Ford Small Block, Block Hugger All tubes are CNC mandrel bent for optimal flow. Made from 304 stainless steel for maximum corrosion resistance. Starting with a CNC cut 3/8″ thick flanges into 1-5/8″ primaries with 2-1/2″ slip-fit collectors. Headers are fully TIG welded and back purged. Mill finish, These, are TIG welded for strength and durability. These kits come with a lifetime warranty and proudly made in the USA. (Flange: HFSBF1).