Assessing mini lathe spindle bearing play questions

I had some misgivings on my 7x and measured the play like you did. I also found it to be quite good, sub 0.0001 TIR. Don't remember the other measurements, but they were more than good enough. So I have left it alone.

Some of my issues were operator error or operator delusional thinking. While one can do creditable work with a 7x within it's operational envelope, it's a fairly limited platform. Once I came to grips with that, my 7x became easier to work with, since I had learned the limits and worked within them.
 
Well, I did what I said I wasn't going to do, I replaced the bearings with a pair of FAG angular contacts :D.

I was planning on replacing the plastic drive gears with metal ones anyway, that requires at least a partial teardown of the headstock anyway, may as well go all the way I figured.

Once I was certain of how angular contact bearings must be oriented in this application, it wasn't any different than rebuilding with OEM bearings, other than the need to creep up on the final seating in order to set preload. I mounted the headstock to the lathe and confirmed a small axial play as well as the sound of bearings bouncing around in the races when spun. Using the c-nut, I pulled the assembly together until the axial play disappeared, at this stage the spindle freewheeled easily. A bit more on the c-nut until the freewheeling showed the first signs of friction. I let the spindle run a bit; freewheeling again became apparent, a touch more on the c-nut took care of that. I think there's still a bit more preload to go, but I'm leaving it where it is for now.

I repeated the radial play test with the DTI mounted to the headstock, indicated off the spindle boss. It is rock solid; total radial play is <0.0002".

Spindle total runout with the DTI mounted to the headstock, indicated off the spindle MT3 inside taper is about 0.0002".

Here's a short video showing the runout (I haven't figured how to turn off audio, may be best to mute your sound):

Angular bearings spindle TIR

If nothing else, I learned a lot about how these machines are built, overall I'm glad I took the dive,
 
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