The head gasket is sandwiched between the Cylinder Head and the Block to seal combustion within the combusion chamber. This gasket is not to be reused after a rebuild, and should be replaced any time the head is removed. Most performance head gaskets on the market today are composed of mulitple layers of steel (MLS) material. You can either select a gasket that matches your bore size, or it is also common practice to go with a gasket that has a bore diameter 0.5mm larger than your piston. A gasket that with a thickness that is larger than stock will lower the compression ratio advertised for your pistons, and a gasket with a thickness thinner than stock will raise the compression ratio advertised for your pistons.