Mainspring strength allowance

[SndChsr]

Greetings good folks. I'm wondering, the watch I'm working on specifies a mainspring - Elgin part#817 with .775 strength. I happen to have part #817 but with .008 strength. Would that be ok to use? Is there any tolerance when it comes to mainspring strength or does it have to match exactly?

Thanks!

[MrRoundel]

Is that an Elgin pocket watch you're working on? I do see some #817's for a 16s Elgin with various strengths. It depends on how many jewels your watch has. The lower the jewels, the thicker/stronger the spring needs to be, and vice-versa.

The problem is that the strengths I'm seeing in a Swartchild catalog from the 50's, are between .185mm and .200mm. Another list I have shows two strengths listed as .190 and .205.

So, is it a pocket watch?  How many jewels?

 

[SndChsr]

Hello MrRoundel,

Elgin pocket watch indeed! It's a 17j, grade 280.

Edited March 11, 2022 by SndChsr

[watchweasol]

Have you tried Ranfft's site  it lists the 218    17jewel    1.65X?X0.14X440  or the pocketwatchdatabase for further information.

[JohnR725]

On 3/10/2022 at 6:32 PM, SndChsr said:

I'm wondering, the watch I'm working on specifies a mainspring - Elgin part#817 with .775 strength. I happen to have part #817 but with .008 strength.

American pocket watch mainsprings can present an interesting challenge today. I  have some images for you to show the problems.  Catalog pages from the Elgin parts catalogs 1915 you can see there are four different strengths of your mainspring. The catalog conveniently gives you a clue of strengths versus number of jewels. They even have a lighter than normal spring because occasionally for some of the higher jewel count watches the Springs would be too strong.

Then I have an image out of a list of modern mainsprings. This is where things get really interesting in that the modern white Springs typically for the same thickness can be stronger than the original Springs. The other bigger problem with modern mainsprings are the availability of them is limited. The one time the readily available but the Swiss don't like to make mainsprings for American pocket watches so the ability to get modern Springs is decreasing. Or for some watches they just consolidated all of various sizes into one spring. So the unfortunate reality can be can have a listing of a lot of different sizes which are lucky to get just one.

 

On 3/10/2022 at 7:39 PM, MrRoundel said:

It depends on how many jewels your watch has. The lower the jewels, the thicker/stronger the spring needs to be, and vice-versa.

If you look at the catalog page that I have it does list the mainsprings suggestions for the number of jewels in the watch. Usually it's not a problem of using a spring of a different thickness.

Then it wrong the strength mainsprings and problems I found have been extremely limited. Occasionally a seven jewel watches unhappy with the week mainspring because basically it's worn-out. Or one occasion I had a Hamilton 21 jewel watch that had too much amplitude. This is where eBay is nice I found the weakest mainspring they made an original spring it solved my problem.