Originally Posted by zz28zz

If I understand the situation correctly, the holes in the coupler are 1/2" dia and the holes in the trailer tongue are 5/8" dia. If thats the case, I'd find another coupler that uses 5/8" bolts. I seriously doubt the coupler manuf will approve drilling out the holes. Why would they? Their risk/reward ratio would be completely out of balance. If they did approve it, I'd want it in writing for sure, but even then, they could day you didn't drill it correctly.

Is regards to grade-5 vs grade-8, there's a lot more to it than which is stronger. Corrosion resistance being one example, ability to bend vs break is another good point made prev.