top dead center

Top dead center, or TDC, is the position where a piston reaches its maximum height inside a cylinder before reversing direction. At this instant, the piston momentarily stops; the connecting rod and crankshaft are momentarily aligned vertically. For a four-stroke engine, TDC occurs twice per cycle: once at the end of the compression stroke (the critical point for ignition timing) and again at the end of the exhaust stroke.

Identifying TDC requires a reference mark on the crankshaft or flywheel, etched or cast into the rotating mass. The engine block or cylinder head carries a corresponding stationary pointer or witness mark. When these two marks align, you are at TDC. Some engines use degree markings around the circumference of the flywheel, graduated in increments of 5 or 10 degrees before and after TDC, allowing technicians to set ignition advance or valve timing with precision.

TDC matters because ignition timing is defined relative to it. Spark plugs fire at a set number of degrees before TDC on the power stroke, typically 10 to 40 degrees depending on engine design and load. Similarly, inlet and exhaust valves open and close at specific points relative to TDC. Mistiming these events reduces power, increases emissions, and risks engine damage. Piston-to-valve clearance is also checked at TDC on the compression stroke, since this is the point of maximum valve lift and minimum clearance.

Mechanics locate TDC by rotating the engine by hand or with a bump starter while watching the pointer and flywheel mark. On port-injected or carbureted engines, TDC is the only reliable reference for setting ignition timing with a timing light. A timing light strobe is fired by the number-one cylinder spark plug or ignition coil; when the strobe frequency matches engine speed, the marks appear stationary, revealing whether ignition is leading or lagging the desired position.

The name reflects physical reality: the piston is at the top of its travel, and the crankshaft is at a dead point, meaning zero velocity. The opposite position, when the piston is at the bottom of the cylinder, is bottom dead center or BDC. Neither position produces motion; all power and mechanical advantage develop between them.