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10BZAC Mainspring

Submitted by wdschell on January 10, 2021 - 5:55pm

Hello All,

I picked up a M1 Bulova at a thrift shop and everything looks really good and is running.  I put it on the timegrapher and the amplitude is only around 200 degrees.  Is that normal for these? I have only fixed a couple of elgin pocket watches up to this point and they had much higher amplitudes.


If it isnt normal do you all know what mainspring would fit this movement?  Thanks

Posted January 11, 2021 - 7:27am

I'm not too sure about the appropriate amplitude range (would also like to know, I have one of these that will not run more than a day); but I believe these use a sealed lifetime barrel, so you would need to find a barrel complete and not just a mainspring - Bulova part# 1S (also used by the 10BZACD)

Posted January 12, 2021 - 8:01pm

In reply to by Andersok

Oh, awesome.  I might try to just take it apart clean it and oil it and see if that helps anything.  Hopefully if I need to I can find that part.  Mine only runs for about 30 hours.  

I wore it for 2 days after I got it and it never stopped but once I took it off it ran for 30 and stopped.

Posted January 28, 2021 - 4:20pm

In reply to by wdschell

I took my barrel apart and cleaned everything. The mainspring also incorporates a separate safety spring that requires braking grease before installing. My watch went from 22 hours reserve to 43 hours reserve. 

Reverend Rob
Posted January 28, 2021 - 5:15pm

Ideal amplitude range would be mid to high 200's, 'perfect' is over 300, but I rarely see that in vintage watches. The better the amplitude, the better the stability of the timekeeping. 

With these sealed barrel types, you can measure the mainspring and find one that is close in tolerance. Width is critical and should be exact, but strength (thickness) and length can vary slightly. That said, some of these Bulova mainsprings have no equivalent that is even close. 

All bridle automatic mainsprings require braking grease (Glissalube 20)  inside the barrel. 

Posted February 5, 2021 - 12:10pm

Thank you all for your help.  I am still on the fence about weather to take it apart and clean it or just leave it as is for now.  I replaced the crystal and it seems to be keeping pretty good time +/- 15 seconds a day