Bearing spacers allow for proper alignment and spacing of your bearings. They are especially necessary when bearings are subjected to hard carving and sliding.
The first number shown in the description is the axle size and the second number is the spacer width.
Most skateboard trucks use axles that are 8mm in diameter, and trucks like Bear may use a 10mm axle. System Skate offers a spacer to accommodate 10mm axles.
Most wheels use 10mm wide spacers, and only a few use 8mm wide spacers.
Kryptonics Classic 76mm - 8mm
Kryptonics Classic 80mm - 8mm
Gravity Fu Manchu - 8mm
Never Summer 72mm - 8mm