If you have a C channel frame, you can drill a 1/2" hole, put the bolts through and tighten them down with a nut and lock washer.
If you have a tube frame, like I do, you can't get to the inside of the tube to install a nut and washer. If you drill a 7/16" hole and thread it to take a 1/2" bolt, you only have about 1/8" of thread in the trailer frame to hold all of that pressure. That is not enough thread to be secure with the stress that it takes. The answer is to drill it out to 11/16" and insert a rivet nut. When you tighten down the bolt, the rivet nut expands behind the steel of the frame side giving a MUCH more secure connection. Google "rivet nut" and you can see how it works.
