If you're lucky enough to make it a few seasons without destroying your rear derailleur from a rock strike, unfortunately the pulley wheels will still wear out. If your bearings are feeling notchy, or the pulley teeth have rounded out, it's in your best interest to replace them to get the best shifting performance out of your drivetrain. These are genuine Shimano replacement pulleys for the XTR M9000 11-speed mountain rear derailleur, sealed bearings included. Each kit includes two pulleys, enough for one derailleur.