Reply To: mainspring.

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chris mabbott
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Identical mainsprings are often difficult to find, I had the same problem many times. You guys are going to get sick of me quoting de Carle but he states in one of his books that for older watches, (this book was published in the 50s so I assume he’s talking about 20yr old pieces and not our, now 80-120 yr old ones)
that older watches, because of lack of available parts, and due to wear, technology etc, CANNOT be expected to keep 100% accurate time as they did when they were new from the factory, there will always be a certain time loss, due to frequent positional changes, shock, dust, temp changes. condition etc etc

I’ve had the same problems finding specific Rockford, South Bend, Seth Thomas parts, specifically mainsprings. So I’ve taken to making my own best fits 😆 I’ve had good results simply by matching width and trying to stay with the consumed space requirements of the barrel (length) i.e. 1/3 for the spring. I also try to stay within the model specs, 18s, jewel count and train type, IF possible.
As Paul states, you can easily achieve the 30hr plus running time by fulfilling these two out of three requirements. The strength… I try to stay as realistically close +_ to the specs, but it isn’t always possible..

If you have the Chicago watchmaking book, there is an in depth chapter on MS, where the author experiments with a correctly sized MS, then proceeds to shorten it and test the reaction, surprisingly, the watch seemed to perform better on a shorter spring, although this is not a reason to start cutting down MS length 😮 ..