Just to elaborate a little on what Panic has said, since it is critical to both valve adjustment and ignition timing. We are talking about a four stroke motor, and the strokes are, Intake, Compression, Power, and Exhaust. The piston will be at Top Dead Center(TDC) twice, at the end of Compression, and Exhaust strokes. Both of these strokes are pushing the volume, one against a closed intake valve, compression, and the other to an open exhaust valve, exhaust. This is where some confusion occurs when re-installing a distributor. Since the cam turns at half the speed of the crank. The distributor can be at either TDC, so the valve action needs to be watched. Hope I didn't confuse things more.
Larry
