Merritt’s Antiques offers the MS310 mainspring, designed for the Seth Thomas No. 89 movement.
The MS310 is specified as 3/4 inch wide by 0.016 inch thick by 108 inches long. Besides the Seth Thomas No. 89 movement, it is good for many other antique American movements that originally had a .0018 or .0017 inch thick mainspring.
Note: Some examples of MS310 spring are stronger than I would like, the Timesavers 15159 may be a better choice if you can get the copper rivet type. See New Batch of Timesavers 15959 Clock Mainsprings for more information.
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