The Shimano Dura-Ace BB-7900 Bottom Bracket is a simple and straightforward component. You'll get two cups with low-drag, high-precision Dura-Ace bearings, plus a cylindrical dust cover that mates the two cups. What's the difference between the Dura-Ace BB-7900 bottom bracket and the previous generation BB-7800? The primary change is in the bearing seals -- they have a new design to reduce drag, to create smoother rotation, and to add durability.