splitting cases

Will be splitting cases in next couple of days to extract a broken or chipped gear. This was the cause of my noisy motor ( swishing noise ) The piston / rings are garbage pail liner. Luckily the 420 rebore I had done over the winter was so tough that there is no major damage. Will have that shop pass stones to remove minor glaze. Asides from the obvious ( such as being organized and calm / patient ) is there anything in particulat to look out for?

Don't forget to put oil in it before starting... :)

Keep track of all the dowels and o-rings, especially on the right cover.

Buy all the gaskets you will need (base, head, left, right).

You may also want to replace some of said o-rings. I had problems with the large one from the barrel water passage into the cases.

Buy some Yamabond for the case halves.

You will need some snap ring pliers (both kinds).

Make sure you have a good, square 5 mm allen tool, you don’t want to round off any bolts. An impact driver (the kind you whack w/ a hammer) may come in handy here, especially if you replace any bearings, they are secured with pretty shallow 5-point torx-head fasteners and an impact is a must for those. If you are replacing may consider new bearings at this point be aware that they need to be pressed in. If you elect to R&R the crank (to get at the main bearings) post another topic, I used an easy and inexpensive alternative to the “tool” called for in the manual. Do not press the crank in. You can use a press to push it out of the left half, but that may squish the threaded end (I’ve seen this happen).

I had some difficulty installing the gearsets. I finally mocked the two shafts, shift drum and forks up together with a big rubber-band (section of inner tube) and installed the whole shootin’ match (save the outermost gear and snap ring on the counter shaft) into the left case half at once. Weird, I know, but the only way I could get everything lined up. You’ll see what I mean.

I’ve done this three times, but it has been a while. If you run into any snags post a question about it here.

Hope this helps.

To add to all of those suggestions:

I did a split on my 01 426 way back !

1. replace all of the bend tab washers on the clutch side. I think there are two or three of them. I'd even replace any snap rings you pull off as well.

2. Don't pry against the clutch basket at any time. Fragile.

3. Use an impact to loosen the clutch nut, the main drive gear nut, and balancer shaft nut. I used a cordless Snap On impact. You don't need a ton of torque to do it, but a ratchet and hammer aren't recommended.

4. Make a square(roughly 10 inches inside diameter) out of two by fours or four by fours. You'll need this to set the motor on whenever you are doing any work on it. You don't want the crank shaft (either end) to have pressure against them if you are hammering on your motor to get something off.

5. You won't need to separate the top end from the bottom end. Just pull the four bolts out from under the cams and I think one 8mm bolt at the base of the cylinder. Take the motor out of the frame before you disassemble this.

6. Have plenty of patience, a manual, and a number for a good mechanic at your side. :) Good luck and let us know if you have any questions.

Ha! Ha! Very funny! :)

When re-assembling, grease all O-rings, dowels and fasteners, and lightly coat all paper gaskets with oil. Everything will go together smoothly, and come apart if the need should re-appear.



01' WR426

99' YZ400

87' FZR1000

Uh, like twist the throttle...

Guys, thanks for the great info. All your suggestions are spot on and look forward to delving into it in a logical, step by step way. It's nice to know you guys are there if I need you!

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