EMPI 21-7420 Cylinder Shim Set, 94mm, .020" / 0.50mm, Set of 4, Compatible with Volkswagen Air Cooled EnginesIf you want to optimize the performance of your air cooled engine, you need to adjust the deck height properly. One way to do that is by using cylinder shims, which are thin metal rings that fit between the cylinder and the case.Cylinder shims allow you to increase the compression ratio, correct the piston-to-valve clearance, and compensate for case machining or stroker cranks. EMPI 21-7420 is a set of four steel cylinder shims that are compatible with Volkswagen 94mm bore air cooled engines. They are .020" (0.50mm) thick and clearanced for standard 8mm studs. They are easy to install and do not require any piston modification. EMPI 21-7420 is a quality product that will restore the power and efficiency of your engine.