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Join Date: Aug 2001
Location: midwest
Posts: 24,939
For every time the cam rotates, the piston reaches top dead center twice:
-once on gas interchange(valve overlap means that both valves are being pushed open at the same time briefly, and the rockers may both be tight).
-and once on compression/ignition. This is when the valves are adjusted.

The engine has to be dead cold because the clearance that your setting is the distance the valve stem will expand when it gets hot so that the rocker and the valve will dance close together.
Too far apart and they will wear, make noise, and not let gasses in or out(power). Too close together and one of the valves stays open-letting combustion backfire through the intake or exhaust and burning the sealing seat of the valve.

Top dead center of the piston/crank/marking pully may not be "TDC" of the camshaft however if the chain is stretched. Because the valve is adjusted on the circular area of the cam(not the lobe) its usually not a problem, but valve timing way out of wack can lead to crashed valves in interference engines and definite loss of power.
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Old 01-30-2004, 03:41 PM
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