Quote from hoolander2
Probably the two biggest mistakes one can make in adjusting valves are:
1. Not letting the motor cool completely before adjusting. This results in a false reading of the gap.
2. Adjusting the gap too small -- this can happen 2 ways. One can mis-judge the feeling of the feeler guage. Or, one gets the gap correct, but then when the lock nut is tightened, the gap changes. Therefore, it is always reccomended to re-check gap after tightening the lock nut.
If the gap is set too small the valves will "burn" or bend. They need to be in contact with their seats for a certain amount of time during the cycle in order to transfer heat to the head. If they don't, they stay too hot. Damage can be done instantly!
Too tight can also damage cam, rocker arm, valve, and more, by putting too much pressure on the entire mechanism.
Oh, and by the way, too large of a gap is not so serious. It may only result in a slight loss of power. If it's way too large, this is when tapping or valve "ringing" is heard. Not terribly serious, but over time can lead to some damage.
It's always better to have an error too large than too small.
