Run an electrical extension cord around the entire path (or string, but the thicker wire is more accurate) overlap the ends and mark, then remove and measure.

Thanks, SteveH....good idea :)

go to the sm shop and buy a 13-5/8 and keep the receipt, try it, too loose, exchange for the next size down,...
you get the idea. But actually wrap the pulleys with something like a clothesline rope, and make 2 marks where they overlap, unwrap and measure.

I know some of you can help me with this: I just purchased a nice little featherweight (finally!). She runs great but has a few little spots of rust on the bobbin casing and on the main shaft underneath. Please tell me how to remove the rust spots. Thank you all for sharing your knowledge!

I use Brasso and a toothbrush or on stubborn spots a brass bristle brush.

Others use Wadding Polish (Automotive stores)
Others use Evaporust

My Video Arrived!!!!!

74 minutes of Silent film glory....

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I think that could watch this video 100 times before boredom would set in....

Watching the sand casting of the arms....
Watching the Japanning process....
Watching the decal application...
Seeing the MOUNTAINS of hardwood stacked as far as the eye can see.....
(I know why Scotland has no forests left now...yikes)
Watching them "fretsaw" three table tops at a time...
Watching them make and test the accessories....
Seeing the QA process.... (RPM testing the motors performance and testing accessories!!)

Seeing the Queen Mary (ocean liner) being built in the background!!!!

Just too much.... (goofy grin)

It was SO worth the money.

and others (ok just me) use a cheap Harbor Freight ultrasonic cleaner with Evapo Rust, letting it soak after it shuts off, overnight. followed by a some time with a 4" dia buffing wheel and some white buffing compound.
Steve H looks like video is something (else)'worth checking into

I talked to the lady who traded me the Singer 15 treadle and did not know it wasn't a clone. It is rather loud and clackity, so she is willing to trade it for one she knows for sure is a clone. I went and looked at it last night and it is a beautiful machine and treadles so easy and so quiet! Is this a good trade? I do want to, but would like your opinion on it first. Especially Miriam and oldsewnsew..you two seem to know a lot about these lovely old ladies. Thank you for sharing your knowledge!

I'm just a beginner, these folks are way beyond me when it comes to machine knowledge. I'm just into the fixin up stuff. But thanks for the vote of confidence.
Post a pic, I'm confused, you're trading for a clone?

I'm thinking the above is a pic of a Featherweight. Then you posted a picture of a Singer 66 on the clone page. Can you show me a picture of the two machines you are wanting to compare? I have a Singer 15 on a treadle - it has a heavy hand wheel so moves very nicely in a treadle. The clone would have to have a heavier wheel to move as smooth as the Singer 15. If you are using electric the clone should be fine. If you are treadling you want one with a heavy wheel. Have you given the machine some Tri-Flow? Some times they just need a few drops and some times they need a good cleaning of dried oil. If an older Singer 15 is on a table top without something to raise it you would be lucky to get it to move at all - they have parts that go down below the risers when the machine turns.