Stronger or weaker mainspring where the on-spec version is discontinued?

[mrkrsl]

I need a new mainspring for my current repair. Unfortunately the exact one I want is discontinued. There are alternatives with the same spec, other than the length.

The specified length for the the correct part is 320 for a 10mm barrel. I have options either side of 300 for a 9.5mm barrel and 380 for an 11mm barrel. As I understand it, the stronger mainspring would increase amplitude and possibly slow the escapement because of it. And the weaker would do the opposite, and may under-power. In case it matters, the movement is a Lemania 1275 chronograph.

Which deviation from spec should I go for – weaker or stronger?

[Kalanag]

I would not exceed the overall volume of the original spring. So if width and thickness of the alternative spring are correct I would not chose a greater length. The shorter spring will just give you a slightly reduced power reserve.

[mrkrsl]

I thought that was going to be the answer. Thank you.

Any contrary opinions? I assume the longer spring would only be marginally longer/stronger. I was also worried that the fact it needs to also power the chronograph could mean that reducing the power would have a negative effect.

[praezis]

1 hour ago, mrkrsl said:

Any contrary opinions?

Not at all.
You don't reduce the power, if thickness stays the same.
Length is the most tolerant size. About 10% +/- makes no difference at all. Bigger differences will gradually reduce power reserve (= running time).

Frank

[mrkrsl]

Ok then. The 300 it is.

Thanks all.

[JohnR725]

On 6/14/2022 at 6:16 AM, mrkrsl said:

Unfortunately the exact one I want is discontinued.

I have a website at the link below that lists your parts. If you click on the mainspring it gives the dimensions and a part number according to the second link it's available.

Then there is a minor problem because the spring they specify is 300 mm long? But you are indicating your parts lists say that it's 320 mm long?

then I do find sometimes references may vary a little bit the third link has specifications for your watch and it still says 300 mm?

okay I think I grasped what the problem is? The website below has an alternative number which is this  GR 3283. But if I go look in the catalog of mainsprings their numbers 3311 at 320 mm long? So the mainspring book is usually right but we have two separate sources it indicates something different. 20 mm in length isn't going to make much of a difference though.

 

http://cgi.julesborel.com/cgi-bin/matcgi2?ref=LEM_1275

http://www.julesborel.com/s.nl/it.A/id.28614/.f

http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&0&2uswk&Lemania_1275

https://jewelerssupplies.com/product10692.html