New Gaskets and New Gas are the problem
This is a problem that has shown up here in the States over the last ten years with Old Chevy and GMC in line engines. One of the problems is the " new gaskets intake exhaust" are asbestos free and they are of a one size fits all construction mean that they fit the newer engines which had larger intake and exhaust ports and manifolds the result being that instead of having a quarter inch of gasket to manifold match instead of about an eight of an inch contact. The exhaust and even the intake manifold are also being subjected to far higher temperature with the new gas which burns hotter and leaner. Once the warp has established in the exhaust manifold tightening the clamps just results in the eventual failure several people including myself have tried adding extra gaskets to the center ports two even three gaskets to make up the warp-age. One of my CMC friends had had the intake and exhaust manifolds milled to restore alignment. I’ve milled the manifolds as a pair and put on hand cut asbestos manifold gaskets. All of the attempts so far been relatively short lived couple of thousand miles, personally I believe it is higher temperature range that is the problem.
I have found only one real fix for exhaust intake leaks that was to replace the exhaust manifold with a two piece hot rod exhaust manifold. This has worked for better than 5 years now on my C60.
But a question how do you know it is a vacuum leak and not a burned valve?
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