I'm currently reconstructing a Seth Thomas 120-J movement that came in missing two wheels and with mismatched mainsprings and 4 screw-in bushings, one of which was loose, and a few other hacks. I have two ST-120 movements that have yielded the missing parts and I planned to set it to running today but noticed there was no end shake on the time side main wheel. The main wheels are scratched "S" and "T" as are the spring boxes (cans) but of course the "T" wheel was in the can marked "S". My reference says that both mainsprings should be 11/16" x .014" x 54". One older spring is, and the other appears to be very new and is just a tiny bit narrower. Both looked good, but the older spring would not "catch" to wind. The "pin" was very worn down, so I just swapped the main wheel and arbor from one of the parts movements and it was in better condition anyway. (I wouldn't be surprised if someone already knows where this is heading)
So I unmounted the spring and just put the arbor and main wheel between the plates and it was perfect, good end shake. The spring had a little bit of coning from a previous repair, so I took care of that and put it all back together and still zero end shake. And yes, the infamous spacer washer between the frame post and the spring box bridge is in place. The spring fits in the can OK, the arbor alone has end shake, coning is not the problem, so why is this making me crazy?
On closer inspection it turned out that the spring hook on the arbor from the donor movement was maybe 1/16" to 3/32" closer to the winding end of the arbor and that combined with the size of the hole in the end of the spring (which was a factory hole, but possibly not a ST) was not allowing pivot part of the arbor to extend fully past the end of the spring box. Solution was to bore the arbor and install a new catch pin in the same location as the arbor that was replaced. All is well, good end shake and all the wheels turning easily so maybe tomorrow she runs?
So, my question is what's going on here? Why did some ST-120s apparently locate the spring hook at a different location? Did some (perhaps newer) specify a mainspring other than 11/16" x .014" x 54"? The donor movement had a riveted in spring hook and the "original" had a struck hook gouged from the arbor. This movement has a notch cut in the edge of the spring box and the two donor movements do not. So what is the purpose of that notch? Do all of the movements that have this feature also have the spring hook placed at a different location? I've seen other movements with this cutout and never gave it any thought because I went back with the same parts.
RC
So I unmounted the spring and just put the arbor and main wheel between the plates and it was perfect, good end shake. The spring had a little bit of coning from a previous repair, so I took care of that and put it all back together and still zero end shake. And yes, the infamous spacer washer between the frame post and the spring box bridge is in place. The spring fits in the can OK, the arbor alone has end shake, coning is not the problem, so why is this making me crazy?
On closer inspection it turned out that the spring hook on the arbor from the donor movement was maybe 1/16" to 3/32" closer to the winding end of the arbor and that combined with the size of the hole in the end of the spring (which was a factory hole, but possibly not a ST) was not allowing pivot part of the arbor to extend fully past the end of the spring box. Solution was to bore the arbor and install a new catch pin in the same location as the arbor that was replaced. All is well, good end shake and all the wheels turning easily so maybe tomorrow she runs?
So, my question is what's going on here? Why did some ST-120s apparently locate the spring hook at a different location? Did some (perhaps newer) specify a mainspring other than 11/16" x .014" x 54"? The donor movement had a riveted in spring hook and the "original" had a struck hook gouged from the arbor. This movement has a notch cut in the edge of the spring box and the two donor movements do not. So what is the purpose of that notch? Do all of the movements that have this feature also have the spring hook placed at a different location? I've seen other movements with this cutout and never gave it any thought because I went back with the same parts.
RC
