This spacer is for the Gen-B versions of O-Chain produced by SRAM. These newer versions of their O-Chain have a slight interference with our MTB Titanium, R-Spec, and Zink cranksets. This 1mm spacer fixes that issue, just install it onto the drive-side crank arm before installing your O-Chain spider and you're all set. 

Two spacer versions available, the 3-Bolt version is for our R-Spec cranks and the 8-Bolt version is for our MTB Titanium and Zink cranks. 

Per SRAM: "To differentiate between legacy and updated SRAM Ochains, please check the model code on the back of your Ochain, updated models are marked “B1”, while earlier models have a numerical model code (01-05), with “05” models also designated as “A1” in SRAM service documents.”