I had a the same problem on the exhaust manifold flange on my 'B. Of the
four holes, I was only able to drill and tap two of them. After trying all
kinds of drill bits, having the flange annealed, and visiting a few machine
shops, I finally took it to a shop where they punched the holes out with a
plasma cutter and welded the studs in place. Apparently, the high nickel
content of the manifold makes the weld incredibly hard in spots, and almost
nothing will cut it. I hope you have better luck.
Calvin K.
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