Bearing spacers precisely aligns bearings, wheel and axle to improve glide speed and free spin. Aluminium provides much better performance than plastic.
Long Sleeved Spacers are for 6mm axles usually used by K2 Skates
UFO Spacers are used for 8mm axles usually in most skates. Spacers come with extra ring to help align holes of the wheels and bearings together. Making it easier to assemble the wheels.
Moving Spacers are used usually in skateboard and scooters. It is also great for metal core wheels. As bearings in metal cores are stuck, the moving spacers are used to knock the bottom bearings out without damage.