I am using 5/16 on my treadle and it's working ok.

Honestly, it depends on the machine. I have machines that have a 3/16, 1/4, and 5/16 pully.

Early machines were made before any real standardization was done.
Later machines were pretty set at 1/4"
Industrial machines tend to use the 5/16"

Yes, any size will "work" but the wrong size will wear out a little faster than the correct size.

I would like to add a bit about the "C" serial number. I recently bought a treadle, the pictures and the ad said it was a C serial number. After getting it home, upon close examination of the serial number and the machine, I determined it was a poorly stamped G and the machine was a 66 if I remember right, which fit with everything else.

Rodney,
Is there a name for the style of decal on your machine? It is lovely!
Dayle

NopahDesertRat: This is a German made machine. I talked with my friend a bit more. I had my story wrong. I thought he bought it to restore but it was actually his great grandmother's machine. They're of German descent and there's a card with a German name and address in one of the drawers that my friend thinks was a dealer tag for the machine. I don't remember if he said she brought it over from Germany but I wouldn't be surprised.

Dalesewblessed: I'm not sure. I'd like to know too.

Rodney

That decal set has two names - Tiffany, or Gingerbread. It's one of my favourites!

Not connected to them but about 3 months ago I suggested to a seller on fleabay that makes repro decals that they should make a 3 color Tiffany set for the the 28/128 machines.. they just released it.

I’ll bet you can get it to sew again. That one looks like my favorite kind of project (rusty & seized). It looks like a Singer 15-30, a really robust sewing machine. I traded the only one I’ve had for a Singer 401A, and wondered if I’d made a mistake while cleaning the 15-30 up and test-sewing with it. Nice stitches.

Most household treadles use 3/16” leather belting. I think that the pulley widths are designed for that size. You can use other sizes, but the larger the belting is, the more effort it takes to bend the belting around the small machine pulley, so you could be causing unnecessary drag.

CD in Oklahoma