I went and stared at some of my 8" drives for a bit this morning.
It occurs to me that if your top guide frame is laterally "warped", the centering of the hub clamp assembly won't work.
If you've got a wiggler gauge handy, you may want to see what the deviation is when the hub clamp mates with the spindle hub assembly. While you're at it, you may want to check the spindle hub itself to see if it's not out-of-round.
There's a patent 3,898,814 on the Shugart clamping system that many manufacturers licensed. Perhaps it can shed some light.
--Chuck
