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Grade 5 or Grade 8 bolts?

I tow my classic Mini with a tow bar occasionally. The receiver for the tow bar is attached to the frame by 1 bolt each side. Last summer while towing the car through a small town the grade 8 bolt broke and one side of the tow bar separated from the car. Fortunately I was traveling slowly at the time and there was no real damage to either vehicle. We were close to our destination so drove both the tow vehicle and the Mini there. While at Watkins Glenn racetrack I was advised by a few people in the pits that I would be better off using grade 5 bolts because even though they were not as strong that they were more flexible. When I went to my local bolt and nut specialist ( a place called “nuts” who sell only nuts and bolts ) I asked them their opinion about which grade would be best the owner just shrugged his shoulders. I did replace my securing bolts with grade 5 bolts and plan on replacing them on a regular basis, but still am uneasy with replacing grade 8 with grade 5. Any opinions?

Grade 8 bolts are great for tensile strength where pulling or compressing forces are applied, But are brittle under shearing forces. Grade 5 bolts are great for shearing forces, but are lousy under pulling or compressing forces because they can stretch or compress.

If you’re concerned about shearing forces, then I would use a grade 5.


I don’t know how you have this laid out but there’s also the possibility of increasing the hole size and stepping up the bolt diameter. An 1/8 or 1/4 of an inch in diameter may not sound like much but can make a huge difference in the failure point.

Also, you want bolts with smooth shoulders, no threads all the way to the head of the bolt. You don’t want any threads in the two pieces being joined, just enough to secure the nuts. I would use lock-nuts…

Thank you all. I have increased the size of the bolts to as big as I can fit, there is limited room on the receiver side for a bigger nut. I am concerned about both the pulling and shearing forces. I tow the car behind my motorhome which has a long overhang. When I make anything approaching a sharp corner it pushes the towed car sideways. I think that I will stay with the now installed grade 5 bolts, watch them closely and change them on a regular basis. The bolt that failed was probably part of the set up for 5 years or more. I have also deceided to keep a spare set with me. It would be nice to have more than 1 bolt each receiver, but not possible. Again, thanks.

My homemade tow bar has a couple of safety chains on it and if yours doesn’t then maybe the addition of chains would be a good idea; just in case. (?)

I don’t think you should be using bolts at all. You should be using some type of pin that is held in place with a clevis instead. The pin should just match the hole size. Also stainless steel would give you the best of both worlds, sheer strength and tensil strength.

All of the above are good answers. Let’s throw hydrogen embrittlement and corrosion cracking into the mix as more possibilities.

The sharp corners pushing the towed car sideways could also be a concern and could lead to flexing the bolts. Bend a paper clip several times and it will also give up.

I’ve heard (but never personally experienced) hydrogen embrittlement problems. Some guys in the antique motorcycle world have reportedly had parts fail after they were zinc plated. Zinc was often done to mimic Cadmium, which is a process used on the parts originally. Cadmium is also much more expensive to have done, assuming one can find a plater to do it.

Cadmium is toxic. It is recognizable from its bluish tint.

Grades of bolts are sometimes counterfeited. You may have had the fake ones to start with. You may just get the real ones at the hardware store this time. If the towbar was made in China, Central or South America, I wouldn’t be too confident that you got the grade of bolts that were specified by the markings on them. Real grade eight bolts may actually be best.

Tester, I don’t recall learning anything like what you said about shear vs tensile brittleness back in my Heat Treatment of Steels and Mechanics of Materials classes in college. I’d be interested in studying any references you have for that.

One bolt on each rail seems unrealistic to me.

Again, thank you all. I tow with safety chains which are each rated for more than my 1400 pound vehicle, wouldn’t tow without chains. Without significant modification I cannot use more than one bolt on each side. I am currently using bolts from a local specialty shop that sells just nuts and bolts, they assure me that they are american made.

Always replace bolts with the same kind or better. Shear and elasticity are really not understood by non engineers. On a tow application if you break a grade 8 replace it with a grade 10. No question. Everything else is just nonsense.