This single crankset features a forged alloy crank arm construction, which makes it great at handling all the abuse that's associated with all-day epics in the mountains. Shimano has also kitted it out with a HollowTech II integrated bottom bracket for increased stiffness, efficiency and additional durability. Item Top Features Continued: Chainring BCD: For use with 148mm O.L.D. frames 5 Arm or 4 Arm: 4 arm Chainring Mount: Direct mount Design: 2-piece Dyna-Sys Compatible: Yes Compatibility: 12 speed Dyna-Sys MTB HG-X transmissions Is BB Included: HollowTech II integrated bottom bracket Part Numbers: 30T 170mm: FCM6120C0; 32T 170mm: FCM6120C2; 32T 175mm: FCM6120E2; 30T 175mm: FCM6120E0 Technologies: HollowTech II: HollowTech II integrated bottom bracket design uses a wide bearing placement with a large axle diameter for increased stiffness, efficiency and added durability.