I recently repaired a Seth Thomas Adamantine mantel clock made around 1905. It has a No. 89 movement. This movement is very efficient in operation, due to the use of a deadbeat escapement. Even with a thin time mainspring, these movements typically take an excellent escapement motion.
This particular example has thin, original mainsprings.
Time Mainspring: 3/4 x 0.0167 inch.
Strike mainspring: 3/4 x 0.0163 inch. (Unfortunately, I had to replace the strike mainspring because it had a rough action when nearly wound, typical of springs that may break soon. I used a new Merritt’s Antiques P-1956).
Many examples of the No. 89 movement have thicker springs, quite often around 0.0175 inch thick, sometimes thicker. After proper repair, this movement will run well with thin springs.
Here is a video of the escapement motion, first run down 6 1/2 days, then fully wound:
Repair Job. 4970.
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