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The real danger is an engine with the valves set in ' the twilight zone'...
At least half the cooling to a valve comes from being in contact with the head...
that is why the minimum valve ' margin' is specified by the factory ... the length of the seating surface on the valve rim... and the valve setting specs are enough so that a valve which is getting longer due to heat still firmly contacts the head when running...
The worst situation is a valve setting which will allow the car to run... and keep running... but allow heat to build up in that valve due to lack of strong contact with the head to convey ( through convection ) enough heat to keep from burning the valve...
For those who can not wait until their engine is cold in mid winter.... at least let it cool over night and set it cold in the morning.
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