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I was holding off responding hoping someone with Porsche specific experience could pinpoint the exact cause and correction.
I personally saw cranks with damage from faulty hardening years ago. Usually from chroming the bearing surfaces, but once in awhile nitriding failures. Gled's time frame roughly coincides with mine, although at the time my failures were seen in nascar engines.
The damage on this crank could very well be arc damage during the hardening process if I were to make a guess. Or some kind of contamination during the process. With the failures that I saw, the cranks looked normal when originally installed, but after running, the damage would occur sometime during use. I would assume this to be the case in this example as well.
Although nitriding has been around for a hundred years, it was only perfected in the last 20-30 years or so. Before then, some parts of the treatment might be hardened to a depth of 0.080" or deeper, while a little ways away the treatment might only be 0.0005" deep.
Remember, some of these cranks are now pushing 50 years old. There was a lot that we didn't even know that we didn't know.
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