Valve Adjustment Question

[BT124]

Okay, so I'm in the middle of adjusting valves on my 1979 Spider. I just used the feeler gauge to determine what the current gap was for each valve. I was surprised to find that for 4 of the valves the gap was smaller than the proper clearance. How is this possible? Shouldn't the clearance be increasing as the lobes on the cams wear down? The only thing I could figure was that the valve face is sitting lower in the head than when it was new. Any thoughts? BTW, one of the shims said "Volvo" on the side.
Thanks

[BEEK]

as the valve pounds itself against the valve seat through normal use, the clearances actually shrink. once the clearances get to where there are none, then the valve burns. this is a process that doesnt happen overnight. if your clearance gets larger then you do have a wear problem, a bad one. the back side of the cam, where the clearances are measured never touches the shim, therefor shouldn't wear

[azruss]

your cam box gasket compresses with time reducing the space between the cam and the wafer. I've heard that as the engine gets old, you can get carbon buildup that will actually increase the distance. I do question that one. I have found that the changes after 10k miles is minute requiring little or no adjustment over the next 100k miles.

[aj81spider]

I had Volvo shims in mine as well. The Volvo shims are the same size, so can be a good alternate source.

[bradartigue]

The valve seats wear faster than that camshafts. You have two types of wear though, the kind you get just after you rebuild a head, where everything is pounded into a more or less constant size, and kind you get every 30,000 miles or so. When I rebuild a head I leave things a little on the loose side (but in spec, just loose spec). Tends to tighten up after some running.

The shims never wear out it seems.