First, a grade 8.8 IS a high strength cap screw. I have searched far and wide for stronger cap screws or any kind of screws and they are hard to find. I know of two possible sources and both are somewhat limited. One is the screws used by Greenlee for their chassis punches. Oddly, the smaller sized punches need to have the strongest drive screws and a grade 8 bolt can just simply break under the stress. They either make them themselves or have them made to their specs. They are significantly better than grade 8 but I can not tell you exactly how much because they do not publish any specs on them. I suspect 150 to 200%.
The other source that I found was from the custom auto industry. There are companies that make replacement bolts that also are significantly better than grade 8. Here is one such source:
I do not think that either of these sources has any left hand bolts. I would drill the hole out and tap it for a larger, English thread, left hand, grade 8 cap screw and be done with it. McMaster has left hand cap screws in English (inch) sizes and you can buy them in quantity of one.
Since you said this was an import compressor, I would strongly suspect that either the screw was defective or it was installed with too much torque. A replacement of the same size and grade would probably work just fine.
Make it fit.
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