Fit Perfectly - ISO 492:2005 Standard, The aperture tolerance range is controlled between 7um, make sure fit great. More than 20 steel balls are filled inside to provide maximum load. 80% of Double-Sided Grease - Pre-filled with high-quality blue grease for increased life. Dimensions Correspond - Inner ID 19.05mm, Outer OD 31mm, With B 7mm, match the point of the bike. Specialized recommends you replace these about once every 6 months with agressive riding and once a year for passive riding.