What I think would make it a project I wouldn't (at this point) tackle would be:
1. Major wiring problems like bare or broke wires inside the motor. Mostly I'm just not sure what would go where, so that's a little beyond me right now and I certainly wouldn't want to make anything dangerous!
2. If all the little coils of copper wire on either the magnet or the armature were messed up. If they were broken, or if it looked like some spots were melted or burned.
3. If the commutator (the copper "barrel" looking thing on the end of the armature) was broken or totally worn down to whatever material it is covering.
4. If any of the mechanical parts, such as the shaft of the armature, were broken.
5. Remember that small parts like the little motor pulleys, carbon brushes, electrical plugs, screws can be replaced. I think this would be important if you wanted to keep an original motor with a machine because of color or branding. Some repair shops might be able to re-wrap the copper coils if something is wrong with them, but I have no idea if it would be cost prohibitive or even, really, if it could be done.

That's just what I can think of right now. If anyone else has any specifics to look for, I'd love to know.

That was so nice of you to take this time for a great tutorial!!
Thanks so much- lots of us are on that vintage machine bandwagon!! :D:D

Thank you for showing your expertise! Just think of the machines that could run again and have a good home....they are just thrown away. Thanks to you, I will be looking at sewing machines in garage sales more carefully....we appreciate the time it took to post your info.
Diana

Thanks. That's a very good detailed instructions. Hope a lot of sewing machine mechanics see it.
Now I see why they charge so much.

Thanks for the tut. I've never seen a motor apart like that. Judy

Are all carbon brushes created equal, where would I buy them if they had to be replaced. All my machines are vintage so I'm sure they need some help. I haven't sewn for 20 years and I'm not sure what to do to the machines before I start to use them other than looking at the wiring and oiling them.

Usually cleaning and oiling is just about what you need to do. :) Most of the machines that I use, I haven't cracked open the motor. I have just had a couple here lately that are so nasty and filthy outside, that I knew the motor probably sucked in a lot of that stuff too.

I have seen carbon brushes at a couple of sewing machine parts places, like Sew Classic, sold either as a pair or individually. I think it's one of those parts where carbon is just carbon. I haven't had to replace any yet, so I haven't had to buy any. I think as long as the little springs are not broken and they have enough of the carbon "block" left to make contact with the commutator, then they are good to go. Also, clearing off the grime helps them conduct the electricity better and help your motor work better. The first motor I played around with, the brushes were fairly worn down, but still good enough that I didn't have to replace them. That was from a 1931 White, if that tells you anything!

Thanks so much for the great tutorial on cleaning the motor, it's terrific to see so many pictures included. And thank you for getting back to me about the brushes.

I haven't touched my machines since I got married, now that we've gotten past the first 20 years, we just bought a house and I'm about to have my first sewing room, once I get everything straightened away in there I figure he can play on his computer and I can play with the vintage sewing machines I've collected since we married. So it should be just a matter of cleaning and oiling I hope. I can't afford to have them all tuned up. DH bought me a treadle that definitely needs some loving care but there's no rust just accumulated dust from sitting in someone's nice dry attic for a lot of years. One of the reasons for getting the vintage machines is that you're supposed to be able to maintain them yourself pretty much (once you learn how).

Thank you for posting this. I took the motor off my pretty turquoise Universal becauue it needs a new cord, but that's as far as I got. Your timing is perfect. I wonder what gauge cord I need to buy so I will be ready Thanksgiving Day after my guests leave. I guess I'll take the motor to Home Depot to see my guys there can help me.