Talked to Charlie Pittman Jr. on the phone one Saturday decades ago. He said any motor that has a side load should be ordered with ball bearings. Lionel and MTH apparently didn't want to add the extra two bucks to their products.
If you ever replace a Pittman, always specify rare earth magnets (a 2 or 3 in the third slot), silver graphite brushes, and ball bearings. The 9x34 comes stock with all that if you ask. Not sure they still sell onesies and twosies direct, but when they did, they were half the price of elsewhere.
As mentioned elsewhere, I started out with 9000 series motors, but in the end would only use 8000 series. I would order a box of 30.
