BDC (Bottom Dead Centre)

BDC (bottom dead centre) is the point at which the piston is at the very bottom of its stroke, furthest from the cylinder head. There it momentarily stops and changes direction. Its opposite is TDC (top dead centre), the highest point.

The distance between TDC and BDC is the stroke. Together with the bore, it determines engine capacity. In a two-stroke, the transfer and exhaust ports are uncovered around BDC; in a four-stroke, the intake valve typically closes some degrees after BDC to fill the cylinder better at higher revs.

Valve and ignition timing are given in degrees before or after TDC or BDC. When working on an engine, TDC is found far more often than BDC, for example when setting valve clearances.