The first question about the valves is a good one. I believe the reason is that the head expansion is the reason you set the valves looser when hot. The rate of cooling of a piece of metal is proportional to its mass. Therefore the valves (light) cool quickly in relation to the head (heavy). Cast iron probably has a higher coefficient of expansion also. At any rate, the distance between the valve seat and the cam grows when hot and takes awhile to cool off, and the valves which cool quickly and expand less, become tighter when the head cools.

I don't have my manuals here to answer the second question about the specs. I would try tightening the head bolts and even resort to Bars leak to avoid taking the head off. But my guess is that may very well be what you have to do to fix it.

Good luck.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold