I have now finally disassembled and assembled a number of Elgin 16s/7 jewels watches as I embark on this new hobby of watch repairing.
After looking though various catalogs, manuals, etc. I do not know how to identify and purchase the right size replacement mainsprings for these PW's. Most of them are 290 Grades (most common)
I understand the serial numbers on the plates, etc. but can't find a way to cross-reference part numbers. I have not removed the springs from the barrels and measured them.

Hi Gregory,
I use an Elgin Genuine Material Catalog. The parts in this book go by grade. It says your grade 290 takes an #817 mainspring. If you have any other Elgin watches you would like mainspring part numbers on, e-mail me the serial number on the movement, or the grade.

Gregory,

just to add to Brian C comment a 817 on my chart is 2.60x20x21 1/2" if you order by size,

Hi all- I am in the same boat as Gregory. Anyone know which spring my Elgin grade 315 takes? Should be pretty common. I have a 1924 size 12 15J Elgin - says 'Diplomat' on the movement (not dial). My research so far seems to indicate a grade 315. Now I am lost in google land searching for new spring.

Wait- in another post here I think I found it.
Elgin No. 2339
Thanks. Marc