30Pcs 693ZZ Ball Bearings, High Speed Silent Small Bearing Deep Groove Double Shield Bearing Miniature Carbon Steel Bearings (3mm x 8mm x 4 mm)

Valued seller, quality goods

Great value for the money, used them in 3D printed toys.

I'm using these as rollers in telescope focusers, and they seem to work well. Perhaps not as precision made as similar products costing ten times as much, but with six bearings per focuser and a saving of £2 per bearing, I'm fine with that. I haven't tried spinning them at 30,000 rpm, and don't intend to.

just what I needed

I measured the ID at closer to 1/8th inch. (3.175) so they're a little slack on a 3mm shaft. They do seem to be smooth and effective though, but you might need to shim the shaft if you're after a precision fit.