Top Dead Centre/Bottom Dead Centre 

TDCTop Dead Centre is traditionally the position of an internal combustion engine’s piston when it is at the very top of its stroke.

BDCBottom Dead Centre is the opposite, when the piston is at the very bottom of its stroke.

In non-destructive testing, top dead centre is used as a datum for the location of indications.

Top dead centre is the position at the top of a horizontal pipe or vessel, on the centre line running vertically through the pipe or vessel, as shown in the figure.

Bottom dead centre is located 180° from top dead centre.

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