Definition
The position of a piston in a reciprocating engine when it is at the very end of its travel within the cylinder, where the connecting rod and crankshaft throw are aligned in a straight line. There are two dead-center positions for each cylinder: top dead center (TDC), where the piston is at its highest point closest to the cylinder head, and bottom dead center (BDC), where the piston is at its lowest point furthest from the cylinder head. At either dead-center position, the piston is momentarily not moving up or down as it reverses direction.
Plain English
The point at the top or bottom of a piston's stroke where it briefly stops before reversing direction. At these two points, the piston, connecting rod, and crankshaft are all lined up straight.
Context Anchor
Seen in piston-engine operation, engine timing, and maintenance discussions involving piston position.
Derivation
Dead' here means 'motionless' or 'without movement' — the same sense used in 'dead stop.' At top or bottom dead center the piston is momentarily not moving, even though the crankshaft is still rotating. 'Center' refers to the alignment of the rod and crank throw passing through the centerline of the crankshaft.
Why Pilots Care
Correct identification of dead-center position is required for accurate ignition timing and to avoid engine damage during startup.
Analogy
It is like a bicycle pedal when it is straight up or straight down. At that exact spot, pushing on the pedal gives very little turning effect until it moves past that point.
Intuition Check
Do not read “dead-center” as simply “exactly in the middle.” In an engine, dead-center position means the piston is at the very end of its travel, either fully up or fully down.
Example Sentence 1
The magnetos are timed to fire 25 degrees before the piston reaches top dead-center position on the compression stroke.
Example Sentence 2
At dead-center position the mechanic confirmed valve clearances with no crankshaft movement possible from piston pressure.