These are Flathead Power, Big Twin cylinders.. Notice how the cooling finn’s go all the way to the barrel flange? This is for strength.. they really don’t do much for cooling. These are the preferred cylinders to use when building a hot rod big twin. Too bad Flathead Power sold out to S&S. S&S hasn’t made any since the purchase. Now these cylinders are “unobtainium”. Breaks my flat headed heart.