I have an Elgin 17 jewel 18s grade 379 class 106 model 5 according to the Elgin Serial Number website. It says I need a Elgin part number 258 mainspring which I can not find any info on. I think I have found that I need the 817. But the 817 comes in 2 different strengths. Does anyone know which strength to use?
Thanks,
Harry

From what I can make out from my pdf of the 1915 Elgin parts book the thicker (.228 instead of .225) and therefore stronger spring is for 7 and 15 jewel watches. The other for 17 and 21 jewel. I have sent you a email with a link to that pdf.
That kind of makes sense as jewels cut down on friction and make it easier to run. Therefore it doesnt need the strongest spring.

I show the Elgin Part Number 812 for your spring. Casker has them in stock.

http://www.jewelerssupplies.co...ing-size-charts.html

Thank both of you Jim and David. You both have been a great help. Jim thank you very much for the email I did receive it and will look at it when I get home.