Over time derailleur pulleys wear out. Either the bearings go bad, or the teeth wear down enough to negatively affect shifting performance. 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 most out of your drivetrain. These are genuine Shimano replacement pulley wheels for the Dura-Ace road rear derailleur, sealed bearings included. Be sure to select the correct option for your derailleur, whether it's Dura-Ace 9100-series or Dura-Ace 9070-series. Each kit includes two pulleys, enough for one derailleur.