Dayco® No Slack® Idler/Tensioner Pulleys are built with smoother surfaces and tighter dimensional tolerances, which translates to less vibration and therefore longer belt life. Lifetime lubricated ball bearings and double high temperature seals assure peak bearing performance, the most critical element of a pulley's service life. This specific "flat", serpentine belt driven pulley contains specially formulated glass-filled polymers to insure a high strength, cooler running, wear resistant "plastic" pulley.