{{{If the valves are too tight (a common mistake; people tighten
the valves until they stop clattering) the exhaust valves don't
close long enough to cool off (remember they spend much of their
time in the direct aftermath of the exploding fuel)}}}

Myth.

If you do a cam timing check with a dial indicator and degree wheel you will see that closing the clearance by 1/3 from .018 to .012 increases time off seat by about 5-15 deg out of 720 (4stroke engine) depending on the brand of cam you use.
In fact, if you can hear the clatter distinctly you will have excessive rocker tip and possibly lifter wear.

Ian

http://www.geocities.com/iadr1/geocitiesintro.html