If you haven't already, check out The Vintage Singer Sewing Machine Blog. (http://vssmb.blogspot.com/) Rain has a very detailed tutorial on how to re-wire a potted motor. I used it again this week as I was working the the Pittsburgh 201-2.
One thing that I do differently. When I am putting the armature back in the motor and lining up the worm, I take out the screw in the end of the motor.
It is important to line the set screws up correctly when re-installing the worm. I completely removed one of the set screws in the worm when I took the motor apart. It just happened to be the one on the flat. The flat on the armature shaft matches the flat on the armature.