do you think grade 5 bolts will be ok for a class 4 hitch?
i got 5/8" dia bolts in grade 5 and by my calculations, since they are rated at minimum tensile strength of 120,000 psi and proofed at 85,000 psi, the area of 8 bolts gives me an approximate capacity (straight down) of 208,600 lbs.

since shear strength is usually 60% of tensile, that means 125,160 lbs (straight back)
which is the same as accelerating a 12,500 lb trailer at 1G

i just wanted to check with you guys if this should be ok...
i probably wont be towing anything over 7,000 lbs anyway, but the hitch was $50 on craigslist so i bought it

 

Why not just get grade 8 and not sweet it? Torque to proper #.

 

For your use, grade 5 will be fine. If going to max rating of receiver, I would swap at least the rearmost bolts to grade 8.

Keep in mind that during a failure mode, the load is not distributed equally among all 8 fasteners, so it is not correct to conclude no failure is possible until 200+K pounds. There are also some other things going on as a result of the clamping forces between the receiver and frame, but bottom line, go back to my first sentence!

 

I think there is something wrong with this math, but the truth is that I have 16,000 lb pintle hook mounted on 4 size 5/8 bolts. They are grade 8, but again, we are talking much bigger load with less bolts.
I still would use grade 8 bolts as they wear out and corrode less than softer one.

 

what kind of torque should i use to torque them down?
i was figuring somewhere around 80 ft-lbs would be fine but im open to suggestions.
i should mention that i got grade 5 bolts, but grade 8 all metal lock nuts, and grade 8 washers (on both sides of course)

the guys at the fastener store convinced me that grade 5 would be better.
while grade 8 has an overall higher strength it can actually fracture at lower stress than grade 5.

the way metals are rated is this:
the 120 kPsi i quoted is the yeild strength, meaning that is the stress that the bolt takes without plastic deformation. at 121 kPsi the bolt will start to stretch permanently and not go back to its original size. however it still hasnt broken yet. it will keep taking more stress until the bolt has elongated by almost 1/2" for a bolt this size. at this point the cross section is reduced to the point where the bolt just snaps. but it usually takes about 2x the yeild stress Psi before this happens.

with grade 8, the yeild point is higher, but the ultimate strength is about the same, meaning that you get much less warning due to the bolt being made from a harder material.

not only that but over time grade 5 will hold up better because grade 8 will tend to harden with more stress over time.

the last reason was that they didnt have the right length in 8 and i didnt want to have to spin those nuts all the way down the extra 1/2" of fastener if i didnt have to.

i think for my application, im fine.
and my math is good as far as i know, but im human, so if i messed up, tell me.
the only thing i didnt consider i guess was the minor diameter of the bolts instead of the nominal size of the thread. the minor diameter of a 5/8 bolt will be about 1/16 or so smaller than 5/8"

 

got it up. looks good. now to find something to tow lol.