M10 and M12 are the bolt size (width), not thread. You are correct in guessing the 10.9 and 8.8 are hardness; 10.9 is comparable to Grade 8, 8.8 is comparable to Grade 5.

I believe (but not positive) that a 17mm hex head corresponds to M12.

If possible, you could go to a nearby hardware store to check out their metric bolt sizer.

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1990 190e 2.6 - The only one I can really call "My Car".

1987 190e 2.3 - The donor car's up and running, only mods are Euros and a Sony headunit. My Dad's runabout now.

1990 300e 2.6 - The parents' ride.