Grade 5 vs. Grade 8:
So which one do you want to use? Generally you should always use Grade 8. The myth that Grade 5 will bend where Grade 8 will break because it is hardened is just that, a myth. A Grade 5 bolt will break well before the point where a Grade 8 will deform in tension or shear, doesn't matter. For reference and Metric 8.8 bolt is equivalent to Grade 5, while a Metric 10.9 bolt is equivalent to Grade 8.