These Hooker RacingHeart headers are for Street Stock cars with Small Block Chevy engine. They fit 1973-1974 GM A-Body vehicles as well as 1970-1974 GM F-body vehicles. They have 1-3/4 inch primaries with 3-1/2 inch collectors, 3/8 inch thick flanges to prevent warping and have merge spears for increased velocity. These are natural stainless steel.