When the front wheel bearing hub is assembled, the bearing outer races are pressed firmly against shoulders inside the rotating hub. There is an inner race spacer between the bearings that is exactly same length as distance between the shoulders in the hub. This allows the bearings to spin freely with no preload. At the inboard end there is another inner race spacer that also serves as a fixed seal surface. From inboard to outboard you have the seal spacer, inner bearing inner race, inner race spacer, outer bearing inner race, heavy washer, and the castellated nut (with split pin locking). When the nut is tight, all of these pieces are pushed together as a single solid assembly, making the spindle assembly much stronger.

Generic dimensions (in inches) for the front wheel bearings are:
Bearing - front hub outer, 1500/1600
ID=0.7874" (20.0-mm) -- OD=2.0472" (52.0-mm) -- W=0.5906" (15.0-mm)
Bearing - front hub inner, 1500/1600
ID=1.1811" (30.0-mm) -- OD=2.8346" (72.0-mm) -- W=0.7480" (19.0-mm)