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Strictly speaking, there is no such thing as a metric AN fastener. The AN series of aircraft fasteners are inch sizes not metric.
There presumably is such a thing as European metric aerospace fasteners, but I've not encountered them here.
Carrol Smith's main beef with non-AN fasteners was two-fold. First was quality control - the AN stuff is typically subject to much tighter quality standards, for obvious reasons - airplanes are held together with a surprisingly small number of pretty small bolts. Second is toughness - the basic AN hardware is tough rather than particularly hard, some SAE Grade 8 is so hard it is somewhat brittle. Grade 8 is nominally stronger and harder than a typical AN, but not as tough.
Roughly speaking (off the top of my head) a metric 8.8 is roughly equivalent to an SAE Grade 5, a metric 10.9 or 12.9 is roughly an SAE grade 8. The 8.8 is a "tough" grade somewhat similar to a typical AN fastener.
Another major problem has been counterfeit hardware - bolts that are marked as graded fasteners but do not meet the specs.
So I think that in practical terms, the closest you will come to a metric AN bolt is to buy brand-name 8.8 or in some cases 10.9 bolts from a reputable source. (And make sure never to buy anything with cut threads).
Most bolts on production automobiles are not stressed particularly highly, but a few are, so be careful!
Al Seim
Action Digital Race Data Systems
www.actdigital.com
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