Mainspring sizing variables

[skridlov]

I'm attempting to do my first repair requiring disassembly. I thought I'd start with a mainspring as a (relatively) straightforward first attempt - on a 1930s Ingersoll 7 jewel watch: I've decided to leave the Patek Chrono until later 

I'm having a bit of difficulty sourcing a matching replacement.According to my measurements the original measures as follows:

Height 1.7mm. Thickness 0.14. Barrel diameter 11.5. Barrel arbor 3.7. Length (approx) 32cm

The spring has a single hole at the arbor end and a foldback of about 6mm at the periphery.

The nearest I can come up with on the Cousins UK site is the GR4938TR (1.70 x 0.12 x 380 x 11.5TR). Of course whether the broken spring is original is anyone's guess - I can't even identify the movement closer than cal 12 (?) if that's what the 12 on the bottom plate indicates.

Overall, what is the flexibility in sizing for a rough and ready job like this one? It would do my aged confidence no end of good to accomplish this successfully so please forgive the questions that will surely follow...

Thanks, Roy

[skridlov]

Just found this alternative on obsoletewatchparts site:

1.70 X 0.135 X 340 X 11 swiss

MS3675

4.50

 

Not entirely clear about the fitting (the site says "hook type").

Roy

Edited May 7, 2017 by skridlov
typo

[dadistic]

Otto Frei has a page that shows mainspring end pieces, maybe it will help.

http://www.ofrei.com/page1597.html

 

Edited May 8, 2017 by dadistic

[skridlov]

Thanks David. How close do thickness and length have to be? I'm guessing that too short is probably worse than (a little) too long?

Roy

[StuartBaker104]

I think I should get commission from the author of this page as I've posted links to it about 3 times over the last month... but it really is very good!

http://www.vintagewatchstraps.com/blogmainsprings.php

So you will see that length is probably the least important characteristic (within reason), but thickness is very important. Use the calculator on the above page to infer the thickness and length from the barrel inside diameter. The height should be about 0.2mm less than the space inside the barrel.

 

[transporter]

Listen to Stuart Roy he knows his onions, and has just helped me out with the same website link

[StuartBaker104]

5 minutes ago, transporter said:

Listen to Stuart Roy he knows his onions, and has just helped me out with the same website link

Onions are not really my speciality, and just to be clear I have no qualifications in horology either.  Like the author of the link I posted, I am an engineer by qualification, and I love his approach to the problem.  Since I was knee high to my grandfather I was taught by him that there was nothing which couldn't be repaired, some things just take a little more learning than others.  I tend to over think and over research every problem I face and so I have read widely around the subject. Like a signature that someone else on here uses, I probably need to spend more time at my bench repairing watches and less time on the internet learning theory.

[skridlov]

Thanks folks! I actually found the nawcc page which has the mainspring calculator. I was thrown out a bit when I looked at Cousins' listing where it was a case of deciding which parameter was least important - and assumed that an incorrect value would result in slippage. I know David Boetcher's site which is invaluable in many respects but hadn't stumbled across the link above as his site lacks a search engine.

I'm really interested in this subject and have a vast number of watches here (literally) requiring attention of all kinds. I've had various involvements with many kinds of engineering albeit without any formal training. Most recently in IT, before that in analogue video electronics and before that with mining equipment. I'd be happy to offer an opinion to anyone needing to extract tracked vehicles from severe bogging. I'm well aware that working on watches is a very particular kind of challenge to take up so late in life...