At least one axial groove is formed on an output shaft which extends from an end of the output shaft to a stop ring groove and has a depth substantially equal to that of the stop ring groove to allow a stop ring fitted therein to be pushed up by a pin or the like inserted along the axial groove into the stop ring groove. A stop ring groove may be formed in a position on the output drive shaft adjacent to an end thereof, and an extension of a pinion drive shaft is provided which extends from an end face of the pinion drive shaft to form a cylindrical protective cover for a stopper fitted by the stop ring received in the groove and protruding from the end surface of the pinion drive shaft when the latter is retracted fully.
