The following is from a forum member that I saved. Hope this helps. I also saved the articles from OGR, but, don't have them on a computer file.
I recently did this repair and learned a couple of things. Bear with me, it takes a little explaining:
Not long after I saw this thread, my sister's Rock Island 4-8-4 started acting up. It had always been noisy, but now it would sometimes stutter and refuse to move. Pushing it a short distance would help. I stuck my little finger into the space between the ends of the stationary field laminations and pushed on the armature. Bonk! Bonk! Sure enough, obvious play, and the source of the noise.
I tore the engine apart. The worm gear looked OK, so we just ordered new bearings, as shown in the repair article. And got the wrong ones. After some sleuthing, I realized that that the diagram in the article, which is the same as the diagram in the Greenberg's guide, is in error:
There are two problems here. The first is that the "Front Bearing," part number 2020M-17, is shown on the wrong end of the motor. It should go on the gear end, which is toward the front of the engine. Not too a big deal if you are ordering both bearings.
The bigger mistake is the "Rear Bearing," part number 671M-20. This does not fit the LTI Northern. If you try to install it into the Northern, you get this:
That's the 671M-20 bearing sticking out of the motor housing, with the ball bearing race ("Thrust Bearing," no. 681-121, in the diagram) between it and the armature. Below is a photo of the 671M-20 (left) next to the bearing which we eventually installed. A big difference!
I seemed to remember hearing that the horizontal motors made prior to 1950 did not have the ball bearings contained in a race: they were loose in the motor. In the picture below, you can see that the 671M-20 is counter-bored, I presume to trap the ball bearings against the armature shaft. So it seems that this rear bearing is correct for pre-1950 engines only.
As you can see in the photo, the correct rear bearing is part number 681-120. The clue to that was the number of the ball bearing race (a.k.a. "Thrust Bearing"): 681-121. When assigning numbers to new parts, Lionel tended to incorporate the catalogue number of the item in which the part was first used. So the race was new in 1950 with the 681 Turbine. Trying to put the 671M-20 bearing into the Northern made it obvious that it and the race were not designed to be installed together. When the 681-121 race debuted in 1950, a new rear bearing must have been designed to go with it; likely, it also had a number starting in 681. There is no diagram that I know of for LTI's Northerns, but they are clearly based on the chassis used by the 736 Berk and the 746 J. But the Greenberg's repair manual shows the 671M-20 rear bearing, together with the 681-121 race, on those engines as well.
I finally found what I was looking for in the MPC parts lists. The 8002-100 motor is specified for the N&W J reissue; there is no diagram, but there, at no. 5 on the parts list, is the missing link, Rear Bearing, number 0681-120!
So we ordered part number 681-120 from a large broker of trains and parts, which shall remain nameless--and they sent us 681-121, the ball bearing races. They eventually made good on it, but it was a long wait. In the meantime we ordered the bearing you see in the photos from someone else. (Not sure who, as I didn't do the ordering; this all started while I was visiting over Christmas, and we just recently got it resolved.) Moral of that story, asking for the right thing only helps if your vendor is knowledgeable.
So now, if you find yourself doing this repair, you know what the part number really is, and hopefully you will be able to get the right part right away. The 681-120 rear bearing is definitely correct for this engine, and should be correct for the 681, 682, 671RR, 736, 726RR, and 746, as well as their MPC and LTI clones.
Oh, just one more thing...
My sister's engine is back on the rails, and runs more smoothly and quietly than it did when new.