10 Loose Ball Bearings 1/4 inch = 6.35mm Ceramic Balls 10 Balls Pack of 10 Loose Ceramic Balls 1/4 inch 6.35mm Si3N4, Grade-5 quality balls, this is a popular size that could be used in many Bicycle, or any application that uses this size 1/4 Inch. Balls are made of Ceramic Si3N4.