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The timing of the distribution will determine the relative positioning of the camshaft! crankshaft. The timing will be characterized by the angle that the opening or closing point of a valve, located on the plane of rotation of the crankshaft, makes with the high or low position of a crankpin of the latter. This position determines the top dead center (TDC) or the bottom dead center (BDC), depending on whether the piston is at the top of its stroke or at the bottom. The valves are generally closed by a return spring.