Trying to understand the concept of shims

I was hoping someone could school me on the way shims work. I just checked the valve clearances on my 82 XJ 750 and they are all grossly out of spec, all at around 0.05mm (could not of done it without BigFitz's thread). As I am ordering new shims I understand that I am installing smaller shims. That makes sense insofar as the "tight" valves don't sit completely when they should be closed. But why are we not installing larger shims to compensate for the erosion of the relevant parts? I would of expected the valves to close with to much force as the seat and shim wear, not remain open.

Second question: if the intake valves are too tight and do not close completely, does this mean that intake mixture would be continuously flowing into the engine?

Thanks to anyone who's got the time to answer

 

The wear of the cams/shims etc is negligible as they're bathed in engine oil when it's running, but valve heads/seats do wear slightly over time as they're not in oil, so that's why the clearances close up. Although you can measure shims with a micrometer to double check if unsure.

If the valves have any clearance at all then they will still close fully, just not for as long. It's only when they have zero clearance that they stay open all the time, then, when the piston comes up on the compression stroke some of the pressure leaks back through the partly open valve (& when the cylinder fires you lose some of the combustion gases past that valve, heating it up)

A tight valve that does have clearance also runs hotter but for a different reason, it spends less time on it's seat, which is the main way the valves are cooled (they transfer their heat to the head), either way tight or zero clearance will eventually damage the valve through it overheating...

Hope this is some help

 

First answer: It's because the valve seats wear and the valves essentially pound themselves deeper into the head, tightening the clearances. If a whole bunch of other stuff was wearing then it might be different; but the only place any significant wear occurs is at the valve/seat so they just get tighter.

Second answer: Pretty much, but not just in; if the valve stays open too long a bunch of the charge can get pushed back out as the piston comes up and puts the mixture under compression. That's why being within the specified range is important; a valve doesn't have to be continually held open for it to affect how the motor runs; once it's open longer than it should be because it's opening early and closing late it's going to begin to affect things. And in a high-rpm motor you don't have to be too far out of spec for it to matter.

 

Thanks for the answers guys. Great help and much appreciated.

Mike