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Found this information about Dilavar:
Dilavar (Ni 13 (Nb NiMnCrV X 68 12 5) - Material No. 1.3937)
It is a nickel alloy.
I also found this text on the Lnengineering website:
On 911 models, certain engines came fitted with Dilivar head studs, which were designed to match the expansion of the cylinders, but also suffered from hydrogen embrittlement, which is a condition that results from the accumulation of hydrogen gas in the atomic structure of the metal.
If hydrogen embrittlement is the cause, and not SCC, it can explain that the bolts fail at the engine stand. In my previous job the same thing happened with new bolts on oil equipment, even before it left the factory.
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