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Seth Thmas 120 series, what is this feature for?

R. Croswell

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Apr 4, 2006
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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

ST-120-spring-boxes.jpg
 

Vernon

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Additionally, the ST 124's that I've worked on has that notch on the chime "can". I've never thought about the reason.
Vernon
 

R. Croswell

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Additionally, the ST 124's that I've worked on has that notch on the chime "can". I've never thought about the reason.
Vernon
Same here, I've always marked (removable pen) these parts and put them back without much thought. One thing I have noticed is that the anchor rivet is always at the notch and is always positioned to face toward the inside of the movement. Pictured here is a ST-124 strike side with notched can, a ST-120 can pair with no notch, and a ST-124 can failure. Seems odd that the can would be notched at the point of maximum stress. Notable that this ST-120 pair has a lot more spring on the strike side, pretty sure that one or both of these springs are incorrect replacements.

RC

124-can-fail.jpg essex-120.jpg st-124-can.jpg
 
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shutterbug

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Maybe it's to let the water out after dunk and swishing the movement :D
(newbies - just kidding!!!)
 
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R. Croswell

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Looks like that notch lines up mitey nice with that 2nd wheel pinion on that 124. Maybe there was a potential for interference at that point?? Willie X
That seems like a possibility. I don't currently have any ST-124 movements in house, but looking through my old photos, here are two different views from two different ST-124 movements. There is not a lot of space, but the main wheel diameter is larger than the can, so it looks like there is clearance, but it is close.

The last picture is the ST-120J as it came in. The cans were marked, and the wheels were marked, but they didn't match, so I just put one spring in each can and the notched can ended up on the time side. As buggered as this movement was, I can't even be sure that both cans belong to this clock. The owner had it to another shop and they lost two wheels and hacked a few other things and gave up. There was no 2nd. wheel on the strike side and the donor ST-120 that provided the replacement 2nd. wheel had no notch in either can. Wheels seem to start moving as soon as winding begins, and the time side is running now. I'll take a look and some pictures in the next day or so.

RC

124-b.jpg 124-c.jpg 120-j.jpg
 

R. Croswell

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This is a close up of the strike side main wheel and 2nd. pinion of the 120J movement. There is no notch in the can, although the notched can may have originally been on this side, or these may not be the original cans. There seems to be adequate space between the wheel and pinion and all the wheels start turning easily, so I don't see any problem. The time side is running fine (it has the notched can but doesn't appear to need the extra clearance).

Notching the can may have been for extra clearance, but really doesn't seem necessary for this movement or the two donor ST-120 movements. I do not believe that Seth Thomas would add the notch for no good reason, but for added clearance seems to be the most reasonable suggestion so far. Perhaps the same cans were used in other movements where more clearance is necessary?

RC

120J-S1-&-S2.jpg
 
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Mike Mall

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Because it is located close to the hook, could it be to help make sure the spring is properly attached? (at the factory)
 

R. Croswell

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Because it is located close to the hook, could it be to help make sure the spring is properly attached? (at the factory)
That is a thought, as sort of an inspection port? It seem like more than a coincidence that the notch is at the spring anchor rivet. Perhaps it is a locating key for one of the manufacturing processes to make sure the anchor is always in the same position in relation to the screw holes that mount the can...... but why? and why do we see clocks that have only one can notched? None of the examples I have seem to require additional clearance, but because the notch is always at the 2nd pinion, somehow that seems to be related.
 

Vernon

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These are before pictures of one that I'm working on. The notch is on the chime side but doesn't occur where the c2 pinion is.
Vernon

PXL_20230117_214012381.jpg PXL_20230114_170630862.jpg
 

R. Croswell

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These are before pictures of one that I'm working on. The notch is on the chime side but doesn't occur where the c2 pinion is.
Vernon

View attachment 748331 View attachment 748332
That is interesting. This 120J that I'm working on ended up with the notched can on the time side strictly by chance because the marks left by a previous repair were mixed up and I basically made a "coin flip" decision, so I didn't mention it before, but on the time side of mine the notch also was not at the pinion location. Everyone seemed to mention the notch on the strike or chime can, so I'm not sure if the time side ever used a notched can. All this seems to suggest that the notch has some significance in relation to the spring or the spring hook, but why not on all the cans in a movement? It's a mystery.

RC
 

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