Abstract: A tool fittable to a spindle of a machine for assembling motor reducers; the machine assembling a pinion onto a shaft of the motor reducer by exerting compression, force from the spindle, to force the pinion onto the shaft and form an interference fit. The tool has a tubular body with a cavity for housing the pinion; and a number of lock devices housed inside the tubular body, each having a movable stop member which projects inside the cavity and against which rests a shoulder of the pinion to directly transmit the compression exerted by the spindle to the pinion downstream from the shoulder which separates the toothed end portion of the pinion from the rest of the pinion.