Installation Kit, LS Acc. Drive Brackets, use with MIDDLE belt alignment (alignment common on F-body & GTO) Holley accessory drive installation kits provide proper spacing for Holley A/C, P/S & Alt accessory drive brackets, so belts ride at the perfect distance on your particular engine. A Fitment And Required Components Guide helps you locate and source components and provides other notes and helpful hints to figure your distance. Three easy steps will get you going. Step one is to determine your belt alignment based on your crank pulley position on the engine you have. Step two is to choose the Holley accessory drive brackets that you need (A/C, Alt & P/S or both). Step three is to see which Holley installation kit your set-up requires to line up your accessories with your crank pulley position. An application specific installation kit (part # 21-1, 21-2, or 21-3) is REQUIRED for most applications.