ACDelco 39155 Professional Automatic Belt Tensioner and Pulley Assembly Ensures proper belt tension, alignment, and damping for superior performance Patented vibration damping system increases tensioner, belt, and accessory component life for long lasting use Torsional round spring design provides consistent tension and does not allow contamination for enhanced durability Provides full line coverage for domestic, import, and heavy duty applications Professional, premium aftermarket replacement Provides the performance and dependability you expect from ACDelco Manufactured to meet expectations for fit, form, and function