Oregon Performance Transmission carries a large selection of servo pistons for many makes and models of automatic transmissions. From stock replacement, molded rubber, to high performance billet aluminum, we have the servo you need for your application. In many cases a servo upgrade can make a very large improvement in your transmission's overall performance for a relatively small cost. The function of a servo in an automatic transmission is to apply the band or bands. For HD or HP uses, upgrading the servo gives the band the necessary clamping power to keep from slipping under heavy loads.