ACDelco 38157 Professional Automatic Belt Tensioner and Pulley Assembly Labyrinth seal stops contamination of internal parts for maximum durability and service life Machined steel pulley reduces surface wear while shielding the internal bearing from outside contaminants Double row bearings minimize friction for reduced bearing wear and noise Round spring wire includes a chrome-silicone torsion spring for less flex-fatigue than flat-wire designs Patented damping mechanism provides maximum stability, reducing vibration and increasing the life of belt, tensioner, and other accessory drive components