Before anything else, the legal caveats:
-- This information is not offered as instruction, I'm simply sharing how I attack an old motor and clean it. I have had good success and no problems doing what I do, but I am not guaranteeing that it will work for you. Please use your common sense and be safe.
-- This information is property/copyright me, may not be copied and/or distributed in other forums without my express written consent.
Okay, the ugly stuff out of the way, let's get started. First, I detach the motor from the machine. On a Nova (or BU or BF), most often it is connected via a three-prong fixture on the side of the machine (note that fixture in the photo is not attached anymore):
Depending on the machine, you may have the motor, the light, and the power cord all hooked into this fixture. Each of these connects to two places: the motor to 1 and 3, the light to 1 and 2, and the power to 2 and 3. Unhook the motor from 1 and 3.
Check that the wires are okay.
In this case, the wires to the motor (left-side set) are fine, but those to the light (set on the right) need attention.
This is how the motor looks detached from the machine. Note the brush cap on top at right. There is a second brush cap directly opposite it, on the bottom of the motor.
Another view of the brush cap... also note the oil port, the small hole just above the dimple on the end of the motor. There is another one on the other side. For general maintenance following cleaning, add a drop of oil in these ports twice a year or so. It's particularly important not to over-oil a motor; too much and it'll burn, at the least smelling bad, and at worst damaging the motor.
I begin by taking out the brushes. Unscrew the caps, being sure to hold them with your finger or the spring under the cap will send cap and spring flying to parts unknown. Half the time, the brushes are stuck in the channel, and all that comes out is the spring. That's okay, we'll get at them once the motor is apart.
Unscrew the long screws that hold the two halves of the motor together...
Gently pull apart the two halves. Chances are good that the washers that live on the end of the shaft will stick and/or fall inside when you pull the shaft out...
You can see them in this picture... I use a tweezers (from my serger) to remove them, trying to keep them in the order they fell off the shaft, so I know how to put them back together properly.
I also use those bent-end tweezers to push the brushes out (remember, they were stuck in the channels?). You can see the end of the left channel clearly in the picture above, it's the small brass box with the wire attached, just left of center. The other is directly opposite. Push the brush (a chunk of carbon) back out the channel, on each side.
The squared-off chunks of carbon above are the "brushes." (No, I have no idea why, but I'm pretty sure it's a guy thing <g>) Ideally, the springs are firmly attached to the rounded end (I've pushed one back on in the upper brush), making it easy to pull the brush out of its channel (to check length) without taking the motor apart... but in an old motor, at least half the time the brush gets stuck and the spring comes out alone. So it goes.
Brushes should be at least 1/4" long, at which point they should be replaced. You can get them at any decent hardware store, they are not "special" equipment. Just bring the old one to match it as closely as possible. They are soft carbon, so if you can't get one exactly the right size, it's easy to sand it down to fit.
Those brushes got stuck in their channels because the channels are dirty... so I clean them with contact cleaner on a Q-tip. Be sure to let the contact cleaner dry thoroughly before replacing the brushes. (I am told by Roy Lowenthal that some "contact cleaners" leave an oily residue, and that "Brake & Parts cleaner" from an auto parts store doesn't. The gold standard is anhydrous isopropyl alcohol, realistically, 90% isopropyl rubbing alcohol works about as well and is easier to come by.)
I also clean the brushes themselves, the sides with a paper towel and the concave ends with a round honing stick. Gently, just enough to remove any oil and glaze.
Next thing I do is clean the commutator (pronounced com-you-tater, it's not a "communicator" <g>). A commutator is made up of copper plates separated by spaces, called micas. In the picture below, the commutator is the round thing with the thick dark stripe around it. That black stripe and the black in the micas is carbon from the brushes (and also gunk on most old machines). The carbon needs to be cleaned off the copper plates and from the mica channels for optimal motor performance.
I use contact cleaner on a Q-tip for my preliminary cleaning, it gets a lot of the carbon and grime off easily.
Next, I clean the remaining gunk, and also polish out any gouges or scratches on the copper plates using a honing stick (which I got from Ray White. Take care when you polish that you are not flattening the brass plates, but maintaining their curve. Gently does it, don't get aggressive removing copper. It's easiest to turn the motor shaft and polish as it turns. (In some machines with an internal motor, e.g., the Viking 6570, the commutator is accessible with the motor installed, so you can hold the honing stick against it while you run the motor.) The copper plates don't have to be bright and shiny, just get the dirt and grime off.
Next, I clean the carbon and crud out of the micas, the gaps between the copper plates. The idea is to clean cleanly along the sides of each copper plate, leaving a squared-off rather than a V-shaped gap between plates. Don't use a sharp tool like the one pictured below (one of these days I'll take a new picture). Roy Lowenthal tipped me off that an X-acto "Razor Saw" is the ideal tool for cleaning these channels, it's exactly the right width and the teeth are alternately set.
Finally, I do a finishing light polish with a swipe of the honing stick (to take off any burrs I might have raised on the sides of the micas), and a thorough wipe clean with a Q-tip and contact cleaner. The idea is to get rid of any particles (carbon, copper, and/or iron/aluminum from your abrasive) that may have deposited in the micas (gaps) during cleaning. Those elements are electrically conductive, and since the purpose of the gaps is to interrupt conductivity, if enough of them collect, effectively there will be no gap. Let the cleaning fluid dry, or dry it off with a paper towel, and the commutator is good to go.
My last step on this half of the motor, I put a drop of TriFlo oil (you could use sewing-machine oil, but I prefer TriFlo which has Teflon in it, and also has some cleaning properties) on the end of the shaft, where it rotates. Again, be careful not to over-oil, and not to get oil on other parts.
If the shaft does not turn smoothly or feels gritty, then I take it apart and clean it rather than just adding a drop of oil as shown above.
The commutator/shaft is held in the motor housing by the motor pulley on the outside of the housing...
The pulley stays put courtesy of a set-screw, tightened down on a flat part of the shaft. Note also the washers on the shaft; they need to be cleaned, too. Again, note their order if you take them off to clean, so you can get them on again properly.
Clean the gunk (I like kerosene) off, rub a film of TriFlo on the shaft and washers, and reassemble.
At the other end of the shaft, on this motor, there's a bit of rust. I'll polish that off with some polishing paper, and apply a thin film of TriFlo.
Put the washers back on the other end of the shaft (thin film of TriFlo, again not too much, especially at this end, next to the commutator)...
Add a drop of oil into the cup/dimple the shaft will sit in...
Then screw the two halves back together. Be sure that both motor-mount holes are on the same side when you screw the housing back together.
To put the brushes back in the machine properly, think about where the commutator is, and note the curve on the end of the brush. That curve was created by the brush riding on the curved commutator. Slide it into the channel positioned accordingly, and replace the caps. Don't over-tighten the caps, they're old plastic and will break if you overdo it.
I've got a rig in my shop to test that the motor works, before I go to the trouble of putting it back on the machine. A foot controller with a couple of alligator clips spliced in.
If you're still with me, I do hope this information is of some help as you decide your own approach to refurbishing an old sewing machine motor.