TDC (top dead centre)

TDC, top dead centre, is the point where the piston is at its very top in the cylinder; it is the reference for ignition and valve timing.

Top dead centre (TDC) is the moment when the piston reaches its highest point in the cylinder and stops before moving back down. The opposite point is bottom dead centre (BDC).

TDC is the reference for engine timing. Ignition is timed in degrees before TDC, and in a four-stroke engine the piston passes TDC twice per cycle: at the end of compression and at the end of exhaust.

For valve clearance checks you usually set the cylinder to TDC on the compression stroke, when both valves are closed. Timing marks on the crankshaft or alternator rotor, visible through an inspection plug, and marks on the cam sprockets show the correct position. Always turn the engine by hand in its normal direction of rotation.

Related terms

Valve clearance Four-Stroke Engine Ignition Timing

← All terms