Ole, respectfully, when a bearing fits tight into the wheel bore, the center spacer has to be absolutely precise, otherwise torque applied to the axle nuts will force the inner bearing to interfere with the outer race because while the outer race is resistance to movement: Pressure is being applied to the inner race to "crush" the center "spacer," but the outer portion of the bearing is not being pulled in. I spent a long time removing .001 at a pass trying to get this perfect fit with tight bearings, and it takes a long time.
What about when bearings are seated in a wheel bore. Theoretically there is zero tolerance between bearing centers and the ends of the bearing spacer. How do you achieve that without leaving some fraction of bearing out of the seat? Who is achieving a zero tolerance interference on these spacers? Wheels that use bearings with flanges are even more difficult to get perfect, because the bearing may or may not be bottoming out within the wheel bore.
Yes, my motorcycle restorations use these spacers, but they are precise lengths. They are also a heck of a lot less robust then what some of these people recommend be used on mini bikes, and the bearings in my vintage dirt bikes are a lot less robust than what we use in mini bikes. When mini bikes start doing 65 MPH, weigh 400 pounds, and encounter leaning turns commensurate with a motorcycle, I'll start using center spacers. Most, and by most I mean every mini bike I've restored, save one- did not have center spacers. I don't know why this continues to be in issue here, since the vast majority of mini bikes never had them, nor needed them in the first place.
For those that are concerned about tightening axle nuts, use a lock nut and double nut the opposite side to get it snug, then repeat for the other side. Now loosen either of them, and spin the wheel, then apply torque until there is no lateral movement, but so as not to restrict the bearing movement. Very easy, and this is how vintage mini bikes for the most part were assembled.