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I don't know if you're all aware of this but G.M. used only silicon on their valve covers in the early 80's. They used a black silicon that was very durable and the valve covers didn't leak a drop.
It's a bit of a hassle using silicon but the results are spectacular. You must clean the head and valve cover with a gasoline soaked rag until every trace of oil is gone. Squirt a 3/16" bead of oil resistant silicon on the head (any of the Permatex "Ultras" will do), smear a 1/32" coating on the valve cover, then carefully mush the valve cover in place and snug the bolts up.
Let the silicon cure for several hours before starting your engine and you'll have leak-free valve covers forever. It takes about one $6 tube to do both valve covers so the cost is actually less than a set of even the cheap cork valve cover gaskets.
It's a bit of a hassle using silicon but the results are spectacular. You must clean the head and valve cover with a gasoline soaked rag until every trace of oil is gone. Squirt a 3/16" bead of oil resistant silicon on the head (any of the Permatex "Ultras" will do), smear a 1/32" coating on the valve cover, then carefully mush the valve cover in place and snug the bolts up.
Let the silicon cure for several hours before starting your engine and you'll have leak-free valve covers forever. It takes about one $6 tube to do both valve covers so the cost is actually less than a set of even the cheap cork valve cover gaskets.