ACB 6805 CC BO- 1 Angular Contact Black Oxide treated Headset Bearing (27.15mm x 38mm x 6.5mm: 36 x 45deg) - Angular-contact headset bearing featuring anti-corrosion Black Oxide/Magnetite coated 52100 High-Carbon Chromium alloy races paired with grade 10 Balls. Black Oxide coating provides an extra level of corrosion resistance and extended worry-free performanceThis size bearing is also known in the bike Industry by these part numbers:, HD 1320K, 428420, TH-373, MR055, BAA1159, MH-P09KThe Enduro line of angular-contact headset bearings start with Black Oxide treated Chromium Steel races, steps up to full 440C stainless steel, and the top of the line is, lifetime-guaranteed, XD15 aerospace-stainless (races and balls). They are always engineered with the largest balls possible, feature dual-lip seals riding in a micro-groove (labyrinth) and are filled 90% with high pressure water proof grease for the ultimate protection against sweat and the elements. Enduro Angular Contact (A/C) bearings are designed so all balls are evenly engaged to spread the force over a larger surface area, minimizing wear and maximizing life.