This little plastic spacer goes on the drive side of a Sram GXP bottom bracket to cover the bearing and properly fit the crank axle during installation.

One is included as part of the Sram GXP Team bottom bracket, but people often break or misplace them when servicing their cranks, and we've seen many a set of used Omniums get passed around without one.

If you're missing one, you'll definitely know it because there will be side-to-side play in your Omnium or S300 cranks and they'll wobble around like crazy when the left-side bolt bolt is tightened all the way.