Abstract: A sprocket comprising teeth for engaging one or more chains is provided with datum connection portions which releasably support a datum body, such as a tensioned wire in a predefined position. The datum body provides a temporary reference or datum element or surface from which the position of the wear surfaces of the sprocket teeth may be determined by eye or by measurement, particularly in the radial direction. The teeth may further comprise a set of markings corresponding to angularly spaced reference planes intersecting at the sprocket axis. The markings are preferably arranged on the crown or radially outward surface of the teeth and are used to determine wear in the circumferential direction.