Fsa is the world leading headset manufacturer and the core of their product line with many years of experience they understand that multiple dimensions and exacting requirements are a must when it comes to headsets the fsa orbit c 40 acb tapered carbon headset is an integrated headset to fit head tubes that are tapered the bearing are 1 18 45 x 45d upper and 15 36 x 45d lower angular contact bearings that sit flush in the frame with head tube outer diameter of 46mm upper and 56mm lower