This is a set of eight precision miniature deep-groove ball bearings, greased and shielded with rubber shields, type 625-2RS, made of corrosion-resistant chrome steel. They are used as V-wheel and smooth idler bearings in the Shapeoko and other MakerSlide applications. With the exception of the single-bearing V-wheels in Bart Dring's early designs of the 2.x Laser, all wheels require two bearings each. The usual MakerSlide carriage has four V-wheels, so a set of eight bearings is just what's needed.
Dimensions: 5 mm inner diameter, 16 mm outer diameter, 5 mm width.
As of February 2017, this is an improved bearing, which significantly less axial play than most 625 bearings out there.
Shapeoko part number: MSK01-07.