This multi-ribbed belt tensioner maintains the correct tension on the engine accessory drive belt. It is mounted on the front of the engine, where it applies a constant force to the belt span between the crankshaft pulley and the alternator or fan drive. The tensioner arm pivots on a sealed bearing; a spring inside the housing provides the preload that compensates for belt stretch and thermal expansion. A worn tensioner allows belt slip, which reduces alternator output and coolant pump speed, and can cause belt squeal or rapid wear. The manufacturer machines the roller surface and bearing bore to the same tolerances as the original assembly, so the belt tracks correctly without additional shimming. On reinstall, ensure the tensioner arm is fully released before routing the belt, then rotate the arm to its locked position.