Alternator "pivot" bolt failure

I have had two bots fail in as many years. The first failure I blamed on re using a 40+ year old bolt that was not in the greatest condition to begin with. This said, its held in there for nearly 10 years. This second bolt (new) has only been in for a year. Both bolts broke right at the cylinder head. I am at a loss as to what is causing this. Everything is in alignment, there is no binding of the bolt when I install it. Anyone else experienced this?

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