yamaha valve clearance

yz450f what makes intake valve clearance too tight. im reshimming. just wondering why they were tight.

Sounds like the valve lip is wearing, the seat or both.

When the valve face and/or seat wear, the valve moves up into the head, and clearances tighten. If your engine needs a shim .10mm smaller than it started out with, or if the clearance tightens up again in a few hours time, it needs new valves.

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