Ultegra FC6600/6603 Crank Arm Fixing Bolt. Also works for the Shimano R600 crank. 1 per crankset, fits FC-6600/3 FC-5600/03 and FC-R700. "B" fit for FC-M600, FC-M580, FC-R600, FC-4500, FC-4550 Shimano # Y1G311000 | Shimano Crankset Arm Fixing Bolt Ultegra 6600/6603, 105 5600/5603