by Bogo » Fri Jan 25, 2013 10:44 am
If you need them, carriage bolts are available in grade 8 strength. Some farm implements use them. BTW, it isn't that hard to square a round hole in a piece of steel. Use a square file, and the job is a bit easier. There are also plow bolts which go into a squared or keyed bevelled hole. I'm not sure how you'd make the bevelled hole on the inside of a tube.... The strongest grade of T-slot bolts would also work, you want 150,000 psi or stronger strength. You would need to file your initial round hole into a slot to use them.
Wow, Fastenall only has one grade 8 carriage bolt. McMaster-Carr didn't have them, only up to grade 5. Hum, looks like Fastenall has them under "Shaker Screen Bolts". Very slightly different head shape than a typical carriage bolt, but they still have the square shank for locking into a square hole. Hum, I don't see any anti-rust coatings, so thread lock them to keep the moisture out of the threads, and then paint over them.
If I had reasonable access for a box end wrench to access the bolt head from the tube end, I'm not sure I'd use a carriage bolt. If for some reason the square hole gets stripped it could be very hard to remove the bolt. Grinding through a grade 8 nut and bolt is hard work. I bet most frames are only mild steel and not hardened steel or high tensile steel which I think would be a minimum for use of a grade 8 carriage bolt. That square shank is small and the area it bites against is also small.