It's a 9/16x3-1/2 (I think) bolt with a nut on bottom to take it loose and lock nut on top. You can use just a bolt and vise grips, but I like being able to put a wrench on there. Just give it a little love tap with a hammer an it fits snugly in there. Note an old worn bolt works best, if you use a new one, grind the edges slightly to allow an easier fit.