All metric nuts and bolts over 6MM are supposed to have a grade marking. Never use a nut or bolt that does not have these markings.
Quick tutorial on bolts:
Specbolt Fasteners Technical Info...learn about motorcycle bolts
All you need to know about metric nuts:
US Grade 2 = approx metric property class 5.8
US Grade 5 = approx metric property class 8.8
US Grade 8= approx metric property class 10.9
US Grade (?) 8+ = approx metric property class 12.9
In general, if a bolt is not marked AT LEAST 8.8, I wouldn't use it.
Bob B