Big Block Chevrolet Square Port Exhaust Manifold GasketsHTX-900 is a composite gasket structure consisting of graphite-coated, high-temperature facing material chemically and mechanically fused to an expanded steel core. It is intended for exhaust manifold, header, collector, and EGR system gaskets, as well as other industrial sealing applications that require high strength, thermal integrity, and anti-stick performance. Port Size: 1-78" SQ.Material: HTX-900Quantity: Sold in Pairs