Which mainspring to use given two options

[goddo555]

I'm servicing a Calibre FEF 350 movement and I'm planning to replace the mainspring. According to Ranfft, the mainspring required measures 1.45 x 0.105 x 300 x 9.5. The FEF spec sheet from the Cousins website says 1.42 x 0.105 x 325. I've confirmed for myself that the barrel diameter is indeed 9.5mm. Cousins has two possible candidates - one is 1.45 x 0.105 x 340 x 9.5 and the other is 1.45 x 0.105 x 300 x 9.

Are either of these options good candidates?

[Waggy]

Hi there, you are bound to get many opinions on this subject, so I'll kick off with my 2 cents:

• Height: this is a showstopper and you need to have it the same or very close to the original or the barrel lid may not close if it is too high or the spring may produce "waves" (AKA sinusoidal buckling)  if height is not enough.
• Type: this is another showstopper, the end of the spring needs to match the original, for example you need an automatic bridal for an automatic watch 
• The thickness of the spring (strength): you can play with this a little but try to keep as close as you can to the original.
• Length: my experience is that you can be fairly liberal with this, maybe if you have a shorter length than the original having a slightly stringer spring can make up for this if you have to pick one that is not the same as the original and vice versa
• Barrel diameter: To be honest this is the dimension I pay least attention to, because as long as you have the above right this will automatically be correct or as close as possible

I am sure there will be lots of people who disagree with the above - but if I were in your shoes I would go with the 1.45 x 0.105 x 300 x 9 assuming the bridal type is compatible of course.

[mikepilk]

I agree with @Waggy, all you loose from the length being a bit shorter is a bit of run time.

[Neverenoughwatches]

1 hour ago, Waggy said:

sinusoidal buckling

Whoa thats a cool technical description of something, i think it happens when i sneeze really hard.

[praezis]

The last (4th) figure is the diameter of the retaining ring, no property of the spring. For easy mounting, it should be equal or smaller than the inner barrel diameter .

Both lengths will produce no noticeable difference of running time.

Frank

[goddo555]

All very helpful. Thanks very much for the replies. Greatly appreciated.

[Waggy]

17 hours ago, Neverenoughwatches said:

Whoa thats a cool technical description of something

Ha ha the engineer in me creeps out from time to time.

[JohnR725]

only two choices for mainspring seems like you should have a third choice. Like for instance at the link below you can look up your watch and see what it recommends. so now we have a third choice where my guess is typically on fractional size mainsprings they're not as common or if you're going to stock mainsprings you'd prefer not the stock fractional sizes. So in other words 1.4 something could be reduced down to 1.40 for instance. The same with the thickness they just nudged it up a little bit. Then the GR number should work on the cousins website especially if you look it up without adding the GR in front of it.

then the first number on top that's the best fit number which looks like it's available at the second link.

MS-510M

MAINSPRING 1.40-.11-320 9,5

 GR 3699

http://cgi.julesborel.com/

http://www.julesborel.com/s.nl/it.A/id.28656/.f