Windsor valve cover rails see plenty of heat cycles and the flanges are rarely dead flat by the time a second set of covers goes on. A cork and rubber gasket with a latex skin fills what is left, then swells slightly once hot oil reaches it, so the seal tightens as the engine comes up to temperature.

Mr. Gasket 5861 Ultra-Seal Valve Cover Gaskets Features:

• Fits 1962-1995 Ford 221, 260, 289, 302 and 351W Small Block engines.
• Manufactured from a high density cork and rubber blend.
• Black latex coat applied to the outside helps seal minor flange irregularities and eliminate oil leakage.
• Creates a positive seal by controlled swelling of the gasket material when exposed to hot oil.