YZ426F rebuild after cam chain jumping a few teeth

Hi my yam skipped a few teeth on the chain yesterday I've stripped it down slight marks to carbon on piston but its fine, looks to be all 5 titanium valves bent, 1 bucket smashed, cams and everything else is fine.

Just wondering what I need to do or more importantly, apart from valves do I need valve seats and guides...

I have the manual and I'm going to check all clearances and tolerances of piston and prob put a new set of piston rings in it. Also if I go down the steel valve routes I know I need new springs but will I again need new guides and seats.

Just trying to work out the cheapest way forward. Its a 2002. Stupid thing is I was going to do a new chain and tensioner on with before a mx race in a few weeks. I've only had it a few months. I mainly use it for green lanning.

Thanks seb

You'll have to have the machinist evaluate the guides and seats.  If the guides are not damaged by the bending of the valves, the seats will only need a minor touchup cut in most cases.  They usually need a deeper cut when guides are replaced.  Only if a considerable amount of material has to be cut away to square them up do they need replacement.

Thanks so even with new valves the old seats can be re-used just with a cut using cutting paste etc. And if the guides look ok then they can be re-used.

Am I correct in thinking if I went to steel valves I would need to have new guides pushed in? Its mainly a green manner now, long gone are the days of a full season of mx so I might just put steel valves in. The bike mechanic I use said its a lot cheaper to that route. He quoted around £500 for a set of titanium valves from Yamaha..


seats can be re-used just with a cut using cutting paste



NEVER EVER USE "CUTTING PASTE" (lapping compound) on any titanium valve, ever.  I know what the book says, just don't.


Titanium is only an acceptable valve material because of a very hard, very, very thin coating of a titanium nitride variant.  Using paste on a new valve attacks that coating and can seriously truncate its service life.  DON'T DO IT.


Valve seats need to be recut with proper seat cutting tools, and/or reground with a good seat grinder. 


Converting to the OEM stainless valves used in the 2000 model 426 is a good money saver without a noticeable performance hit.  Just be sure you get the springs from the same year to match.  If the guides haven't been damaged as I mentioned before, they don't need to be replaced in order to convert it.

Ideal that sounds brilliant. Thanks for the help will go for the stuff from the 2000 model then. Cheers

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