On your 671M-1 motor, I checked some of the parts in the Lionel Engineering Standards. The rear bearing is 671M-20 and, by design, has an ID of 0.191”. The front bearing is 2020M-20 and, by design, has an ID of 0.1875”. The armature is 671M-6 and has shaft 671M-7. The design of the worm on this shaft shows a Do (maximum diameter) of 0.186”. So the worm should fit through both bearings, but the front bearing is tight. You may want to examine the worm to see if it has developed a slight bur or lip at the edge where it slides on the worm wheel. If so this could be cleaned up with a fine flat file.
The balls that belong in the motor for the thrust bearing are 671M-21. They have a diameter of 0.0625. The caged thrust bearing is 681-121. The balls in this bearing are 0.062” in diameter. These parts should be interchangeable if the OD of the cage fits in the housing. With these tiny balls wear is possible. When I work on a motor I like to change all of them to be sure they are all the same diameter and will share the load. Mc Master Carr carries precision balls in this size for a very reasonable price.
