3rd gear - tooth missing & then some

Well, my mechanic split my engine last night & found 3rd gear missing a tooth and 3-4 other gears messed up. Also a coupla shifting forks are beaten up. Luckly the crank & such look ok. Can't wait to get this bill. Are there any after market manufacturers for 426 tranny parts? If I have to go w/ OEM how can I be sure they last longer???? What causes this problem? Riding style (or lack of)??



I read that the gears are not made by Yamaha and that early shipments of these parts for 2000 YZ (426 & 250) did not meet the hardness spec. and were replaced in later production runs. I bought my YZ new in December and the tranny blew up the next fall. I lost third gear dogs which ejected bottom shift fork into the crank, breaking the case and bending the crank. When I split the cases I discovered both fourth gears were pitted badly on the thrust face of every tooth.

So hopefully this was a one-time glitch in the Yamaha supply chain. But who really knows for sure?

I spent less than $300 replacing four gears, all the bearings, the shift fork and assorted washers, circlips, gaskets and o-rings. The crank and cases were what killed me.

I saw an ad once a few years back for wide ratio tranny sets available for the big four 250 MX bikes, that is the only time I’ve heard of aftermarket gear sets and I can’t remember the company. This doesn’t appear to be a widespread problem so I just crossed my fingers and installed OEM stuff.

i thought that yamaha's prices were quite reasonable. i had my bike jump out of fifth gear. turned out the fork had bent over allowing the dogs to come out of the 3/5 connection.

when the new bits turned up the selector fork was VISIBLY beefed up. even the dogs had been altered slightly.


Thanks guys! Hey Hick! double thanks - you keep replying when I post the same dang questions over & over about my engine. Thanks a ton. Gil - my mechaninc - found the missing tooth sitting in the bottom of the case, didn't touch much anything on the way down. Looks like 2 shifter forks are scard, we'll replace. A coupla of other gear's dogs are getting rounded so we'll replace them too.

Piston mic'ed at the big end of normal (.06 mm I think w/ .15 being too much) Debating whether to replace piston & rings or just the rings.

Good thing though.... clutch & basket look fine! woo. hoo.

Thanks again! CYa!

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