When in doubt, Grade 8.

You can practically design for any fastener you want, just be sure your parts match up with your assumptions and analysis. Or if in doubt, Grade 8.

My Heidts IFS/IRS uses some stainless bolts and nuts which are no better than grade 3 probably. Thing is, they are larger than normal and stress is proportional to area or diameter - squared .............. Oh yeah, and they won't rot out for a long time.

One should never design a fastener to operate beyond its yield point so elongation is usually not as important as strength; = Grade 8.

Kitz