These nifty shims allow you to fit a hub with 40mm bearings into a frame designed to accept 42mm bearings. Very handy indeed!
CNC-machined aluminum spacers adapt 40mm bearings (also known as 6203 bearings) to 42mm ISIS bearing size. This allows you to use a hub with 40mm bearings, such as a UDC Chromoly Hub, in a frame such as Nimbus II ISIS frame, or a KH frame.