First time splitting cases. Tips, advice, words of wisdom?

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2/24/2018 3:36 AM
Edited Date/Time: 2/24/2018 3:38 AM

Hey everybody I have a project 98 YZ250 im currently working on. (Check out my bike build thread.) I bought the thing to tinker with and the next step is to go through the bottom end.

Ive maintained my two other bikes so Im not too intimidated by the thought of splitting my first cases.

I plan on getting the proper tools for the job like a case splitter, clutch holder etc. in the next couple of weeks.


I just wanted to know if you guys had any advice or words of wisdom or anything for my first go at it. I have a manual and everything but I know theres always something you werent planning for or dont expect to happen.

Is there anything you wish you knew or were taught the first time you split cases? Somthing a newbie would overlook?



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2/24/2018 4:15 AM

Just did my first one on my sons 16 yz250f. The crank puller/ installer tool was definitely needed. I bought one from rockymtn and am seriously glad i did. Everything else was pretty straight forward . I was a little skeptical but after doing it i wont hesitate next time. We just sent the crank, cylinder and head to millineum for rebuild/ machining, replate and everything came back mint. Next step was to break out the manual and go to work. Im really happy on how everything turned out. I hear how alot of guys put the cases in the oven to remove/ install bearings. I did not do that. I had the main bearings in the freezer and used mapp gas on the cases. Struggled a little with that. The interference fit is a pain so i can see why people put the cases in the oven. I didnt want to mess with that. I left all the seals in and transmission together and didnt want to disassemble everything. Maybe i should have because i did ruin a main bearing. Everything is back together now and bike is running great. Im really happy to have tackled that job with success. Only other "issue" is having the cases up off the bench being supported properly while installing/removing crank and bearings etc. I will probably use a couple 2x4s next time. Good luck with your project and i will be curious to hear how everything goes for you.

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2/24/2018 4:49 AM

Buy the right tool for the job. Crank puller, case splitter, flywheel puller, etc. ziptie the transmission together so you dont forget how it goes. Follow the service manual. Every bolt gets torqued and something put on the threads.

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2/24/2018 5:08 AM

Make sure you mark your shifting Forks not sure if it's like a ktm but if they are there different and there's nothing in the manual to tell you which one goes where and no identifying marks. I also Mark the location of the shift Forks on the shift drum just makes it easier when reassembling. You can wrap zip ties around the ends of shaft hold the gears in place when you're pulling shafts out and you got to watch first gear will fall off the bottom on you also.

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2/24/2018 6:17 AM

I have found that on the YZ's you will need to reinstall the complete transmission including the forks and the drum as a unit. You can the install the fork shafts. It is almost impossible to get the forks back in place doing it any other way. Most bike you can install the drum, then the trans and then the forks and pins with no issues, but the YZ's are different.

Paw Paw

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2/24/2018 6:31 AM

Paw Paw 271 wrote:

I have found that on the YZ's you will need to reinstall the complete transmission including the forks and the drum as a unit. You can the install the fork shafts. It is almost impossible to get the forks back in place doing it any other way. Most bike you can install the drum, then the trans and then the forks and pins with no issues, but the YZ's are different.

Paw Paw

KTM claims you can remove the forks and reinstalling with the Drum in ,I never found a way to do it ,I put the two shafts in with the forks on them and then install the drum.

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2/24/2018 6:44 AM

Split a few cases. On the yz125. I started assembly of the tranny on the stator side. Way easier. On my yz250 the clutch side. Yes it was a little rough getting the whole tranny installed in one unit. Next time I will try on the 250 from the stator side.

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2/24/2018 3:52 PM

I wrap the transmission in plastic stretch film from Home Depot so pieces don't get away. YZ 2 strokes trans are tricky to assemble. It seems like they like to be slid in as a unit.

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2/24/2018 5:55 PM

Put the tranny in without the crank and put the cases together with no sealant the first time. Make sure it shifts easily and you can find neutral. Then resplit them and install the crank.

There are no center case gaskets, so ensure the mating surfaces are clean and use Yamabond when you do the final assembly.

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If you're not mixing gas, you're not haulin ass.

2/24/2018 5:56 PM
Edited Date/Time: 2/24/2018 6:00 PM

Grab a Tusk crank puller in addition to your splitter and flywheel puller. This makes reassembling SO much easier. If its your first time, dryfit the cases back together without the crank to get a feel for how the transmission fits in. Once you have done it a few times, it becomes an easy afternoon project.

Edit: and if dont already have one, buy or make a two stroke leak down tester (different that a 4T one). You can make one from box store components for less than $40. Silly to do a complete rebuild and not ensure its sealed properly.

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2/27/2018 6:49 PM

Seems simple, but take pictures as you go. Sometimes the pictures in the manual aren’t the greatest, and I know I’ve done this plenty of times before so I have a reference point on reassembly

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RPM Performance
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2/27/2018 7:26 PM
Edited Date/Time: 2/27/2018 7:26 PM

If you're going to replace any bearings, definitely use the heat/freeze technique. It takes patience and a few tries, but don't give in to the temptation to press or hammer any bearings in because you will dramatically increase the risk of damaging the cases. And when you're reinstalling the crank, be aware that any side force on the main bearings can shorten their life. So to prevent this, pull on the the crank (with a crank puller) while pushing only on the inner race of the bearing. You may have to track down some washers and odd bushings to make this work.

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