Replacement Bearings for Cane Creek Headsets.Cross compatible between all Cane Creek headsets. This means you can upgrade your 40 Series headset to a Hellbender or AER bearing etc.AER - Lightweight hybrid Steel/Aluminum. Maintains the quality and durability our bearings are known for, while shaving 35% of the weight.47mm (1"1/4) Single AER (Hybrid Alu/Steel)