Pulley alignment is very important, especially when the engine is run at 6000rpm and above. These shims are designed to properly align the pulleys so the belts will stay put. They are installed between the water pump and water pump pulley. Made from aluminum sheet stock, each kit contains two 1/16 inch and one 1/8 inch shim. They fit Ford and GM with 3/4 inch or 5/8 inch center shaft.