I just overhauled a Seth Thomas 89C movement for an Adamantine mantel clock (my job no.4401). Both the time and strike mainwheels had significant tooth wear. The mainsprings had the ST logo and may be original, and the thicknesses were .071 for the time spring and .073 for the strike spring. To reduce future wear, I replaced both spring with the Merritt’s P1496 mainsprings, measuring .0155 inches thick. They provide noticeably less force, yet are plenty strong enough to run the clock, provided that the movement has the pivots polished and necessary bushings installed. See this post for information on these mainsprings.
Note 9-11-07: The strike train must be almost perfect to run reliably with this thin of a mainspring. My customer brought the clock back into the about shop two weeks after he had picked it up, saying that it did not always strike. I first established that the mainspring had enough torque by letting the spring down 10 turns from fully wound up (simulating a running time of 10 days). The striking still ran. I wound it back up, and found that occasionally the strike would not go into warning, and that it was always when gears S3 and S4 were in a specific relationship. I let the mainsprings down, and removed gears S3 and S4. S3 had significant wear on the tooth tips, and there was slight wear on the pinion wires of S4. (With a stronger mainspring, this would not have caused any problem). I reversed the wires in S4 so that S3 would engage with the new portions. Then I removed gear S3 from its hub, turned it over and re-installed it, so that the unused side of the teeth would be employed.
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