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smdonald

Rebuilding Transmission

8 posts in this topic

I need to rebuild my transmission. 01 YZ426F. I really would like to do it myself not only to save the money, but also to gain the knowledge. Should I attempt it myself w/ a Clymer Manual? Anyone have any ideas on what to do or how to do it?

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Thanks GrayRacer. I've done everything else on 2 and 4 strokes but never a transmission. Just wondering how comlicated it was. Im confident I can do it but just thought I'd ask before diving into something i would regret. Dealerhips hate engines coming in in a million different pieces! :bonk:

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The Yamaha manual is really top notch. Just look through the transmission section step for step and see if you think its within your ability. Its not really that hard of a job, but it needs to be done right the first time.

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SMDonald, let me give you some tips since I have done this job twice on my 01 yz250f.

- The yamaha OEM manual was great, you follow it, you can do the job.

- If you are a smart mechanic, you will lay your parts out on a clean bench, top end parts with top end parts, etc.

- Don't forgot to check the inside of your flywheel, chances are there are lots of metal chunks from your transmission failure in there

- Don't forget to check your frame filter screen incase anything is blocking it

- You may and probably will have a few swear words burst out when trying to assembly the shift cam mechanism (2 springs, 2 spacers, other little gadgets that have to go in as one piece), yup.... its a B*tch but you will get it I'm sure

- Inspect EVERYTHING, you probably have some bad bearings in there, case bearings particularly where the transmission shafts go through, there are probably crunchy from metal fibers stuck in there..... replace them. Inspect all top end parts incase a metal chunk tore something up there

- replace the timing chain and oil pump regardless if they look fine

- Use a good permatex or yamabond to seal the cases

- Use a complete gasket set to put the engine together

- When putting the cam chain and cams in, after you think they are set.... make sure there are I believe... correct me if I'm wrong, 12 teeth between the 2 dots on the cams.

- Don't become overwhelmed.... you can't do this all in one shot, take your time, come back to it, take pictures or do drawings in you have to because let's face it, there are lots of moving parts in there, and sometimes the manual diagrams don't show everything well.... but usually it does a pretty good job

- Good Luck!

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SMDonald, let me give you some tips since I have done this job twice on my 01 yz250f.
Good advice. Allow me to comment:

- If you are a smart mechanic, you will lay your parts out on a clean bench, top end parts with top end parts, etc.

If you're a smart mechanic, you won't need to do this. If you are a wise mechanic you'll do it.
- Don't forgot to check the inside of your flywheel, chances are there are lots of metal chunks from your transmission failure in there

- Don't forget to check your frame filter screen incase anything is blocking it

Add to that, flush both oil lines, the tank, and inspect the oil pumps. Look for metal everywhere. It'll be there.
- You may and probably will have a few swear words burst out when trying to assembly the shift cam mechanism (2 springs, 2 spacers, other little gadgets that have to go in as one piece),
That's AFTER you've held two shafts, ten gears, and 3 shift forks in one hand and slid them into 5 holes while holding the case with the other. But it's not as bad as it sounds. Really.
- Inspect EVERYTHING,
Bearings that can't be cleaned to run perfectly smooth should be replaced. Bearings you aren't 99% certain about should be replaced, too. Any shift fork that has wear in the center of the 'C' is worn out or bent, get rid of it. The locking lugs on the side of the gears should not be chipped or rounded at all.
When putting the cam chain and cams in, after you think they are set.... make sure there are I believe... correct me if I'm wrong, 12 teeth between the 2 dots on the cams.
Use the timing marks and the head to time the cams, not the number of teeth between cams. Chain between the cams will tell you if they are in the righe relationship to each other. What's important is their relationship to the crank. Anyway, stock timing on a 426 should put 13 pins between the cams. If you do the 450 decomp cam mod, follow the instructions in Common threads.

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Wow. Lots of great info. Actually, I had problems getting the drive sprocket off as you probably have read in my other posts. Finally tonight I got it off and split the case. I couldnt tell when the case was together but it doesnt look like a tranny failure. The right side crank bearing is shot. I dont know if The metal from something else got into it and caused it to fail or if it just happened and the bearing itself caused all the metal. either way im sure ill have to replace pretty much all the bearings down there to be safe.

Im not really worried about how much it will cost, I just want to make sure I know all the facts before its to late and do everything the correct way. Grayracer, does this sound like a good plan to replace all the bearings? Also, I dont really know what the gears should look like. When I hold the shaft from the tranny that connects to the clutch inner hub and hold the output shaft, there is very noticable play between them. Should it be this way?

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Rotating play? Yes. Look at how the the trans works. You're probably seeing the backlash between the locking lugs. The lash in the gear teeth themselves is less.

If the metal is a whole bunch of fairly uniform flakes, they're from the bearing. But that doesn't change the recommendations for inspection and flushing previously given in this thread.

Yes. I would replace the whole set of bearings, including those for the balancer shaft.

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