I've replaced mainsprings in a NY Standard 18s and a Seth Thomas 18s with used Waltham 18s springs. And they both seem to run okay! This was just an experiment to see if it would work, and it does. I full intend to change them out.
Just wondering what the consensus is on this? Looks to me like if it fits it will work. Correct tensile strength, length, and other factors can result in inaccuracy I'm sure, but appearance wise they all look very similar.

My opinion . . . Yes . . . usually . . . if . . . the thickness and width of the "new" spring are the same. Many, including me use 18s double brace Elgin mainsprings which seem to have a usable width fitting many other models. The double brace design leaves little really that cannot be facilitated with the outer end parts of that Elgin mainspring.

If the spring is in good condition and the watch is adjusted with that spring the only possible variation might be the run time.

16s watches have more variable widths and can be problematic for other reasons. 48 and 60 hour Illinois springs ARE NOT interchangeable!

There is a interchangeability chart that was made available of all the different mainsprings. I forget who was emailing them. I have uploaded it to my Google drive as its a picture thats just to large for the forum. Mainspring Chart

Thanks Dave and Jim,
I tried the chart but it requires a password.

It shouldnt need a password. I have set it as open to the web so try again Dave.

Jim, it opened for me but it was too small to read.

Did you download the file from the File menu, or right click on the image and save it?

Jim,I just right clicked and saved it, and it worked. Thanks.

Jim,
No matter how I try to save the spreadsheet, all I get is a google login page, asking for a password.
Could someone please attach it to an email and send it to me.