Josh
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My curt hitch also game with grade coarse threads. 3 of them sheared off with in a week towing a trailer way under max weight for the hitch. Switched em to fine thread and have yet to have a problem.I use my judgement, I try to use the original bolt type but sometimes you can't find or wait for an order of grade 8 fine threaded bolts. What ticks me off about rusty fine threaded bolts is the threads can break and the bolt just spins in place. Good if you want to keep the part together, bad if you want to remove the part!
I recently did a tow hitch install and noticed the Curt 18,000lb trailer hitch and Reese 16,000lb ball and pintle both came with grade 8 coarse threaded bolts for the install. The ball however was fine threads. If grade 8 course threaded bolts are good enough for towing...well....? I do make sure to use lock washers or thread locker after a bad experience loosing a leaf spring bolt on a trailer once.