Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
In part, it depends on the way the joint is designed. A lot of guys will attach a tow bar using a bolt as a pivot pin. Instead of being tightened down, it's double-nutted or a Nyloc nut is used just to keep it from falling out. That's a bad design, using a bolt in a way it's not designed for. A clevis or hitch pin should be used there, but if you do use a bolt that way, a softer grade 3 is a better choice.
If you're doing it right, where the bolt gets torqued down and only provides a clamping force, grade 8 is OK, but probably overkill, and grade 5 might actually be better.
With cyclic loads, if the bolt can see some beam load, softer bolts can actually last longer because they're more fatigue resistant.