We have a few sets of various thickness spacers.We usually use steel shim head gaskets between the block the spacers and aluminium or iron heads.The steel shim head gaskets seal great on the smooth surface of the spacers and don't dig into the aluminium head surface like a gasket with a fire ring.It works similar to a Cometic MLG.We used these on old race blocks from the era that were decked close to .250 to make them usable for low compression street use.