Valve Clearances tightening

[pekingduck]

The common thinking is that valve clearance clearances increase as cams and bearings, shims, and rocker arms wear. And it's true to a small extent. But the clearances usually come from valve seat inserts in the head getting their sharp edge surfaces constantly hammered by the edges of the valves. Over the long haul, the valve seats will get wider and the valves will lose shape, becoming "tulip" shaped in extreme cases.