I looked and I'm going to sleep on it. But, I have a theory. Looking into the bore hole, it looks like there might be a metal washer. It makes some sense because it would center the rod in the hole so the nut would slide on easier. But it might be creating friction and the washer might be hung up on a thread.
I'm thinking of taking a wooden dowel and try tapping on the head of the rod to see if it causes things to listen up. That's where I am tonight.
No scotch involved, have to think clearly! Maybe more to come?
