The Hollowtech II arms have been reshaped with an asymmetric design that's different yet evokes the sophistication of XTR. The right arm is pressed together with the hollow 24mm spindle at the factory, and the splined left crank arm mates to the spindle during installation and is secured with perhaps Shimano's best detail — two pinch bolts to secure the arm once the bearing preload is dialed in perfectly. In our experience, it's a superior design in terms of installation, performance, and durability. The bottom bracket is improved, as well, with lighter weight, improved durability, and reduced seal drag.