It's often necessary to split the cases on a 2 stroke dirt bike as doing so is the only way to enable replacement of the crank bearings, the connecting rod, big end bearing, the crankshaft as a complete assembly, or to service the transmission. Additionally, splitting the cases is often necessary to properly repair case damage (IE: Damage from a derailed chain or broken transmission, A stripped oil drain, Piston shrapnel that broke the cases in the area of the crankshaft counterweights, or damage from rocks and other external hazards).
Below we'll go over the steps necessary to disassemble and reassemble the bottom end of a 2 stroke dirt bike's engine while attempting to broadly cover how to get the cases apart without inflicting damage to expensive parts, as well as we'll cover how to put the cases back together without getting things in a bind, or creating other problems that may include knocking the crank out of balance or being left with a locked up transmission that doesn't shift among other problems.
If you have not already done so, you're going to need to remove the engine from the frame to be able to split the cases and service the transmission or crankshaft.
Removing an engine from a dirt bike is a task that can be accomplished by most people with commonly available tools, although it is strongly recommended that you have a factory service manual specific to the motorcycle being serviced as this is necessary for torque and wear specifications of internal engine parts.
If you're unfamiliar or feel intimidated with the thought of removing the engine, don't sweat it as it's not difficult and you should be able to have the engine out of the frame within an hour's time. However if you're not sure what needs removed, or you don't want to end up with a mess, here's an excellent article that will walk you through how to get the engine out of the frame with minimal mess.
If you're skipping the article on removing an engine, at the very least drain the transmission oil and ensure that the transmission is in neutral, then remove the shift lever to prevent accidental shifting of the transmission during engine removal.
Once you have the engine out of the frame and on the bench, it's recommended that you perform these steps below before actually opening the engine up.
Before getting started with the actual disassembly, it's important that you've cleaned the exterior of the engine as mentioned previously, and that the area you choose for disassembly and reassembly of the engine is a clean, well lit and organized working environment.
Additionally, don't forget that a factory service manual specific to the motorcycle being serviced and an engine building box will prove invaluable during your rebuild and both should be a must have part of your tool collection.
The first step in disassembly of the engine should be removal of the top end, otherwise the cylinder and reed cage will prevent separation of the cases.
If you're unfamiliar with removing the top end, be sure to review our article on rebuilding the top end of a 2 stroke as there are many useful tips there related to disassembly, cleaning, inspection and reassembly of the top end which includes the cylinder & cylinder head, the piston and powervalve.
Once the top end is removed we can begin disassembly of the bottom end in preparation of splitting the cases.
Even though splitting the cases on most 2 stroke motocross bikes is usually possible by only removing the left engine case without so much as even touching the RH engine / clutch cover it's important to realize that removal of only the left engine case only allows limited access to the transmission and will not enable any crankshaft services, or removal of either transmission shaft.
If you're reading this so you can replace the crank bearings, or perform any type of crankshaft service, it will be necessary to completely disassemble the engine as discussed below.
However, if you only need to remove the left case to repair a leak between the cases, to fix case damage, or to enable a visual inspection of the transmission, you can get by with just removing the flywheel, stator plate and countershaft sprocket as discussed below, then jump directly to the section on splitting the cases.
If you're splitting the cases to replace the crank bearings, put a rod kit in it, or replace the crankshaft as a complete assembly, complete disassembly of the engine will be necessary, and the steps below will guide you through the steps necessary to ultimately enable separation of the engine cases, which will allow unrestricted access to the crankshaft and all of the transmission, clutch and shifting components.
If you're working on a motorcycle with a quick access clutch cover, removal of the right engine cover is possible without removing the outer / quick access cover beforehand.
By this point the engine should be completely disassembled and ready for the cases to be separated. Although if you reached this point and you have not yet disassembled the engine in preparation of splitting the cases, be sure to review the section on engine disassembly just above before continuing.
If you do not have a crankcase splitting tool, splitting the cases is possible using a commonly available harmonic balancer puller (available at auto parts stores) along with 2 (long) 6mm x ~150mm bolts and a dead blow hammer.
If you're not using a case splitting tool, you may experience difficulty in finding the 2 long bolts necessary to work with a harmonic balancer puller. However, stopping by your local motorcycle shop and asking to dig through their bolts bin, or by robbing the thru bolts from an old motorcycle, or import automotive starter, you should be able to obtain a couple bolts that will work.
After ensuring that all of the case screws have been removed, and the engine has been disassembled in preparation of splitting the cases as discussed in the section on disassembling the engine, place the engine on an engine building box so the crankshaft and transmission shafts are vertical then let's get started.
With the engine on the engine building box, begin by fully retracting the center push bolt of either a case splitting tool, or a harmonic balancer puller, then place the center bolt of the puller directly over the end of the crankshaft.
Now with the puller centered over the crank, position the arms of a case splitter, or the flange of a harmonic balancer puller appropriately, then thread the long bolts into the engine case until bottomed (finger tight) while using the mounting holes for the stator plate as the pulling point near the crank.
Next, thread the center bolt of the puller into the end of the crankshaft until the puller's center bolt is snug against the nose of the crankshaft, and the puller is taught against the bolts threaded into the stator plate mounting holes.
As a final preparation before applying any pressure to the puller with tools, it's important to ensure the puller remains parallel to the crankshaft once tightened.
If necessary, adjust the length of either of the bolts threaded into the engine case by unthreading one or the other, or by adjusting the stop nuts on a case splitter until the puller remains parallel to the crank's end with the center / push bolt snug against the crankshaft.
While adjusting the length of either bolt, ensure that 4-5 threads remain threaded into the engine case. If this is not possible it may be necessary to use shims (Additional Washers or Spacers) under the bolt heads, otherwise case damage (pulled threads) is possible.
Finally, if using a case splitting tool, thread the 3rd arm's bolt into the engine case as far from the crank as possible, but do so while ensuring the puller remains parallel to the crankshaft once under load, otherwise if you're using a harmonic balancer puller, you don't need to do anything more except skip to the section that shows you how to actually get the cases apart.
With the engine positioned on your engine building box, and with the puller attached to the engine case and leveled as discussed above, begin threading the puller's push bolt in against the crankshaft while watching and listening for the cases to begin separating or binding.
A moderate amount of force may be necessary to get the cases to separate, however if you have to really crank down on the puller's center bolt before anything moves, or if something doesn't seem right, STOP and recheck to ensure there are no case screws remaining.
With the puller tightened sufficiently, if the case halves have not already begun to separate evenly, GRASP THE PULLER with one hand and slightly lift the engine out of the engine building box, then with your other hand gently strike the end of the countershaft, as well as any protruding areas of the opposing case with a dead blow hammer but do so without much force, and without lifting the engine very high out of the engine building box as the case halves may suddenly separate.
After a few gentle blows with the dead blow hammer, if the case halves do not suddenly separate (fall apart), set the engine back in the engine box and attempt to apply more pressure to the puller's center bolt.
If the engine case appears to be separating evenly, continually tighten the puller, and if necessary strike the opposing case with the dead blow hammer continuing this process until the engine case is clear of the case locating dowels.
As the engine case begins to separate, continue tightening the center bolt of the puller, but stop frequently to inspect and ensure nothing is binding, then continue pressing the cases apart until the engine case is separated to a point where the puller is no longer needed.
Once the engine case is free of the crank bearing, or the crank journal is clear of the crank bearing, you'll simply be able to lift the case half straight up and off by the puller, however while doing so be careful to ensure that any thrust washers which may remain stuck to the transmission bearing(s) in the case half you just removed don't fall off and get lost.
Now with the engine case's separated you'll be able to perform the necessary crankshaft, crankcase or transmission services, but before you get too carried away, be sure to check out our tips below that detail removal and installation of the crank bearings, the crankshaft and putting it all back together.
Generally, people split the cases on their dirt bike to remedy the following:
If you disassembled the engine cases due to a leak and the cases aren't damaged (or have been repaired), it will still be necessary to replace the crank bearings and seals as we'll illustrate below. However, after that sealing the cases will come naturally by following the advice we'll be providing upon reassembly of the cases.
If you have a worn rod bearing, this article doesn't cover installing a rod kit. Alternatively any issues with the connecting rod or big end bearing we feel is best remedied by replacing the crankshaft with a new, fully assembled and trued crank & rod combo. However, we will show you how to install the crank bearings and crankshaft into the cases while ensuring everything still rotates smoothly.
In coming steps it will be recommended that you remove the transmission, as well as the shift shafts and forks. However, upon reassembly if you need help on the stacking order of transmission components you're going to have to refer to an OEM parts diagram, or a service manual specific to the motorcycle for those details.
Unfortunately there are way too many different gear and shim arrangements among manufacturers to be able to provide exact details on how to stack the gears or which way the shims, collars and spacers go on the transmission shafts and it's for this reason that we recommend you refer to an OEM service manual specific to the motorcycle during disassembly and reassembly of the transmission.Don't Forget About the Case Dowels,
Removing the case locating dowels will enable you to install new dowels, or provided the existing dowels come out easily and are in good shape, at reassembly you'll be able to coat either dowels with anti-seize reducing any chance of these dowel(s) becoming seized, potentially creating future problems.
To remove the case dowel(s), attempt to gently pull each dowel from the case using needle nose pliers. However, if the dowel(s) don't EASILY come out of the case(s), carefully heat the case in the area surrounding each dowel to enable removal of the dowel from the case although if the dowels become distorted during removal be sure to replace these with new case dowels during reassembly.
After separation of the cases, if the crankshaft remained stuck in one of the case halves, this would indicate the crank bearing on that side is tight in the case, and the crank is merely press fit to it's bearing as they both should be.
If the crank and it's accompanying crank bearing DID remain in the case half which houses the transmission, it is highly recommended that you pick up a factory service manual specific to the motorcycle, then remove the shift shafts, the shift forks and the transmission before attempting removal of the crankshaft or bearing.
With both of the engine cases cleared of any loose parts (transmissions and such) place either case half in a press with the bearing or crankshaft facing down, then heat the case in the area surrounding the crank bearing to a temperature of roughly ~150 degrees Fahrenheit.
Once the engine case has been heated sufficiently, if you're working with the side which the crankshaft remained in, press the crank and it's accompanying bearing out of the case together, but while doing so support the crank with your hand from the other side so it doesn't fall out and hit the ground.
With the crank and / or it's accompanying bearing now removed from one case, if you have not done so already remove the crank bearing from the other case half in a similar manner as that of what you previously used to remove the crank and crank bearing, then remove the crank seal from either side.
Once the crankshaft and the old crank bearings & seals are removed from the engine cases, be sure to check out the tips below where we'll go over installing new crank bearings, the crankshaft and closing the cases back up while ensuring the transmission is working properly and that the crank isn't in a bind.
Whether you've been following this article from the earliest steps of disassembly, or you just landed here looking for how to put a 2 stroke bottom end together, at this point you should have two engine cases devoid of the crankshaft and crank bearings, or the transmission and related shifting components.
If you have the cases apart for any reason other than transmission and shifting issues, or to repair a leak, it is recommended that the bottom end is renewed with a complete bottom end kit which includes a crankshaft with the rod installed and the crank trued, as well as both crank bearings and crank seals and all of the necessary gaskets to facilitate a complete rebuild of a 2 stroke's bottom end.
Why Do I Need to Replace the Entire Bottom End?
Oftentimes by the time the bottom end of a 2 stroke engine needs crank bearings, the big end connecting rod bearing is usually worn of which to repair will require disassembly of the crankshaft to enable replacement of the connecting rod, big end bearing, pin and thrust washers.
However, taking the approach of installing a rod kit doesn't address a common wear area on the crank journals in the area of the crank seals, and locating someone knowledgeable in rebuilding motorcycle crankshafts then sourcing the rod kit, as well as the crank bearings, crank seals and gaskets is often a PITA and doing so usually cost's nearly as much as a complete bottom end kit so that's why we suggest replacing the entire rotating assembly with a new and complete bottom end kit.
By this point you should have a complete bottom end kit, or at a minimum a gasket set or a tube of Yamabond #4 and new crank bearings, new crank seals and a crankshaft which was rebuilt or checked to ensure it spins true.
Note: Yamabond is only applicable if the motorcycle manufacturer calls for non hardening sealant between the cases. If there was a gasket between the cases at disassembly you do not need Yamabond.
With the crankshaft and bearings in the freezer, the first step in reassembling the engine should always be to ensure that all of the parts which are to be reinstalled are clean and have been inspected.
Although inspection of transmission and shifting components are beyond the scope of this article, due to the differences among manufacturers a factory service manual specific to the motorcycle is essential for these details.
Cleanliness during an engine's assembly is paramount to a quality build so if you have not done so already, ensure all the parts you'll be re-using are clean and have been inspected, then lay these parts out in a clean and organized manner.
Next, using a piece of scotch brite, along with a can of brake clean or 2, a wire brush and compressed air, clean the engine cases while paying particularly close attention to the mating surfaces and the holes where the dowels fit.
Once the exterior, interior and sealing surfaces of the cases are clean, focusing on the case half which will house the transmission, either install new dowel's with a light coating of Anti-Seize on each, or if the existing dowels look good, reinsert those with a light coating of Anti-Seize on each beforehand.
After cleaning and inspecting all of the parts, it's important that the transmission and shifting components are properly reinstalled (provided they were removed from the cases) and that neutral can be easily obtained as we'll go over below.
As mentioned during disassembly, due to variances between manufacturers or models of motorcycles it's wise to have a service manual specific to the motorcycle and for this reason we're unable to provide exact transmission details.
If you disassembled the transmission it's critical that the transmission shafts, shift shafts and shift forks are reinstalled properly but whether you disassembled the transmission or not, before continuing, ensure the transmission is in neutral as we'll discuss next.
With the transmission assembled in one case half, to determine if the transmission IS in neutral, position the engine case housing the transmission on an engine building box with the transmission shafts vertical, then hold the transmission's main shaft stationary while attempting to turn the countershaft. If the transmission is in neutral you will be able to hold the main shaft so it doesn't turn, yet be able to turn the countershaft with little to no resistance.
If the transmission is in neutral you can skip to cleaning the cases in preparation of assembly, otherwise it is strongly recommended that you ensure the transmission is in neutral and that the shaft's turn freely before continuing assembly of the engine, otherwise you may be facing the possibility of a transmission that won't shift properly, or may lock up blowing a nice sized hole in the engine case.
If the transmission is NOT in neutral, you can locate neutral by rotating the shift drum while turning the transmission shaft's simultaneously until the detent (arm with a heavy spring on it) is positioned directly over the smallest or irregular detent on the shift drum as this will be neutral.
If it was necessary to manually place the transmission in neutral, after repositioning the shift drum, check to ensure the transmission is actually in neutral by holding the main shaft from turning while turning the countershaft back and forth vigorously.
Once the transmission is assembled and it's in neutral, all that's left is to install the crank bearings and crankshaft, then close up the cases as outlined below.
With crank bearings that have been in a freezer for several hours, position either engine case in an engine building box where the cavity for the crank bearing is facing up, then heat the case in the area surrounding the crank bearing's cavity to roughly ~150 degrees Fahrenheit.
Once the case you're working with has been heated sufficiently, remove the bearing from the freezer and QUICKLY place it into the engine case followed by repeating the heating process with the other case, then installing that bearing.
Once the crank bearings are installed in the case, dry the bearings of any condensation from being in the freezer with compressed air, then apply a light coating of 2 stroke oil to each crank bearing.
Next ensure that the sealing surfaces of the engine cases are clean and dry by wiping each with a clean rag and brake clean, then install any case o-rings (If present) followed by installing the center case gasket over the dowels, or coating the sealing surface with Yamabond #4 in preparation of closing the engine cases.
Note: Yamabond should only be used where the manufacturer specifies a non hardening sealant between the cases.
With the crank bearings installed in each case half, the crank still in the freezer and the case gasket or Yamabond in place, before moving onto installing the crankshaft you'll need to heat the inner race of each crank bearing as doing so will allow the crankshaft to be installed without the use of a crankshaft installer.
To heat the inner bearing race, fabricate a shouldered steel slug that has an area approximately 1 inch long with an OD (Outside Diameter) slightly smaller than the ID (Inside Diameter) of the crank bearing, then place this in a vice and heat it to a temperature upwards of roughly ~600 - 700 degrees Fahrenheit.
With the slug nearly red hot, VERY CAREFULLY remove the slug from the vise and place it inside the bearing within the case which the assembled transmission is in allowing 10 - 20 seconds for the heat within the slug to sink it's way into the inner race of the crank bearing.
Once the inner race of the crank bearing in the case with the transmission has been warmed, pull the slug from the bearing, quickly reheat the slug, then place it in the center of the crank bearing which has not been heated.
At this point you need to be moving FAST, so when you remove the crankshaft from the freezer, QUICKLY insert the crankshaft into the previously heated bearing ensuring it goes in straight.
If the crankshaft is lowered into the bearing race anything but perfectly straight, it'll be necessary to slightly wiggle the crank and work it down into the bearing by hand until it's fully seated but do so quickly before the crankshaft warms.
Once the crank is seated in the crank bearing, remove the heated slug from the opposing crank bearing and quickly place that case half over the crankshaft and transmission shafts while working it down by gently tapping the engine case in several areas using a dead blow hammer.
As long as the case is moving continue carefully working the case half down until the case halves are either meeting, or within a 1/4 inch of being in contact with each other
With the engine cases nearly in contact with each other insert the case screws and carefully tighten each of these in steps using a criss cross pattern while slowly pulling the cases together until the case screws are tightened and the case is sealed.
Immediately after closing the cases, reposition the engine so it sits upright in your engine building box, then trim any excess gasket material, or clean up any excess of Yamabond.
Next stick your finger in the small end of the connecting rod and attempt to spin the crankshaft.
If the crankshaft spins freely, great you don't need to do anything more except reassemble the remainder of the engine. However, if the crankshaft feels tight, use a dead blow hammer and FORCEFULLY strike the crankshaft from the right side, directing the blow inwards at which time you should hear a "Crack" at which point the crankshaft should then spin freely.
Once the case halves are together and the crank spins freely, lightly oil the crank bearings with the same 2 stroke oil that the motorcycle will be running, then install the crank seals while ensuring each seal's installed depth is correct.
Finally reassemble the remainder of the engine but be sure to check out our articles that detail inspection and assembly of the clutch and it's various components, as well as a separate article which details inspection and assembly of the top end and breaking the motor in.
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