Abstract: An indexing mechanism comprising a body (10) mounted so as to be rotatble about an axis; means (3) for rotating said body about said axis; first cam means (9, 10) for displacing said body along said axis away from an axial rest position during rotation of said body in a first direction; biassing means (11) for restoring the body to its axial rest position on completion of a rotation or a predetermined portion thereof of the body; means (8a) for detecting the axial movement of the body when it is being restored to its axial rest position; and second cam means (2, 9) permitting said body to be rotated in a second direction opposite the first direction.