In spite of what people think, you can rebuild - to an extent - many Shimano components. The Shimano 7900 Rear Derailleur, while not being completely rebuildable, has many easily replaceable parts. Such are the pulley wheels. Shimano finally caved and ditched the ceramic bushing on it's upper pulley for the new Dura Ace pulleys. Both pulleys now run on sealed cartridge bearings. The upper retains it's floating ability, and both bearings have added sealing to keep them from being contaminated.