Abstract: A motorized reduction gear unit for any purpose having a crank and connecting rod mechanism incorporated in it, but especially for use as a vehicle screen wiper drive unit, comprises a hollow casing having a cover plate, with, within the hollow interior of the casing, a toothed wheel which is rotatable about an axis. The toothed wheel carries a gudgeon pin parallel to the axis of rotation of the toothed wheel but eccentric with respect to the latter. The pin drives a crank and connecting rod system in which the crank and connecting rod are articulated together about a pivot pin, so that continuous rotary motion of the toothed wheel is converted thereby into alternating rotary motion of an output shaft. It further includes a peripheral gasket disposed between the upper edge of the casing and the cover plate, together with a friction plate element in the form of a thickened friction portion disposed fixedly on the inner surface of the cover plate facing towards the casing.