The everyday phrase 'dead centre' originates in engineering, where it means "any position of a crank where the applied force is straight along its axis, meaning no turning force can be applied."
In a piston engine, there are two dead centres: top dead centre (TDC), where the piston is furthest from the crankshaft, and bottom dead centre (BDC), where it's closest to the crankshaft.
When an engine has more than one cylinder, the connecting rods are arranged so that the dead centre for each cylinder occurs out of phase with the other(s).
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