The valve train will tighten up as much as .002" going from cold to normal operating temp, which is why I always suggest setting lash fully hot. It's also why I don't make much sense out of the "hot or cold" in the Green Book specs.

I don't understand how seat pressure is affected by thermal expansion, as long as there is some lash. Surely you don't mean heating ate up anywhere near .018"?

There is a related problem with seat recession, though... once the seat starts receeding at all, you lose spring pressure. Reduced spring pressure accelerates the rate of recession (reduced heat transfer? Valve bounce?). You can lose all the lash in that vicious cycle.

This also seems to relate to wiped cam lobes -- I often see bad lobes and receeded seats match in any particular motor. Not sure end is the chicken and which is the egg.