Enduro Specialty Headset Cartridge Bearings. Features: Sealed chromium steel bearing with black oxide treatment on the outer surfaces of the bearing races AB-2050 is the upper headset bearing for Trek Speed Concept frames and has dimensions 14.2x25x6.5mm, 45x45 deg AB-2052 is the lower headset bearing for Trek Speed Concept frames and has dimensions 27.1x38x6.5mm 36x45 deg AB-2054 fits Felt TT frames and has dimensions 19x30x6.5mm 36x45 deg Specs: Description:ACB 6805 black oxide Fits:Trek Speed Concept Package:single Upper Lower:lower