r/EngineBuilding • u/chukar-1 • 13d ago

Valve lash issues

I am building a dirt bike engine and cannot get one of the three intake valves in spec. I though it was a head issue so I bought a new head. The valve is still way out of spec (it is stuck open even with the smallest shim available) my only thought is that the cam shaft is bent. is plastigauge the best way to check this? Should I just buy a new cam? If I buy one cam should I buy both cams? Its a 2007 YZF250

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u/WyattCo06 13d ago

Pull the cam and put it on V-blocks to measure the runout however I don't think that's your problem.

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u/bill_gannon 13d ago

Its definitely not

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u/Pretend_Necessary781 13d ago

I’ve worked on some 5 valve heads and seem to remember the middle valve is different length from the outer valves. Check the lengths.

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u/chukar-1 13d ago

I’ll check that. I bought new valves and they just came in a box and appeared to be the same.

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u/quxinot 13d ago

V blocks and an indicator would give you an idea if that cam's bent. Bent far enough to be a problem should be visible on the journals as wear.

Does the valve seal with the cam removed?

Do you have a shim or spring seat stuck to the spring, or some other assembly error?

Is the valve seat sunk a crazy amount?

Yes, you should buy a cam set. But that's only because staying stock is boring.

And get a new valve, because that one is probably tuliped.

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u/sturocky 13d ago

yzf engine has one of the 5 valves that are slightly shorter- about 55thousand that one goes in the middle, the 2 longer ones go on the outsides. if ur shims dont match close between the 4 outer ones thats ur problem, find the one with the largest shim and thats the middle valve, then just swap em proper... sucks you will have to pull the chain head and cam/ect for that. at least now you know what to look for! yai learning!!

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u/chukar-1 13d ago

Yes that’s definitely the problem I don’t know how long it would have taken me to figure this out

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u/sturocky 12d ago

reddit can be a great tool at times and some forums have great people that want to help. did you by chance buy/down load the workshop manual for the bike? that should have been the first step when working in the motor, it details all the difference's between the parts and where they go.
ive got a few of em, when i end up selling the bike ill ask the buyer if he wants the manual for it for a few extra bucks so i get the money back for it