TENSIONER AUTO/LT TRUCK, DAYCO. Dayco No Slack Automatic Belt Tensioners are built to outperform and outlast competitive OE parts. This specific Dayco Automatic Belt Tensioner is manufactured with a specially coated patented flat wire spring to allow the internal tension to be concentric, thus providing a more even degree of constant tension. The heavy duty cast aluminum spring case resists cracking and fatigue. Each component is built to resist rust, endure the toughest of conditions while promoting quieter performance and longer life. .