replacing bearings, seals, gaskets, and refurbishing the crankshaft either by rebuilding or replacing it is essential. It’s recommended to peruse your service manual or microfiches ahead of the rebuild to generate a list of replacement components.

Selection of components and sourcing them should also be planned out. In addition to OEM options, brands such as ProX offer OEM quality parts at more affordable prices. Complete crankshafts, connecting rod kits, bearings, seals, and many other components can be found for a wide variety of engine models that can make the rebuilding process easy and affordable. All ProX bottom end parts are made by OE manufacturers and suppliers to OE standards, so your mind can rest easy that your rebuild can retain OEM quality and longevity.

Click here to see more about ProX drop-in ready complete crankshafts.

prox_crankshafts-16.JPG.349a1fa07e6417ee1065ae5dd2e3916b.JPG IMG_9951.jpg.413ab00e729fbcb4d60bb6f0b3e66e2a.jpg

ProX bottom end components are manufactured by OE suppliers to OE specifications. Crankshafts, gaskets, seals, and bearings are available to cover your bottom end rebuild with OEM reliability and performance.

Find all the OEM replacement parts you need for your bike here.

Keeping Track of Hardware

As the engine is torn apart, you will amass a significant number of components, bolts, nuts, and miscellaneous hardware. Properly keeping track of these items is critical. I prefer to lay sub-systems out on a large table, remove the bare minimum of components/hardware to get to the items I’m servicing, and stick bolts through cardboard in the pattern they were removed from components (think crankcases and covers). This methodology reduces the number of mixups that can occur and ensures bolts of varying length will be reinstalled in their original location. While my method is far from the only one, make sure you have a robust and sustainable system for keeping track of everything. 

Flywheel Removal

Commonly, two specialty tools are required to remove the flywheel - a flywheel puller and a flywheel holder. It is imperative that both are utilized. Many rebuilds have gone awry because a flywheel holding tool was not used during the rebuild. Instead, the crankshaft was secured from the primary drive side when the flywheel nut was removed/installed. At face value, this may not seem like a big deal, however, when the flywheel is removed, or more importantly installed, in this fashion, a torque is exerted across the crankshaft. While it may seem implausible, the twisting force that is exerted can actually alter the trueness of the crankshaft.


Using proper tools is critical. Pictured here is a clutch holder, used in aiding in clutch component removal, which also doubles as a flywheel holder. The small dowels on the back of the arms of the tool sit inside the recesses in the flywheel to hold it in place while removing the flywheel.

Crankcase Separation

There are a few noteworthy items to discuss when separating the crankcases. First off, I always recommend blocking the crankcases so that the split line lies horizontally, and confirming which side should be oriented face up. Doing this will reduce the likelihood of components falling out and ensure that subsequent removal of components goes smoothly.

When installing the crankcase splitter, make sure a protective cap is used to cover the end of the crankshaft. This applies to both two and four-strokes, but is especially critical on four-stroke engines that pass oil out the end of the crankshaft.

Be sure to position the crankcase splitter arms as close to equispaced from one another as possible. Also, ensure that the splitter studs utilize thread engagement at least 1.5 times the diameter of the bolt. For example, most crankcase bolt holes are 6mm, so the stud should be screwed down at least 9mm to ensure adequate thread engagement.


Removing the crank after the cases have been split is another critical job that requiring a special tool. We always recommend using a crank puller. The puller can also be used to install the crankshaft.

Once the crankcase splitter is set up, it is imperative that separation happens evenly around the periphery of the crankcases. Screwdrivers and the like should never be used to facilitate separation. Instead, a rubber mallet can be used to encourage separation.

Seal and Bearing Removal

The use of seal pullers to facilitate seal removal is not completely necessary, but is definitely recommended. Their use reduces the likelihood of bore damage during removal.

Bearing removal can be done with or without heat. However, the former seems to be a better method. Using heat to remove the crankcase bearings reduces bearing bore wear and work on part of the rebuilder. I do want to note that your heat source and surrounding area can become odorous due to the residual oils that become heated during the process. For this reason, it is advisable to thoroughly clean the crankcases prior to heating them up, as well as keeping ventilation in mind.


After heating the crankcase halves, use a hammer and punch to remove any bearings that did not come out on their own. Be careful not to damage the bearing bores when doing this.

To remove the bearings by heating the crankcase halves, position the crankcase halves split line down on a pair of trays. The trays will catch the bearings and any oil that did not get cleaned out. The oven, grill, or heat source should be set at 350 degrees Farenheit, and the crankcases should be heated for around half an hour. After, the majority of the crankcase bearings should fall out of their bores. Any bearings that did not drop out should be carefully tapped out with a punch and hammer. Bearings situated in blind bearing bores that did not fall out should be removed with the assistance of a blind bearing puller.

If you do not want to or can’t use the heating method to remove the bearings, an arbor or hydraulic press may be utilized to aid in removal. A blind bearing puller will also have to be heavily relied on to facilitate removal. Due to the unevenness of load distribution that can result from pounding the bearings out with a hammer, we caution against this as a primary form of removal without the aid of heat.

Cleaning, Case, and Component Inspection

At this point, it is my recommendation that all components that originated inside the crankcases be thoroughly cleaned. Clean components will ensure easy and accurate inspections. On four-stroke engines that have oil passages running through the crankcases, cylinder, and cylinder head, it is imperative that these are cleaned and blown out. This is especially true if the engine suffered a major failure where oil contamination was a resulting issue. Similarly, on two-strokes, the passages that lead to the crank bearings should be cleaned.

Component inspections should be conducted to assess the condition of the gearbox, crankcase bearing bores, and crankshaft. The crankshaft should either be rebuilt or replaced depending on the severity of wear and desires of the builder.

Should you determine you'd like to rebuild your crank, ProX offers OEM quality connecting rod kits that can be used for a crankshaft rebuild. Unless you are experienced in this field and have the tools, crankshaft rebuilds should be trusted in the hands of a reputable shop.


ProX connecting rods are double-forged, Japanese steel, and are heat treated and shot peened for strength and longevity.

Read about the advtantages of ProX connecting rods here.

Crankshaft Trueness

Regardless of whether a new or rebuilt crankshaft is utilized, the trueness of the crankshaft must be checked. This can be farmed out to a competent shop, machinist, or if properly equipped, performed in-house. While it should normally be expected that new or rebuilt cranks are within runout specifications, the trueness of the crankshaft is imperative to long-term durability. Checking is insurance that our postal system didn’t drop your crank, and that the factory or rebuilder did their job correctly.

Bearing Installation

Similar to removal, heat can also aid in bearing installation. The same heating recommendations apply, and once at temperature, the majority of the bearings should fall to the bottom of their bores without any input. The caveat to this is the bearing dropping into the bore cock-eyed. When this happens, a punch and hammer should be used be help square the bearing to its bore. Be sure to tap on the outer race of the bearing. To ensure the bearings are at the bottom of their bores, they should be tapped to confirm they are fully seated.


Bearings should be tapped on their outer races only to make sure they are completely and squarley seated in their bores.

Alternatively, if you don’t want to, or can’t use heat, an arbor or hydraulic press should be utilized to install the bearings. Be sure to load the bearings through their outer races when pressing them in place.

Seal Installation

Seals can be tapped into place with a seal driver or socket and hammer. Alternatively, a press can be used. The important checks to perform are to ensure the seals have been installed squarely in their respective bores, and at any prescribed depths outlined in the service manual. Any seals installed cock-eyed will wear out prematurely.


When installing new seals, make sure they are completely seated by using a seal driver or socket and hammer. Use caution and make sure the tools don't come in contact with anything but the seal.

To prepare for the new bearing and seal installation, complete crankshaft bearing and seal kits are available through ProX. ProX uses reputable, OEM bearing and seal suppliers for all parts in order to maintain OEM quality. Ordering in kits creates less hassle and is a budget-friendly step in your rebuild.

Click here to check out ProX bearings and seals.

Crankshaft Installation

Crankshafts that utilize an interference fit with their mating crank bearings can be installed two ways. Shrinking the crankshaft in place using a combination of heating and cooling of components works well. Alternatively, utilizing a crankshaft puller is another great way to install the crankshaft. Pounding or pressing the crankshaft into place should never be considered because the trueness of the crankshaft can be affected.


While your crankshaft cools in the freezer, carefully heat the inner race of the crank bearing, paying careful attention not to damage any new seals.

To shrink the crankshaft in place the crankshaft should be set in a freezer for about an hour, and the inner race of the crank bearing should be carefully heated with a torch. The cautionary point here is to use care when heating if the crank seals have already been installed. Inner race temperature can be gauged by applying a drop of water to the surface of the race. If the water droplet sizzles, the inner race is plenty hot. At this point, the cooled crankshaft can be dropped through the heated bearing. Once seated, work to button up the crankcase assembly should progress quickly, and the remaining inner race can be heated and the crankcase installed.

Using a crankshaft puller is an equally acceptable method and is incredibly straightforward. The puller is threaded and seats against the crank bearing or crankcase and attaches to the end of the crank via a nut or retaining ring. Once set up, the puller is tightened and the crankshaft is pulled through the bearing. 


Your cooled crankshaft should fit through your heated bearing fairly easily. A crank puller can be used for installation to make sure everything is square. Also, be sure to use assembly lube in critical areas during assembly.

At this point, your crankcases should be buttoned up and you should be well on your way to rebuilding the rest of your engine. Upon installation of your flywheel, be sure to use a flywheel holding tool to secure it in place when torqueing the nut.

Images provided and owned by Kelsey Jorissen Photography, LLC.

Crank Removal.jpeg

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 Very good job at documenting a lower end rebuild. My only comment would be anyone using a press to install bearings in a case probably shouldn’t. Just do a final wash of the cases in dish soap and water and put them in the oven. Heating the cases to 350 is the way to go to remove and install bearings. Also freezing the new bearings while the cases are at 350 will allow them to drop in with a threaded rod, nuts and washers to hold them straight. Just went through it on my Yamaha 450. ?

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I heat the cases and freeze the bearings and they drop right in. The seals go in afterwards. The seals can prevent you from being able to see to verify they bearings are fully seated in the saddles, let alone get damaged by trying to heat the inner race to drop the crank pin through, instead of side loading the roller bearing trying push the crank pin through with level of force, by it pressing or pushing or pulling with a crank installer. KX500's are a different story and some of the earlier two strokes that oiled the right side bearing with the transmission oil instead of two stroke oil. But even when I heat the cases in the oven, I've never gone hotter than 275F. So even if I'm forced to install a seal prior to the crank, the seal is ok at 275F. Then, there's always the "Whack" to center the crank after the cases have been assembled.

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Amazing write up and great follow on tips.  I know where to go next time I need a bottom end rebuild.

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I just completed my first bottom end rebuild.  I had lost a lower rod bearing and decided to replace every bearing and seal in the motor while I had the crank getting rebuilt by Andrew Cooksey.  Andrew used all new parts from ProX and was able to get my crank balanced out to .0005 which is crazy.  My KTM is so smooth.  If not for the two stroke sound, you would swear it was as smooth as a four stroke.

With some advice from Andrew, my first bottom end was easier than I would ever have imagined.  I would do it again in a heartbeat.  By the way, I heated my case in the oven at 250 degrees for about 10 min and my bearings and seals come out smoothly.  Repeated the process for installation.

Edited by rholbrook
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I would be leery of a mechanic if he were wearing those thick ironworker gloves and swinging a claw hammer to remove a small bearing as illustrated 

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I rebuilt my 08 450R over the winter.  New crank, piston, rings + Hotcam.  Really impressed with the HotRods kit.  I went on my own and got an OEM piston but the kit came with both top/bottom seals/gaskets but only the crank bearings.

Here is what I can think of..

1. Clean EVERYTHING!!! FIRST.   Go ahead and buy a case of brake cleaner spray and nitrile gloves

2. Every time you open a cover or expose something for the 1st time take a picture at a couple of angles.  Only takes a sec.  I guarantee you will go back and see if you've configured something correctly at reassembly.

3. Take something off...  screw the bolt back in partially where it came from before setting aside.

4. Covers with different length bolts.  turn a big box upside down and lay the covers on it, gasket side down with the bolts loose in the holes.  Stab holes in the box all the way around next to the bolts and poke the bolts into the corresponding box holes making a stencil of the bolt pattern... with the bolts themselves to keep track of where they came from and keep them stores safe.

5. Go buy 3 folding tables.  Store items by category.

6. Break loose the flywheel and the clutch nut with the engine still in the frame. You will thank the leverage.  If it's out.. no big deal. Go wedge a corner of the engine under your truck tire and hold it still with your foot while you bust the nut loose.  Other wise it wants to spin around on the floor.

7. Crank Bearing seals:   I did not want to chance damaging the seals built into trans bearing by putting everything in the oven so I used a heat gun on the Aluminum around the crank bearings.  When a drop of water would just bubble and boil I knew I could tap the old one out and slip a new frozen one in.

Edited by Whopharded
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One thing I wish I would have thought to do was weigh the old crank and the new one.  I swear the new one feels like it's got more fly wheel effect.

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I have a Yz250f engine that I am rebuilding (my first four stroke dirt bike I am rebuilding) and a piece broke through the case and I can't get it to TDC. I don't think it was the crank cuz if it was it wouldn't turn over as far as it does. I don't want to mess up when I take it apart without it being at TDC compression stroke cuz there are some engines that need the oil pump to be aligned when you take it apart.

I tried doing the timing on my 600 thundercat but I failed and it was off a notch and it broke.

Have you guys rebuilt a four stroke dirt bike without taking it apart at TDC compression stroke? Any tips on timing?

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On ‎5‎/‎3‎/‎2019 at 12:07 AM, Shawn_Mc said:

I heat the cases and freeze the bearings and they drop right in. The seals go in afterwards. The seals can prevent you from being able to see to verify they bearings are fully seated in the saddles, let alone get damaged by trying to heat the inner race to drop the crank pin through, instead of side loading the roller bearing trying push the crank pin through with level of force, by it pressing or pushing or pulling with a crank installer. KX500's are a different story and some of the earlier two strokes that oiled the right side bearing with the transmission oil instead of two stroke oil. But even when I heat the cases in the oven, I've never gone hotter than 275F. So even if I'm forced to install a seal prior to the crank, the seal is ok at 275F. Then, there's always the "Whack" to center the crank after the cases have been assembled.

I would never whack the crank to center it. I will measure and shim then pull it with a crank installer. 

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I dont either. I'll use a plastic mallet on the bearing boss on the case. You're not looking to "move" the crank as much as give everything the opportunity to find it's own center. I use a crank puller when it's necessary, or there's no way around it. Pulling a crank through a bearing center works if you've got access to the inner race to pull against, but usually isnt the case. So you are forced to put a lateral load on the bearings. The issue there is, there is way more flex in the cases as that load gets imposed than you'd think. You want that radial bearing to run in it's center. Not on the inside of the inner race and the outside of the outer race. It'll even feel free, but it's tough to tell if the mains are running free when the seals are dragging, even if they're oiled or greased. But more often than not, you'll find the crank runs freer after a wack, than before. If I do need to wack the crank itself, I'm hitting the primary drive gear, or putting the flywheel nut on the stator side to protect the end of the crank and the mallet. I'd NEVER use a metal, any kind of metal hammer. A hard plastic mallet or a plastic dead blow.

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