Top dead center, often abbreviated TDC, is the condition when a piston is at the apex of its travel in the cylinder. The air chamber in the cylinder is at it's lowest volume. The piston head, connecting rod and crankshaft journal where the piston rod connects are all in a straight line thru the center line of the cylinder.

Top dead center must be accurately located when adjusting the timing on an engine (usually expressed in degrees before or past top dead center - this figure is when the spark plug fires, causing the compressed gas/air mix to explode). There are usually marks on the crankshaft pulley and housing for a good reference as to when you should be close to top dead center on a particular cylinder. A timing light is necessary to insure the timing is properly adjusted.

This is only applicable to piston powered internal combustion engines. Rotary and jet engines don't have pistons.

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