2009 YZ450 valve shim questions.

2009 YZ450, intake valves are as follows. Left side (looking from back to front of engine) specs out at .12 MM, middle is .09 MM and right side is .10 MM.  Exhaust both spec in at .21 MM.  My question is could I just take the loose .12 and move it to the tight .09 on the intakes?  Or would a better option be to re-shim them all since they are all on the lower side of spec to get them more in the middle of their range (.10-.15 MM for the intakes).  And what shim sizes would be my best option?  Had the bike a few years now and although bought used I feel pretty comfortable saying this is the first time they've been out of spec when I check them quarterly and so also this would be the first shim adjustment.  Thanks for the replies.

Shim the middle intake only.  It's the only one out of spec.  The whole range of tolerance is only .002", and they are originally built as close to the low end of clearance as they can be.

My shims ended up being (L) 180 (M) 185 ® 180.  With the digital calipers they came out to be a smidge higher at (L) 182 (M) 186 and ® 182.  Since those are only in increments of 5 I can assume they're 180, 185 and 180?  Also with the middle one being so far off the other 2 would that mean it may have been shimmed before?  I bought the bike used and it was low hour so I suspect not, but wanted to get your opinion.  Thanks Gray 

Edited by Yzflier977

I used the shim calculator that I found on here and plugging my numbers in it looks like I need a 180 to get the middle one back in spec.  That sound about right?

Edited by Yzflier977

The OEM shims are not limited to increments of .05mm.  The factory uses them in increments of .01, and they set the clearance as near the minimum numbers as they can.  But if your shims are marked 180 and 185, and you're reading 182 and 186, maybe your calipers aren't zeroed.


And yes, If you have a marked 185 in it, a 180 will increase your clearance to .14mm, which will be in spec.  Recheck after you install the shim.

There was only one shim that I could read the number on and it said 180 but miked out at 182.  (The other 2 the numbers were wore off.)  I checked it twice and zeroed the calipers both times.  But be that as it may that valve was the best clearance wise at .12.  And a thin sliver of metal (feeler gauge) doesn't need to be zeroed so I'll move on.  I assumed the OEM ones were in increments of 5 because on Motosport and Rocky Mtn's website they only offer OEM shims in 5 incriments.  I picked up the needed shim today and should be good.  Thanks Gray for the help.

The available service shims are in fact only sold in increments of .05, at least by Yamaha and Hot cams. Only the factory has the .01mm incremental stuff. Honda, on the other hand, sells them in increments of .025mm, unless they eschew carrying the OEM shims in favor of aftermarket, and the ones for the CRF450 are the same 9.48mm diameter.  They almost always have a good selection on hand, too.  ;)


Readings on vernier calipers are subject to variances of .01-.02mm or more based on nothing more than the touch of the user.  As far as feeler gauges not needing calibration, better check that.  Not only can there be considerable discrepancies between the marked and actual sizes of the less expensive ones, I have $5 that says I can have 5 relatively inexperienced people to measure the valve clearance on a YZ and get a minimum of 3 different answers.  Just sayin.  One thing I haven't ever seen is an OEM Yamaha shim that was bigger than its marked size. 

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now