Sourcing a replacement mainspring

[rduckwor]

I'm looking for a replacement mainspring for an old Elgin movement.  CousinsUK seems to me to have the best "look-up" feature UNLESS your search comes up empty.  The original spring is 0.1MM X 1.25 MM X roughly 270MM in length.  this of course turns up nothing.  What variable do you start with that is least injurious to the movement and accuracy when you must locate a substitute mainspring. I understand that heaight and length are rather fixed variables.  Can I go up slightly in strength without too much badness happening?

Thanks,

RMD

[Khronos]

The Elgin should have a serial number.  You can look it up here: https://pocketwatchdatabase.com/

Once you know the 'Grade' you can look up the correct mainspring either with a online scanned material catalogue or even just searching for the grade on eBay.  Also, the pocketwatchdatabase.com folks will more than likely know the required mainspring once you bring up the movement there will be something like: "Need a mainspring?" or some such codswallop. 

[praezis]

Apart from the best reply of Khronos:  exact length is least important.

Frank

[rduckwor]

Thanks.  Ebay list a number of Elgin main springs, all for other movements.  I will keep poking the interwebs.

RMD

[rduckwor]

O.K.  I found a suitable main spring on CousinsUK, but, I have a question.  The Elgin mainspring has the DBH bridle, the CousinsUK listing does not specify how to order in that configuration.  also, the listing is

1.25 x .10 x 280 x 8.5 Non-Automatic

GR3109

 

What does the "8.5" designate?

 

Thanks,

RMD

[StuartBaker104]

Rule number one when replacing a mainspring is that the one you take out is not necessarily the correct one, and therefore should only be treated as a useful reference which probably worked rather than being an absolute.

The 8.5 is the barrel internal diameter in mm. Everything you need to know about why that is important and how you can use it can be found here: http://www.vintagewatchstraps.com/mainsprings.php, 

So I had a quick look here https://watchguy.co.uk/cgi-bin/mainsprings and there are just a few Elgins with DBH barrels in the GR catalogue. I may be taking a leap of imagination here from the info you have provided, but the closest to what you have measured is cal 6099, although there are 2 options for that. If the pocketwatch database doesn’t help, you should measure the barrel i/d to work out which is most appropriate. Cousins does have the GR3137DBH in stock if you have a 9.5mm barrel, but not the 3123 for the 9mm barrel.

When searching for mainsprings on Cousins where the exact item isn’t available I usually limit my search criteria to the correct height and end type on this page https://www.cousinsuk.com/category/filter/mainsprings-by-size-watch-pocket. This gives 4 options in your case, so you can select the closest - if you have the smaller barrel I would be inclined to go for the GR3095DBH if you can’t find the 3123 elsewhere

Searching ebay for Elgin 6099 mainspring brings up some NOS parts for 6099-299 and 6099-300 matching the GR sizes.  Personally I'm not a fan of buying NOS mainsprings as you have no idea of how they’ve been stored so they may be corroded, cracked, set or worse, but I have read posts from others who have used them successfully.

[rduckwor]

Thank you for the explanation.

 

I do appreciate the help.

 

RMD