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Well, a lot of those header manifolds use a thin material where they bolt to the head. The original manifolds were cast iron and much thicker. On a stock MGB engine, the original cast iron manifold works very well and changing over to a header system has very little improvement in exhausting gasses, in fact hardly noticeable. If your engine is completely stock, you might be better off getting a stock manifold, their not that expensive and there's quite a few of them used out there, then put a stock exhaust system back on it and eliminate the problem. JMHO. PJ