The 608 Z bearing is the designation for a standardized type of ball bearing used in inline skate wheels. The numerical designation '608' specifies its dimensions: an inner diameter of 8 mm for the axle, an outer diameter of 22 mm for seating into the wheel, and a width of 7 mm. The letter 'Z' in the name indicates that the bearing is protected on one side by a metal shield. This shield, also called a dust cap, prevents larger debris from entering the bearing and thus protects its internal mechanism with the balls. Two bearings are always required to complete one wheel.