Loose Ball Bearing 1/2" inch = 12.7mm Ceramic Balls ZrO2 G10 One Ball Loose Ceramic Balls 1/2" inch 12.7mm ZrO2 (zirconium Oxide), this is a popular size that could be used in many Bicycle, or any application that uses this size 1/2" Inch. Balls are made of Ceramic ZrO2.