ACB 3645 1125 BO - 1-1/8" Black Oxide treated Headset bearing Angular contact Headset Bearing (30.5mm 41.8mm x 6.3mm; 36 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.lack Oxide coating provides an extra level of corrosion resistance and extended worry-free performance. All Enduro Black Oxide headset bearings carry a two-year warranty.

This size bearing is also known in the bike Industry by these part numbers:
IS42
ACB30418K
MH-P03K

This size bearing is also known in the bike Industry by part number HD 1408K

The Enduro line of headset bearings consists of two basic performance levels that are a substantial step up from standard bike industry specifications. Enduro "entry-level" headset bearings feature premium materials like Black-Oxide treated races and carry a two year warranty, while our premium 440C stainless headset bearings carry a full lifetime warranty. 

All Enduro bearings are always engineered with the largest balls possible and feature dual lip labyrinth seals for maximum protection against sweat and the elements and to contain the 90% high pressure water proof grease, keeping everything running smoothly for the duration. The Angular Contact (A/C) design of Enduro headset bearings allow all balls in the bearing to engage the load, spreading the force over a larger surface area minimizing any possibility of premature wear and greatly extending bearing life.

Static load - 935lbf / 4.15kN 
Weight - 19g