Seth Thomas Pocket Watch Mainspring Part Numbers and Sizes
The mainspring provides power to the mechanics of the watch. When a mainspring breaks or no longer offers enough energy for precise timekeeping, a replacement will be required. Measure the dimensions of the old mainspring using a micrometer, and use the chart below to find proper replacement for any American movement. Note: Using proper tools for mainspring installation is highly advised.
Professional Installation Strongly Recommended
Mainspring replacement requires specialized tools and expertise. Improper installation can damage your watch or cause injury. Consider having a professional watchmaker perform this repair.
| Manufacturer | Movement Size | Part Number | Alt. Part Number | Length (in) | Width (mm) | Thickness [Strength] (mm) | End | Purchase on eBay |
|---|---|---|---|---|---|---|---|---|
| Seth Thomas | 18 | 536 | 141 | 21.5 | 3.30 | 0.19 | T-Brace | |
| Seth Thomas | 18 | 729 | 141 | 30 | 3.30 | 0.22 | T-Brace |
Data Source: BestFit Mainspring Chart