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Valve clearance and adjustment

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I got a 2009 yz450f, i took the readings for the valve clearance but here is my question. The feeler gauge is supposed to slide under fully through but not easily. From what i understand you want it to be snug but not so much you cant pass. Does it get tighter or more loose fitting as they wear? Should i be concerned if i cannot get the feeler through underneath?

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If you lay the feeler gauge on a bench, then rest the end of a 3/4 or 7/8" box end on the blade, the drag you feel as you pull the gauge out from under the wrench is what you should expect.  Any tighter than that and you'll be starting to lift the valve open.  The gauge should slip all the way under without being forced in any way. 

Normal wear causes the valves to tighten as the wear progresses.

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That depends on how tight it is, and whether it stays adjusted.  Rule of thumb is that once you reduce the shim size by two steps or more (.10mm), the valve will need to be replaced. If you don't have to go that far with it, but checking the valve again in less than 5 hours finds that it has already gotten tighter again, that means the hard coat is worn through, and it needs replacement.

So, you'll need to start by finding out what shim will bring the clearance into specification and go from there.

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