Yes, of course, you're right. The shaft does not bear against the ID of the thrust bearing. I could replace the thrust bearings with regular ball races, but don't know who well they would stand up to the thrust.

I'm planning to use dual 1/4" x 20 set screws at 90 degrees on the outer collars to maintain pressure against the thrust bearings. I'm kind of worried that these might keep working loose, especially after your experience on the ball nut mount...

Dang, nothing's ever simple, is it???

Chuck