What does 'TDC' stand for in engine timing?

In engine timing, TDC stands for Top Dead Center.

Top Dead Center is the position at which a piston is farthest from the crankshaft at the top of its stroke. It is a reference point for describing crankshaft position and ignition or injection timing. For example, a spark may be specified as occurring a certain number of degrees before TDC, written BTDC, or after TDC, written ATDC.

A piston reaches TDC twice during a normal four-stroke cycle: once at the end of the compression stroke and again at the end of the exhaust stroke. Those positions are not functionally identical because the engine is in a different part of its valve and combustion sequence. Mechanics therefore identify the relevant cylinder and stroke when setting timing.

The opposite position is Bottom Dead Center, or BDC, where the piston is nearest the crankshaft. TDC marks may appear on a crankshaft pulley, flywheel, or harmonic balancer. In multi-cylinder engines, more than one piston can reach TDC at the same time, depending on the engine layout and firing order.