Dead Centre (TDC and BDC)

The piston moves up and down and momentarily stops at each end of its travel. The highest point is top dead centre (TDC) and the lowest is bottom dead centre (BDC). The distance between them is the stroke.

TDC is the reference point for engine timing: ignition advance is given in degrees before TDC, and cam timing is checked with the piston at TDC on the compression stroke, when both valves are closed.

When checking valve clearances, find TDC using the timing marks on the flywheel or alternator rotor, as described in the service manual. On a four-stroke the piston passes TDC twice per cycle, so make sure you are on the compression stroke.