ROTOR PF4124 BB. You had your choice of any bike in the shop but you made your selection based on potential upgrade options. The Press Fit 4124 will easily accommodate your handpicked rig with its press fit, 41mm diameter with 24mm axle BB shell. ROTOR conceived its Universal Bottom Bracket (UBB) concept to eliminate the complicated bottom bracket/crank compatibility challenge that has increasingly frustrated the mechanics at your LBS.