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Mainspring tolerances?

GuyMontag

By GuyMontag
in Watch Repairs Help & Advice

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GuyMontag Super WRT Addict

GuyMontag

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Posted (edited)

I need to replace a mainspring in a Bulova 10AN movement. Cousins lists the mainspring for this movement with these dimensions :

1.40 x .10 x 300 x 9 Non-Automatic
GR3664

I'm trying to source this locally in the US. I do get some hits on ebay for a 10AN mainspring, but the dimensions are slightly off. For example, this listing for the 10AN has dimensions of 1.4 x .15 x 290 and this listing has slightly different dimensions for a 10AN mainspring of 1.4 x .1 x 292.

Both of these come in branded packaging with the Bulova 10AN movement listed on them, but they are both different than what Cousins has. Are the differences big enough to be a concern?


 


 

Edited by GuyMontag
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ChrisRadek Member

ChrisRadek

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The .15 instead of .10 will be WAY too strong and will cause galloping (too high amplitude).  The 292 vs 300 length makes no difference, go ahead and use that one.

Swigart shows the 10AN as 5x11x11 3/4, which as you say is 1.4 x .10 mm.  Interestingly, Bestfit shows it as 5x10x10 1/2.  This makes sense, if you use a thicker/stronger spring you have to make it a little shorter so the barrel doesn't fill up too much.

The 11 3/4 inches is 298.5mm, but again the exact length doesn't matter.  Looks like Borel has a suitable part MS-511K. (or 510H if you prefer the stronger but shorter length)

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