ACB 3645 1125 SS - 1-1/8 stainless steel, Angular Contact Headset Bearing

ACB 3645 1125 SS - 1-1/8 440C Stainless Steel Angular contact Headset Bearing (30.5mm 41.8mm x 6.3mm; 36 x 45 deg) -

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

Not all stainless steel is created equal.
Enduro only uses 440C stainless steel balls and races to provide long life and superior corrosion resistance in applications and environments that may be less than ideal. 440C has the highest strength, hardness, and wear resistance of all the standard 440-series stainless alloys with high carbon content. As expected, it is also the most expensive of the 440 series alloys and requires multiple rounds of heat treatment and cryogenic treatment to achieve its superior mechanical properties. Though stainless steel has 20% less load capacity and is slightly softer than our standard 52100 chromium steel used in most of our bearings, stainless steel is an excellent alternative for use in wet conditions, if you wash your bike often, or where sweat can be a problem. Enduro Bearings also uses LLB type seals and CRC Marine grease in these bearings to additionally water proof against the elements.

The Enduro line of headset bearings begin with chromium steel, Black Oxide finish A/C type and then offer full 440C stainless steel at the top. They are always engineered with the largest balls possible and dual lip seals riding in a micro-groove for ultimate protection against sweat and the elements. They are filled 90% with high pressure water proof grease, so you never have to guess if there is grease inside. Enduro angular Contact (A/C) bearings, all balls are engaged to spread the force over a larger surface area, pressure water and minimizing wear.

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