Last week I put new valve cover gaskets in the 383 in my 66 Polara. The old gaskets were originals and petrified. I went with the fel-pro high temperature rubber ones instead of cork. After putting them in all was well but after a few days I noticed the drivers side leaking again. I tightened the bolts and the leak was gone for a few days. Basically I have to retighten it every couple days. These gaskets when I got them were pretty stiff and not very pliable, more like a stiff hard rubber. Maybe it just needs a few heat cycles to conform? Has anybody else used these ones or should I stick to the cork ones? Thanks.