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Ok, so I am assuming that since the bolt heads in queation do have grade 8 markings that you are using SAE fasteners. Can you confirm if the factory bolts were SAE or metric? I can only speculate that putting a standard shank in a metric hole is leaving just enough slop to cause small movements of the engine, which is putting a lateral loading on the bolt shaft. High grade fasteners are very weak to this type of strain, and it could be a contributing factor. Like I said, just speculation at this point since I am lacking some information.
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