Valve shim??

[vitesse7749]

Being quite familiar with valve trains in general I wonder , how does the clearance "tighten" or reduce itself on a bucket and shim system?
The valve stem length at TDC is at it's min. valve to seat full contact. The cam lobe is on the min. round of the profile. This being the case neither can advance towards each surface until the cam turns to begin it's contact with the shim in bucket in order to push the valve to it's full open state.
I imagine in time that the contact points would wear, the lesser metal being the shim would decrease it thickness thus requiring a thicker shim , I hear that this is not the case but the gap actually reduces. Only a valve bedding into the seat could allow this as the cam is in a fixed position.
Someone much more intelligent can explain this please.
Thank you

[djsbriscoe]

Hi,
It doesn't make sense to me that the gap reduces because of wear, but I could be wrong. Like you say a valve bedding into the seat might reduce the gap slightly, but not much.
I feel that wear would occur on both the shim and cam about equally. However proving this would be down to experience and having a feel for a particular engine.
The forces involved are quite complex. However the oil is the important thing here as it's there to separate the two surfaces.
The only thing that makes the gap between the cam and shim reduce is expansion due to temperature.
This is why the exhaust valve clearance is larger than the intake valve clearance, because the exhaust valves get hotter.

David

[DAVID THOMPSON]

i let the engineers think about the hard stuff
i use shell rotella t oil so it gets a good lube
and check it every so often with a feeler gauge
and if it's close ....i stop wasting time and go ride

dave