Abstract: An impact wrench (1) comprising an output shaft (5) for receiving a socket or other fastener engaging element for tightening a fastener is powered by a pneumatic motor (11). Apparatus (20) for determining when the fastener has been tightened to a predetermined tightness comprises a body member (21) extending around the output shaft (5), and an eccentrically disposed member (24) on the output shaft (5) located within a central bore (30) of the body member (21). A pair of photo sensors (33a, 33b) in light accommodating bores (34a, 34b) receive light through the bores (34a, 34b) from corresponding light emitting diodes (36a, 36b).