If an Overhead Cam Cylinder Head has been excessively machined the distance between the Cam and the Crank centerline is too small. This will affect the valve timing and the engine will not run properly. There is also a very good chance that the engine will not pass emissions tests. SBI Head Saver Shims are designed to replace this lost height and restore the centerline distance. These Shims are 0.020" thick and are made from steel. They should be installed on the block surface directly under the Head Gasket.