Square Port header gaskets for Mark IV big block Chevrolet engines, built using a high-temperature expanded-steel core for stability, torque retention, and long-term durability under extreme exhaust heat. The graphite-impregnated facing conforms to minor flange irregularities, transfers heat efficiently away from the cylinder head, and prevents sticking during removal. The high-recovery design helps maintain a seal even if the header flange experiences clamp-load loss. Can be trimmed for modified port shapes when needed. Suitable for OE replacement, performance street use, drag racing, and oval-track applications.