YZ426F dropped valve

Greetings everyone. I'm new to TT and hoping you can help me out. I just picked up a 2000 YZ426F with a seized engine. I got a pretty good deal on it, but I'm not too familiar with the bike.

Anyway, I got the bike back to my shop and tore it apart to find out what the problem was. As it turns out, it had dropped an intake valve, and proceeded to run just long enough for the broken valve piece to destroy the head and piston, and put a few small nicks in the cylinder wall. Other than that, everything (bottom end and the rest of the bike) is in great shape.

Questions: What would be the best way to repair it? I'm looking to get a big bore kit and turn it into a 444. I think the over-bore will clean up the cylinder just fine (Of course I'll check with a machine shop prior to purchasing any parts).

I'll also need to replace the head and I've been looking on ebay. It seems like there are more heads for a 450 than for the 426. Would the 450 head work as a direct bolt on? What other things should I be looking for in the head?

What would have caused this to happen? The guy I bought it from said that it was idling in his driveway and just shut off, seized. Sounds a little suspicious to me.

Is this a common problem? I haven't seen any other threads about it.

Thanks for all your help!

ya sounds suspicous...it could be bad maintence or just cause its an old bike..either way its not real common on yammies.

btw i think with the 444 kit u get a new clyinder anyway...dont quote me on that but every other big bore kit ive seen came wtih a new cylinder

Send the head to Engine Dynamics to get the damage repaired. Then use your damaged cylinder as a core for a Luke's Racing big bore kit.

Typically, dropping valves is caused by fatigue, which is usually exacerbated by wear at the valve face. If the valves are adjusted more than twice, it's a good idea to replace them. Fortunately, the 2000 models used the less expensive stainless steel valves.

Thanks. I'm going to make some calls on Tuesday and see what I can find out.

I can not wait to get this bike running again and go ride. The weather is surprisingly tempting for riding here in Colorado.

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