just visualize that the springs are always pulling the valve stem toward the cam. When the cam is not touching the valve stem, the only thing stopping the valve is the seat in the head itself. As this wears down (and the valve wears too) the valve stops deeper and deeper in the head. Eventually(if not adjusted) it will touch the cam all the time, not closing completely, and then it's valve burnin' time.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.