The NordiK Supersonic bearings have integrated spacers built in which allows the wheels to always be in alignment. The Supersonic bearings let you tighten down the axle nuts while keeping your wheels rolling perfect. These bearings are designed to allow your wheels to go as fast they possibly can, and to spin longer by cutting down the vibration and friction. The self alignment feature also cuts out the rattling that most bearings do. They also feature removable shields which makes cleaning them quick and easy. The Supersonic bearings are fast, strong, long lasting, and a great price for what they offer. 8 bearings come in a set. These bearings are Abec 7 rated, and are 8mm steel bearings which is the industry standard.