If you look at the roller skating history years ago, when we used Jam Nuts instead of the Nylon inserted Lock Nuts used today.
A Jam Nut must be locked down to keep from loosening and coming off.
To achieve good Roll from this type of configuration, the sides of the spacer must be absolutely parallel, without any burrs and just
a couple of thousandths of an inch wider than the web (that smaller center portion) of the wheel or space between the Bearings.
The sizes of the web of the Wheels vary between manufacturers and even between various wheels from the same manufacturer, in some cases.

Many years ago, the wheel manufacturers used to supply the Spacers with the Wheels.
When the Axle Nut is tightened down on the wheel (with bearings and a spacer), it creates a solid boss of material
(the inner race of the inside Bearing, the Spacer, and the inner race of the outside Bearing)
that effectively increases the Axle size from 7mm to approximately 14mm.
The increasing of the Axle size greatly reduces the possibility of bending or breaking any Axle.
By locking the Bearings and Spacer down, it also assures that the center race of each Bearing is not spinning on the Axle,
but that the balls of the Bearings are actually doing the work that they are intended to.
Tightening the Axle Nut on a Wheel without a Spacer can severely mis-align the center races of the bearings to the outer races
of the Bearings by compressing the inner races towards the center of the Wheel.
The Locked Down approach also requires near perfect alignment of the Wheel bore and the web of the Wheel must have very
clean square corners to provide a seat for the outer race of the Bearings.

Today, Lock Nuts are accepted, most Wheel manufacturers do not even supply the Wheel Spacers, and the tolerances on the width
of the web of the Wheel are not held as tight as they once were.
To my knowledge, Roll-Line is the only one still making Spacers for Wheels.
They normally come with a complete NEW Frame as an accessory.
Rollerskateusa.com does have spacers available in sets of 8 from Roll-Line.
(Just a Note: not every spacer will make every wheel work properly. Sometimes there are spacers that are non-parallel,
have a burr, or are just too small to match the web of a specific Wheel.)

The spacers have always been a part that the manufacturing cost must be minimized, or for some manufacturers, completely eliminated.
They are made from a long tube that is sawn into the Spacers, and the tolerances are not usually as rigorously controlled as on other parts
of the Skate. To get the Spacers, Bearings and Wheels to work, means sometimes using multiple Spacers to get a specific wheel to roll properly.

When everything goes together well, there should be little to no variation in the speed of a free spinning Wheel
(with the Axle Nut (Lock Nut) is just backed off) and when the Axle Nut (Lock Nut) is tightened down.

To achieve a Free Rolling complete set of Wheels in this manner will take time and patience and plenty of Spacers along with a good Bearing Press
like the our new Roll-Line Bearing Press/Puller.