We took the tandem out today for 25 miles. But as I was putting the bike in the car I noticed that the captain's saddle and the stoker's handlebar weren't quite straight. So I grabbed the 5mm Allen wrench and loosened the bolt, straightened everything, then tried tightening it up. The bolt bound up as if similar-metal threads had developed burrs. Well, it wasn't loose enough to prevent us from riding, but since I had another bolt I figured I'd just swap it when we got home.
The problem was that it moved only enough to loosen up the seat post a little and then it wouldn't budge. It wore open the hex-shaped inside of the bolt instead. Dang. So I worked the seat post out and then tried putting vice grips on it. That effort just scratched some of the paint and mangled the key slot on the opposite side's flange! Eventually I pulled out a hacksaw and carefully cut through the bolt through the expansion slot, and fortunately doing no further damage to any paint. Fortunately too I had recently picked up some touch-up paint to treat a few other scratches.
That binder bolt, something I'd picked up from a LBS when I built up the bike, was labeled just "cro-mo". Really? I wonder if it was chrome-plated and the plate broke off in the threads. I don't expect the Sugino bolt in there now to do the same thing.