1940s Seth Thomas 8 Day Time and Strike Clock Movement

The Seth Thomas No. 89 clock movement was used in many mantel clocks from ca. 1900 – 1938. Around 1939, a cost-reduced movement was introduced. It uses smaller gears and plates, and has rack and snail striking. To save cost, 5 gears are the same. It has a deadbeat escapement with diamond-shaped steel pins (whereas the No. 89 movement has a strip deadbeat escapement). Each gear train has 6 gears (compared to 5 in the No. 89).

An article in the January 2002 Clockmakers Newsletter covers this movement. An example dated 12 48 with model number A200-018 is shown (in a Sharon steeple clock). The mainsprings are listed as 11/16 inch wide, 0.018 inch thick, and 80 inches long, but I recommend thinner (0.0165  inch) springs when a replacement must be made.

An example I owned several years ago (in a tambour case) had a weaker time mainspring, but the strike spring felt very strong (I didn’t take it apart to measure the springs). The strong strike spring could lift the hammers one hammer head’s distance and strike for over 2 weeks.

Notes on job 2959 completed 1-27-03:

The movement is dated 41-8  (August 1941) on the back plate behind the time mainwheel, and has the numbers 45-03 on the hammer lifter bridge. It appeared to have had both mainsprings replaced with 11/16 x 0.015 x 108 inch springs. The very long springs barely fit in, and made it difficult to use mainspring clamps to let the spring down (I had to use cable ties on the strike spring).

After repair, the time train ran very well, with a great overswing. The striking stalled, even after adjusting the hammers to lift 1/2 hammerhead distance. After rounding the pawl that the lift points lift, and smoothing the lifting points, the strike will run for over 2 weeks with 1/2 hammer head lift.

Notes on job 5216, completed 12-13-10 (illustrated above):

This clock did not receive a complete repair, I just disassembled and cleaned it, and replaced the broken time mainspring. The original time spring measured 11/16 inch wide and 0.018 inch thick. The replacement mainspring is from Empire Clock, Inc., part number 280-17-505, 11/16 inch wide (specified as 11/16 inch wide but actually 23/32 inch wide), 0.0165 inch thick, and 96 inches long). I cut 16 inches off the outer end to make the spring 8o inches long. This spring provides plenty of power, with a good amount of overswing (supplementary arc).

At the top of this article are photos of the clock and its movement. The case is 20 inches long, and the dial’s minute track is 5 inches diameter.  The movement is dated 10 – 40 (October 1940) on the rear plate behind the time mainwheel, and has the number 4503 on the hammer lifter bridge.

Mainspring Recommendations: Leave the original springs in if they are ok, and not causing excessive mainwheel tooth wear.

If a replacement must be made, use a mainspring such as Empire 280-17-505, Mile Hi CML 175.3, or Timesavers 20506; 11/16 x 0.0165 x 96 inches, shortened to 80 inches. (A 0.015 inch time spring may be acceptable, but I need to verify this in more examples – however, I don’t have this type of movement in for repair very often.)

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  1. The large end (for winding) is size 6 (American size). The small end (for regulating) is 0000.

  2. What size key does this clock need? I have one and love it but do not have the key 🙁

  3. I have a Seth Thomas mantel clock that’s was my father’s . It says on back it’s a 8 day clock. I can only get it to keep time for 24 hours. Is there a solution.

    Thanks for a reply.

  4. If it is not clicking when winding it, the ratchet may be about to fail, which could cause the key to unwind, injuring your fingers. It is best NOT to wind it – it should be overhauled for safety. See my website for details: https://billsclockworks.com/repair/sethThomas/index.html These clocks were give very strong mainsprings when new, and I’ll install a weaker one as part of the overhaul.

  5. I just picked up one of these clocks from Goodwill for $15. It runs very well and seems in very good condition. When I wind the time side, I can’t feel “teeth.” It winds with tension, but smooth. I can feel some teeth when winding the strike, but they aren’t as obvious as when I wind my other antique clocks. Do I likely have one of these with the “too strong” mainspring? My movement is marked 4505 and 12 40, which I know is December 1940. There appear to be good teeth on every wheel I can see from the back. I love these clocks and have learned a lot, but I’m no expert.

    If I have a too strong mainspring, will I hurt the clock by keeping it going? Not knowing how recently it was serviced, I do plan to have that done. But I hope I’m not hurting it by winding it and letting it run. I am enjoying it so much!

  6. I need a strike spring for a Seth Thomas Mantel clock with the number 110 and 3 under it. Do you know where I can find this spring?

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