I suggest you see if the tightening of the valve cover nut is really doing anything. In my case I found that the underside of the casting where the bolt goes through, was making contact with the head, thereby not allowing the gasket to sqooooooosh to the point of a seal.

I cut about 1/4" to 3/8" off the underside of the each casting point where the bolt goes through and now it seals really nice.

Don

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it