New valve shims, no clearance change?

1999 YZ400F

I checked the valve clearance and they were all tight, one of them extremely tight (I just bought this bike so don't flame me). After carefully measuring the existing clearance, doing the calculation between the existing shims and the clearance specs, I ordered new shims and installed them, torquing the cam caps carefully. The new clearances are not what I expected with the difference between the old shims and the new shims. For example, the left intake was at 0.10mm, so I changed the shim from 175 to 165 and the new clearance should be 0.20mm, right? I measured the new clearance at 0.13mm. Even worse, the center intake valve clearance did not change at all even after changing the shim from 166 to 160. What gives?

Hi there

Did you measure with the cam chain at tension?



Did you measure with the cam chain at tension?

Yes the cam chain and guides and tensioner are all installed, and the timing marks are perfect.

The center of the shims can wear on the top of the valve, making the shim thinner than it is marked. It is possible that your old shims were worn, so when you bought the new thinner shims, they actually weren't as much thinner than you thought. Look at the old shims and see if you can see a circular wear mark in the center of one side.

One of the other two things that can happen are to get the shim cocked in the spring retainer so it doesn't settle all the way down on the valve stem.

The other is that you may have had less than zero clearance. Once the valve reaches zero, the pressure holding the valve head into the seat drops way off, and the valve will start vibrating around on the seat, wearing it even farther. The is aggravated by the fact that the clearances are actually tighter when the engine is cold, so it may wear down to .001" while running, and then have -.0005" when cold. If your shims are all seated, try the next smaller size.

I'll add that I've noticed differences in thickness when mic'ing individual shims that are "marked" as being the same thickness. This is when using the HotCams kit.

I mic my old shims as well as new ones before I install them.

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