Mainspring help (again)

[transporter]

Hi guys, been awhile I know, another question, I need a new mainspring for a nice old cylinder fob watch im working on, 

My springs dimensions are as follows, 1.29mm height x 0.16mm thick x 300 mm length, barrel diameter is 11.31mm

On cousins the closest that my untrained brain can find is 1.25 x .115 x 340 now I know these measurements are abit off from my original ones but could someone please tell me what measurements are allowable to change so you can get a spring as close as possible if an exact replacement is not available? 

I can hazard a guess that I could drop a little on height as I have done so before after a kind member here pointed me in the correct direction for a new spring once before, but before I continue to ask the same quest over and over I need to learn this stuff once and for all so I don't have to keep annoying you all.

cheers guys

[jguitron]

Interesting situation. I'm no expert by any means but it looks like you should give it a shot. The replacement spring may be a touch underpowering but it's also longer, so it may work to fit it in the barrel. I figure the power difference would be low enough that you could compensate by regulating the hairspring.

The alternative is a thicker hairspring which would over power a bit which is find unless the movement knocks.

Let us know how it turns out!


Sent from my iPhone using Tapatalk

[oldhippy]

One thing to remember a third of the barrel too be taken up by the barrel arbor a third by the spring and the other third left for space.

With many of these old pocket watches, you will never find the exact match. It is better to be a little weaker then to strong, providing it will run for 30 hours unless it is of 8 day duration.  

[StuartBaker104]

Probably me that pointed you at this page last time... http://www.vintagewatchstraps.com/blogmainsprings.php

I wouldn't assume the watch has the correct spring fitted, but would work backwards from what you know... barrel diameter and arbour diameter, and use the calculator.

If you can't find a spring on Cousins, then try John at obsoletewatchandclockparts

 

[transporter]

Gents many thanks for the help, and yes Stuart it was you before, so again cheers, oldhippy thank you, away I go with some new found knowledge, ill let you know how I get on.

[praezis]

Thickness 0,16 is much too strong, 0,115 much too weak.

Hight and length are no critical sizes, but thickness should be right. My calculator says thickness  0,135 ... 0,14  length 350 ... 360.

I will post a little program for this and other calculations soon.

Frank

[transporter]

Manyy thanks guys, sorry was away with work again and haven't been online for a couple of days let alone at the bench trying to solve the mainspring problem, will hopefully be on top of this in the next few days and have one on order and soon fitted back in the movement

[praezis]

Posted the mentioned app here:

find mainspring

Frank