Small screw for used to attach the rotor valve stop plate (aka cork stop) to the valve housing. The stop plate is the part that holds the two rubber or cork bumpers. Two screws are required for a complete installation. Used on numerous Benge, Conn, & King horns, trombones and tubas, plus two specific Bach models (A47BO/MLR trombone and B1112 horn only). Sold as a single screw.
NOTE: Not for use on Bach rotary valves other than the A47BO (A47MLR) trombone and B1112 horn. -cdp