How to find a good main spring replacement

[glnds]

Hi All,

I’m a Lemania collector and as a hobby I try to service some watches myself. I still have a lot to learn and currently i’m a bit stuck with finding a decent main spring for a Lemania cal 3600 (automatic).

Here’s my problem:
I ended up with 2 different specifications for a Lemania 3600 mainspring based on the following pictures (see attach).
Maybe the Marco one is less reliable because it has a hand written specification instead of a printed one….?

Now, there is a wide variety of auto mainsprings available here: http://www.ofrei.com/page_112.html and here: https://www.cousinsuk.com/category/mainsprings-by-size-watch-pocket

The thing is that  they all differ on at least one of the specs. I can imagine that the width is crucial, otherwise it will just not fit, but other things like length, is some margin allowed there? For example instead of 12,5 inch is 12 or 13 inch also ok? If i have to choose, which specifications are more important, for example strength is more important then length,..…?

Would be nice if somebody could give a good hint to a mainspring to would fit in a cal 3600 .…

Thanks!
Gert

[praezis]

Funny that inch is called metric...

Most important is strength, Its influence is to the power of 3! (my book says 0.11 btw)

Least important is length. Differences of 10% have no influence at all.

Frank

[StuartBaker104]

Gert,

There is a really helpful explanation of mainspring design and selection here...

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

You can find the Lemania 3600 in the GR catalogue here...

https://www.cousinsuk.com/PDF/categories/7808_GR Pages 121-130.pdf (under 25.6A) which gives the same dimensions as the Marco part above

The closest which Cousins has in stock is the GR3257X, which is 40mm longer, than the correct GR part.

As Frank says, this difference in length is insignificant, and the thickness is much more important, but I wouldn't be worried about going up to 1.1.

S