Header flanges warp, and once they do a composite gasket will blow out again within a few heat cycles. Dead soft aluminum takes the shape of whatever it is clamped between and stays there, which is why these can come off with the headers and go back on again.

Mr. Gasket 7401G Aluminum-Layered Header Gaskets Features:

• Fits 1955-1991 Chevrolet 262-400 Gen I Small Block Chevy engines with square ports measuring 1.45 in. x 1.48 in.
• Manufactured from dead soft aluminum layers, 0.125 in. thick.
• Compresses down to 0.060 in. to create a tight leak-proof seal.
• Conforms easily to surface irregularities such as warped flanges.
• Reusable header, exhaust manifold and collector flange gaskets.
• Withstands intense exhaust heat and resists deteriorating.
• For high performance street, drag race and oval track use.