2006 YZ450F Valve Clearance

I was under the impression that the valve clearance for a bucket and shim set up got tighter over time due to valve stretch. On my 2006, I went from .009" on the Exhaust to .010" for both exhaust valves in about a year with 50 miles or so every other weekend. The intake on one stayed the same at .0055", one went from .0055" to .008", but the center one went from .0055" to ,010". Spec is .00039 to .00059.

Strange thing is, I measured the gaps, then measured the shims to see what I needed. I was able to swap two, and needed to buy two. When I re-assembled, 3 of those measured as expected, but the the center Intake, which shim was measured at .074" and replaced with a .079" still had a measured gap of .008", which is .002" out of spec.

Pretty weird that the one valve that was almost .005" out of spec, did not work out when I replaced the shim wit one that was .005" thicker, going from a 185 (.074") to a 200 (.079").

Anything I should be looking for on this valve that I may be missing?

Two things come to mind. Generally, in order for a YZF valve train to gain clearance, something has to wear in a way that allows the cam and lifter to be farther from each other. Normally, the heel of the cam never contacts anything, so only the lifter becoming worn would do this, and that rarely ever happens to any significant degree. What can happen is for the upper half of the cam bore to wear in the cap, allowing the cam to lift away from the tappets as you check clearance.

The second way excessive clearance can occur is for the valves not to close correctly. Weak springs, carbon near the valve faces or on the stems, or bent valves are the usual cause. Carbon seems a likely suspect because the clearances are evidently not stable and consistent. If it's carbon, you can often bounce the valve open a time or two using a punch and cause it to settle in.

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