These unique 4-layer head gaskets are designed to interact with the mating layer in the stack. The 2 outer layers are designed with a reverse embossment to interact with the inner layers. Once compressed, the outer layers are forced into the bore-stop layer. This provides a superb combustion seal, even with high-compression applications. Kit for Evolution Big Twin contains 2 multi layer steel head gaskets and 2 silicon beaded metal base gaskets. Kits for Twin Cam contain 2 multi layer steel head gaskets and the cylinder base O-rings. Each gasket is coated on both sides and embossed around each oil passage, thus no additional sealers are required.