Features and Benefits: Releases tension on serpentine belts with a spring loaded idler pulley Used to remove and install serpentine belts Fits idler pulleys requiring 13mm 15mm 16mm and 18mm hex or 3/8 square drive socket sizes. Removes and installs serpentine belts. Releases tension on serpentine belts with spring loaded idler pulleys requiring a 15 16 and 18 mm hex or 3/8 in. square drive. Provides extra coverage for most applications.