Selecting A Replacement Mainspring

[Don]

I've decided to replace the mainspring in my current project watch and I'm a little confused about the measurements. On the Ranfft movement archive, the mainspring dimensions are given as 1.15 x 10.0 x 0.11 x 320mm. From what I've read, I'm guessing that 1.15 is the width, .11 is the metric strength?, 10 is the Dennison strength? And 320mm is the length. Does this sound accurate?

 

I'm hoping to order from a local supplier who lists mainspring length in inches. If 320mm is the metric length, then I should buy a new spring with a length of 12.5984 inches or less?

 

The closest they carry is listed as:

Metric Width: 1.15 mm, Metric Strength: 0.11 mm, Length in Inches: 10.5
Dennison Width: 2 1/2, Dennison Strength: 10

 

Would this be a suitable replacement? The next size up is longer than the original:

 

Metric Width: 1.15 mm, Metric Strength: 0.11 mm, Length in Inches: 13.5
Dennison Width: 2 1/2, Dennison Strength: 10

 

Any help would be appreciated.

 

Don

[bobm12]

Hi Don,

 

I believe you can use a slightly shorter spring as long as all the other measurements are the same. Only you will have a slightly less reserve. But don't quote me on this one, let's wait for another member(s) who have more experience in this.

[anilv]

Dont forget that these measurements are not the whole story.... the end where it grips the arbor should not be too far off...(you can't bend it to close it up but you may be able to stretch it if the arbor is too big).

 

The other end where it locks into the barrel also differs form watch to watch Some lock into the barrel wall, some lock into the top and bottom.... best bring a sample or at least replace from same manufacturer from the same timeframe (eg ETA for ETA).

 

Anil

[WillFly]

+1 for Anil's advice about the arbour and barrel end. I needed a replacement spring for a wartime Unitas 173 movement. Not only were there two slightly different sized replacements for this movement available from Cousins, but there were 4 different arbour barrel ends to choose from!

Edited October 14, 2014 by WillFly

[bobm12]

Hi Will,

 

may I ask in the case you describe, what's the best way to find the one you need, considering it is all on line, no actual physical comparison? Thank you in advance.

[WillFly]

In the end, with the Unitas, I managed to source a complete replacement movement - so I didn't actually have to solve that problem!

[bobm12]

Thank you Will, got it! I haven't found this problem yet but it is good to be prepared!

[Don]

And here I thought this was the easy solution. I have no idea how long the existing spring has been in the watch, but it looks pretty good. I was willing to give it a try, but my attempt to hand wind a practice spring was no good. Then I tried a mainspring winder I bought off the Bay. That was a complete disaster. I would love to buy the Bergeon #5355 set, but I haven't found it at a price I can pay right now. Cousins lists the mainspring as obsolete. Jules Borel has it in their database, so I may try ordering it from them. Has anyone tried them before? Their site is possibly more confusing than Ofrei.

[Charlie]

I usually can call J. Borel and tell them that I need a mainspring or any other part for a particular movement and that's enough information for them. That works quite well for me.

[Charlie]

I order from J. Borel on a regular basis lately and have used their service for years now. I have almost no complaints.

[Don]

Thanks very much for the responses.