I used 7/16" just like the originals - don't have a lathe to turn down the body. As you can see from the picture my originals were badly worn around the square shoulder. Surprisingly the square hole in my mount is pretty good. Combine that with the new bolt and it is completely solid. You may want to try a NEW 7/16", it may give you what you need to tighten it up. 1/2" wouldn't fit the sleeve down thru the rest of the mount.

Of course the biggest pickle of this whole thing is the fact that you have to take EVERYTHING apart to get the durn bolts in or out.

Don't be tempted to use grade 2 carriage bolts from the hardware store - too soft. Get at least the grade 5.

PRW