An under travel actuator, particularly intended for an automobile clutch, including an engine (12) for the translation movement of a mobile gear (14) acting on a push stem (16) for the opening and closing of the clutch, the mobile gear including screw-nut means (22, 24) cooperating with two combined means of unidirectional driving in order to regulate the length of the push stem (16) according to the wear of the clutch surfaces subject to friction.