Stanley JFP59800 Proto Master Serpentine Belt Tool Spring loaded idler pulley releases tension on serpentine belts. Use to remove and install belts on serpentine belt systems. Fits idler pulleys requiring 13mm, 14mm, 15mm, 16mm, 18mm, 19mm, hex and both 3/8-inch and 1/2-inch square drive sizes. For applications with limited access; 13mm and 16mm and 19mm sockets are built into the handle. Covers most domestic and some foreign applications. Comes with extension for hard to reach idler pulleys. Eight low profile sockets and extensions help to fit into tight places.