Abstract: An automatic washing apparatus designed to travel on a single I beam track that can be easily mounted in an existing self-service car wash bay. This apparatus is unique within itself since the trolley apparatus that supplies water and chemicals to the water boom manifold is driven entirely by a screw gear apparatus. The water boom manifold swing is also unique, being rotated by a single half spur gear that is rotated by striking a gear rack at either end of the I beam. An improved embodiment features an adjustable gear rack assembly at one end of the track which is adjusted to shorten the linear travel between end turns in response to signals from vehicle position indicating devices in order to efficiently wash smaller cars. The improved embodiment also includes a smoother vibrationless screw drive and a more rigid one-way clutch bearing support for the rotating members.