valve clearance increased during break-in?

3 replies to this topic
  • jameskiehl

Posted March 19, 2008 - 04:03 PM


hey guys,

rebuilt my top end last summer, had to take it apart this spring to repair what looks like a bad seal on the coolant race. Anyway, when taking it apart after having broken in the top end rebuild (brand new head, valves, cylinder and piston), i took the cam lobe-valve lifter bucket clearance measurements.

I was surprised to find that they appeared to be WAY out of tolerance, with too much space between cam lobe and bucket. I know i sized the shims correctly when assembling it the first time, and i know over a long time the valves should actually STRETCH, not compress, so if anything, running the engine should make the tolerances smaller, not larger. Even with that, i would not have expected any change at all, much less this large. I've certainly double- and triple- checked that i am using the correct units on the feelers and in the manual. only difference might be that there was a lot more oil on the feelers and in the cam lobe area than when assembling it for the first time, so maybe this made one or the other of the readings less accurate? easier to slide the gauges in with a little oil?

anyway, any thought would be appreciated. i'm going to put her back together and give her a shot this weekend, either way.


  • Zaj 243

Posted March 19, 2008 - 06:38 PM


I am very meticulous with my dirt track bike and between the middle of last summer and just last week when I rebuilt my bike, the shims I placed in the engine last year had worn .03mm. Not saying this is absolutely your problem but between that and wear on the buckets and lobes, it could combine to change the clearances enough. My case I account to the high continuous load at extreme rpms to result in the wear. If you have a micrometer measure and record your shim thickness. You might be surprised at how much it can change.

PS- My shims were from a Hotcams kit, not oem replacements.

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  • dazzabb

Posted March 19, 2008 - 09:23 PM


Hey Jim,

I had the exact same problem after my last adjustment with the valves being 0.1-0.20mm out of spec.

I too was worried about it and posted a thread on this forum, see this link

http://www.thumperta...=BIG valve gaps

My mechanic (ex Honda Japan) assured me the while valve stretch is the norm it is not uncommon to have to shim for bigger gaps and not to worry about it, just rebuild it and ride.

So thats what I did and after putting it back together glad to report I've had about 700 trouble free km's.
Next valve check coming up soon.

Hope this helps.

  • jameskiehl

Posted March 20, 2008 - 09:00 AM


thanks for the link to the other thread. seems like using the feeler gauges takes the right touch. i will re-check the assembly and make sure my gauges are giving the proper readings. i will also do a leak test on the valves, make sure they are seated properly. maybe my tdc was slightly off, too.



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