I have a Sz 18 Seth Thomas that will only run 20 hrs. Is this reasonable considering the age? It seems to be in pretty nice shape with good motion on the balance. It keeps good time for the hrs it does run. Should it run at least 24 hrs? I guess if the experts think it should run longer, it is in need of a COA and/or new spring.

Another view.

3

I have the #225173 Grade 260 that only ran for twenty hours.

I had it cleaned and a new mainspring installed, it still only runs for twenty hours.

If that is the only problem, I would leave it as-is until you decide to use it as a carry watch. The full plate Seth has an in-line escapement and the mainspring has to have a lot of juice to keep that oscillating well.

A white steel Illinois main might get you to 28-30 hours.

Thanks gentlemen. David, did all the Seth Thomas' have that kind of escapement? Is there one that was better designed?

The In-Line escapement was the best design and was used right up to now. In the larger, heavier balance wheel movements it was at a slight disadvantage for forcing the balance wheel into oscillation over the Lever type used by Waltham and a few others.

Not surprisingly, the Seth Thomas Full Plate movement has a remarkable resemblance to the Illinois 18s movement to the point that Illinois Balance staffs have been used to repair the Seth Thomas.