I can't imagine how the eccentric pinch bolts could shear off...
George, I wondered the same thing, so I tried to play forensic analyst...
I went to the linked profile of the frame and note Thorn is (currently) selling the frame alone and says, "These frames are made to a price, finish is not perfect but completely functional". This indicates the frames weren't made to the same spec as Thorn's higher priced offerings. The rear dropouts support this, as they differ from the other tandems Thorn produced.
In Cedric's photos, the sockets for the machine screws aren't milled from billet and circumferentially welded as on our Nomads. Rather, they are nuts tack-welded to the eccentric BB shell. I'm not sure the screws are the same diameter or fine pitch as ours. If the pitch is more coarse and the diameter smaller, then any shearing might have been the result of the previous owner's attempts to get the eccentric screws tight enough to hold against the force of two people's pedaling torque.
It is odd they'd shear at about the same point, but I suppose the nuts could have served as a collar to isolate the threaded shaft until it emerged and twisting forces concentrated on the unsupported shaft. A bit hard to tell, but I think the break doesn't look clean enough to be sawn through, and my first thought -- that they'd somehow hit something and sheared from impact -- isn't supported by the photos.
Just some thoughts on a Friday morning that saw me awakening to reddened leaves on the trees -- strong hints of Fall!
Best, Dan.
