Mechanical WW USA vs Swiss stem sizing

Discussion in 'Watch Repair' started by Roy Horrorlogic, Aug 28, 2019.

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  1. Roy Horrorlogic

    Roy Horrorlogic Registered User
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    Mar 7, 2017
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    I recently bought both a new stem and a couple of (ludicrously expensive) gilt crowns for an early Gruen Curvex. I checked the old stem size first and the threads appear to be tap 10 (1.0 mm). As is the new stem. Neither of the crowns fit, having threads approx 0.8 mm.
    A wiser man than I has told me that there's a discrepancy between Swiss and US "metric" measurements for stems. Uh?
    So what size Gruen crown do I need to find to fit this size stem? Assuming I can raise a mortgage to buy one.
    If anyone has a suitable crown I'd be happy to swap for the two I have - or buy it.
    Roy
     
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  2. Chris Radek

    Chris Radek Registered User
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    Apr 13, 2014
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    I wish we all had the good sense to just use mm measurements everywhere!

    If the stem measures 1.0mm with a micrometer I think that's generally called tap 9, not tap 10. This might be your error?

    Although I can't find confirmation right now, I think I have seen two "tap" scales for stems/crowns where one increases tap number with increasing size, the other increases tap number with decreasing size (which is the commonly used system). I would not be surprised if there were a good handful of systems over the years/countries.
     
  3. Roy Horrorlogic

    Roy Horrorlogic Registered User
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    I should have added the word "nominal" to the figure of 1.0 mm. Measuring with a caliper the diameter is a little over 1 mm. Which, I assume, relates to the relative diameters of outer and inner thread. The crowns fit a test stem which measures a nominal 0.8 mm.
    Does this clarify the problem?
    Roy
     
  4. Al J

    Al J Registered User

    Jul 21, 2009
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    Tap Size Chart.jpg

    Can't recall where I got this from, but it may help. To resolve the issue you have, you can either get a stem with a different thread is it's available (some companies offer more than one tap size) or you can use a stem extender that also converts one size to another...or of course get a different crown.

    Cheers, Al
     
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  5. Roy Horrorlogic

    Roy Horrorlogic Registered User
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    Thanks Al
    i have lots of extenders but the movement and case only allow very short threads so that's not an option.
    That chart is, to put it mildly, very odd. If the intention was to ensure confusion it couldn't be improved upon. I have a huge number of miscellaneous crowns so sizing has always been a matter of "which one fits". But in this case I'd like to put a Gruen signed crown on the watch. Nothing is easy...
    Roy
     
  6. Barney Green

    Barney Green Registered User

    Jan 19, 2017
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    Hi Roy,

    if you get a Gruen signed crown for this watch it might not be original. We discussed this in another Gruen forum and it turned out that it looked like that the crown of the early Curvex watches were not signed. It would really ne nice if we could either find original signed crowns for these stems or even better, a NOS watch which should have the original crown.

    Barney
     
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