Is a loose valve a happy valve?

4 replies to this topic
  • aj_yz426

Posted April 15, 2009 - 05:49 PM


I have a 2001 yz426 with stock Ti valves, one exhaust valve clearance is at the upper limit of normal 0.25mm and the other is 0.30mm (normal range 0.20-0.25mm). By the way, the bike is running fine, I've been running this way for a couple short rides since I added a hotcams exhaust cam. Clearances haven't really changed after a couple checks. I know, shame on me for running it out of spec:bonk: , but now I have the time to get around to taking off the cams to get to the shims.

Should I adjust the 0.30mm down to 0.25mm or should I adjust both to around 0.20mm? In other words is it better to shoot for valve clearances on the tight or loose side?

Will the valve clearances generally get alittle tighter over time as they wear into the seat? If so, adjusting towards the upper limit of 0.25mm would probably be better-right?

The other factor is that all the current shims are original factory (so I have some weird sizes not available to the public, like 184's and 188's). The one with 0.30mm valve clearance has a 184 shim, so if I go to a 190 shim my clearance should be 0.24mm, but if I go to a 195, then I'm just alittle too tight at 0.19mm. I've heard a loose valve is a happy valve, so I'm going to shoot for valve clearance on the upper end of normal and use a 190 shim unless advised otherwise.

I read another recent post with a question of whether its OK to run slightly loose, but didn't answer if its better to adjust on the loose or tight end of normal.

Thanks for any help.

  • grayracer513

Posted April 15, 2009 - 06:03 PM


The usual pattern is that he valve will tighten, yes. Nevertheless, it is technically better to run then at the tight edge if the clearance "envelope" if you can.

If these are valves that have never been shimmed, you may find odd number shims in there, as the factory has access to shims in .01mm increments to do the initial setup with, and if so, that will make your job a bit easier as you attempt to get it within the adjustment range. One thing to remember is that the entire specified range is only .002" wide, so don't twist yourself in knots trying to get exactly on a certain number.

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  • 642MX

Posted April 15, 2009 - 06:53 PM



  • SamHard

Posted April 15, 2009 - 10:41 PM


your valves open on your pistons intake stroke, the tighter your valves are the more they open, the more open the more air you get in your combustion chamber, which leads to more power. trust grayracer and snug those things right up to the tight end of spec

  • grayracer513

Posted April 16, 2009 - 06:33 AM


So, you you think .05mm (.002") extra valve lift will make a difference in power? :p

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