Mainspring vintage or new

Discussion in 'Watch Repair' started by piedmontg, Feb 16, 2020.

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  1. piedmontg

    piedmontg Registered User

    Jun 14, 2009
    #1 piedmontg, Feb 16, 2020
    Last edited: Feb 16, 2020

    I am new and am about to replace a mainspring in a Hamilton 992B circa 1940. I searched through the site and found 111 msg on mainsprings, read about 15 of the threads. None of the threads came right out and said do not use a vintage ie an original in the bag spring rather than using a new alloy spring.

    My inexperienced logical conclusion would be that a vintage spring wound up for 50 or more years would very likely have a set similar to the spring in the watch now ie about 2 1/2" in diameter.

    My goal for the watch is to make it my carry watch, so it will be used every day. With that said I would think a new alloy spring would be the way to go, however putting an original in the watch has some appeal, ie keeping the watch as close to original as possible.

    Could you provide your experience with NOS vintage mainsprings?

    Also in the book I have there are two mainspring numbers - 534B, Str. 15 1/2 MM or 35435 (standard) str. 370,Dynavar. I have found all on ebay white alloy, blue, brown. Which would I use and what are the differences?


  2. richiec

    richiec Registered User
    NAWCC Member

    Feb 24, 2007
    automotive warranty inspector
    Brick, Ocean, NJ
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    Use an alloy spring whenever possible as the older blue steel springs can set right in the packaging, are susceptible to rust which causes premature breakage possibly with catastrophic results and kink when bent, again with breakage. There should be plenty of alloy spring available at reasonable? prices, expect to pay anywhere from 20-35 per spring these days. Don't worry about using the oem type of spring, it does not affect the value, actually might bring it down as the next owner will have to change it. It is a definite crap shoot when buying a blue steel spring, I have purchased packages where every spring was already broken right in the package. I only use them if there Is nothing else available, like on some Waltham 14 sizes or early 10 sizes.
  3. GeneJockey

    GeneJockey Registered User
    NAWCC Member

    Mar 2, 2012
    San Francisco Peninsula
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    You should have no difficulty finding an alloy spring for a 992B. There are at least half a dozen on Ebay right now.
  4. karlmansson

    karlmansson Registered User

    Apr 20, 2013
    Linköping, Sweden
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    In my understanding the only reasons not to use a modern springs is in watches with a build in compensation mechanism to deal with the imperfections of the older type, spiral only springs. A fusee watch would be such a case.

    Also bear in mind that the modern springs are usually slightly stronger for the same thickness so going down a hundreds of a mm or so in thickness is probably closer to the original power specifications of the watch than the specified spring dimensions would be with a modern spring as a replacement.


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