the LFP Alternator Puley is used with aftermarket Crank Pulley Kits to reduce alternator revolutions. As you increase the diameter of your Crank Puley, your alternator puley will have more revolutions. Over time, the increased revolutions may cause your alternator to over spin prevent that before it starts by adding an oversized LFP Aluminum alternator Pulley. Alternator Pulleys come complete with installantion spacer. Most cases will require use of a slightly longer belt, usually no longer than .5" Required when upgrading to a 4# lower crank pulley.