Stainless Works BBC Round Port 304SS Exhaust Flanges 2-1/2in Primaries (Raised Port Heads Only)Big Block Chevy round port header exhaust flanges. These flanges come as a pair and are designed for 2 1 2" primaries, also these are used only for raised port BBC heads, laser cut from heavy duty 3 8" thick 304 stainless steel for superior resistance to warping from high heat, and comes with a limited lifetime warranty.