Why the vertical play in the motor stems?The Hurst motors have a single friction bearing at the top of the motor housing; the magnetic rotor hangs below it and is unsupported at the bottom. IME, this is a source of a lot of the vibration in these motors. The 55mm series are supported on both the top and bottom, but the motors have a 0.25" shaft rather than the 0.1875" shaft of the PA/PB series (59mm).
Why the addition of the small brass collars to the motor stems?
The stock Hurst motor has a "C" type snap ring attached to the shaft that rides on the top of the bearing, with a wave type spring washer in between, suspending the rotor at that position. These motors usually have a fair amount of vertical play. I believe the brass collar was a VPI specific mod that allowed you to take up some of the slack in the vertical play and try to dampen the vibration by pulling the rotor semi-tight against the bearing using the wave washer as suspension. On the motors I have, it has limited affect.
IME, the Hurst motors aren't cheesy because of where they are manufactured, they are cheesy because of how they are designed and built. A 3 phase BLDC motor with precision bearings is a much better solution:
http://www.diyaudio.com/forums/analogue-source/288730-3-phase-bldc-motor-turntable-use.html
I still see a lot of apparently unresolved complaints on VPI's forum regarding motor vibration and the Classic turntable. I wonder if the problem got worse when they started using the 49mm series motor which is quite a bit cheaper.
I also have come to understand that the Classic line will be discontinued next year. Coincidence?