Over time derailleur pulleys will wear out, even if the derailleur itself is perfectly fine. Usually it comes down to the bearings going bad, the pulley teeth wearing out, or maybe both. If your bearings are feeling notchy, or the pulley teeth have rounded out, it's in your best interest to replace the pulleys to get the best shifting performance out of your drivetrain. These are genuine Shimano replacement pulley wheels for the Ultegra road rear derailleur, sealed bearings included. Be sure to select the correct option for your derailleur, whether it's Ultegra 6800-series or Ultegra R8000-series. Each kit includes two pulleys, enough for one derailleur.