ACB 4545 S8551 BO 1-3/8" Angular Contact Black Oxide treated Headset bearing (36.8mm x 45.8mm x 6.5mm: 45 x 45 deg) -

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 performance

This size bearing is also known in the bike Industry by these part numbers:
S8551
S182500006
S172500014

The Enduro line of angular-contact headset bearings start with Black Oxide treated Chromium Steel races, steps up to full 440C stainless steel, and theop of the line is, lifetime-guaranteed, XD-15 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, decreasing friction, minimizing wear and maximizing life.

Static load - 660lbf / 2.93kN
Weight - 20.05g