All tubes are CNC mandrel bent for optimal flow for the J-bends in this header builder kit. Made from 304 stainless steel for maximum corrosion resistance. The diameter of tubes is 1-5/8" with .065 wall thickness. This kit includes (2) 3/8" thick CNC laser cut flanges, of your choice. Also comes with (12) J-bends, (2) formed collectors and (2) filler diamonds to connect the tubes together. We recommend the use of 308L SS welding rod. These kits are proudly made in the USA.