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Crzy Joe Devola

Valve Replacement Question

5 posts in this topic

03 YZ 450. Total Valve replacement, valves, springs, guides, head cleaned up by machinist etc. I assemble everything and on the intakes I am already down to a 160 shim at .10 & 155 on the exhaust at .20. Is it normal to have to run such low numbers on the shims right out of the gate on new valves? I guess I expected to be more towards the 200 range. Also, is there a rule to running them a little tighter or loose when they are new?

Thanks in advance for any help.

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The low shim numbers are a result of material having been removed from the seats when they were refinished. Either the machinist was a bit heavy handed, or the seats needed an extra bit of touching up to get them squared away.

The angst over whether to run the valves at the big or small edge of the specified range abates a little when you remind yourself that the whole range is only .002" wide. Yamaha always builds to the tight side of the spec, though, because it's overall easier on the valve train to do that.

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Thanks Gray, certainly makes sense. I figured the new valve would maybe make up more of that difference being it isn't all hammered out like the old valve. I ended up going with a 155 at .12 Intake & 145 .22 Exhaust. So being new components, I shouldn't see too much play in clearances anytime soon?

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Normally not, but be aware that the normal pattern is that they close up as they wear, not get looser.

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