Originally Posted by
Hondo6
If the cups are still good, you may be able to replace the spindle with a 5- or 7-series JIS square taper spindle - if you can find one with the correct length - and use 15/64" or 6mm bearing balls.
This may work, but not well or for long.
Smaller balls will change the contact angle, making it less in line with the principle forces (radial for pedaling, not axial for locating the axle), so increased wear and chance of failure.
Which of couse also means the ebaring tracks will be higher up on their curves, and that portion of the axle/cup may not be finished.