I had to replace one of the Alt bearings in my Cube E mount (it was “grinding” and causing star trails at 1-5 second exposure times due to vibration). I made a video of the procedure here:
The iOptron Cube E 8500 that I have uses 2 sizes of bearings:
1x 6804z bearing (20x32x7mm) for the Alt axle closest to the telescope.
3x 6803z bearings (17x26x5mm) for the ALT axle nearest the “lock” handscrew
and for both the top and bottom of the AZ axis in the bottom.
I purchased and used this NSK brand bearing.
You’ll also want a 14mm or 9/16th box end wrench to remove the AZ axis bolt head if you need to access the bottom.
Before/After results (click to enlarge):