Sram Spare - Rear Derailleur Chaingap Adjustment Gauge - B Gap Tool - Compatible With All Etap AXS Cassettes 26T-44T 2021:

OVERVIEW
The new B-gap adjustment tool is simple and easy to use, allowing for a quick set up experience. Small enough to be able to pack in your ride bag or stash in your toolbox for repeated use, it is also industrially recyclable if you find you no longer have use for it. This is the most precise tool for setting up and adjusting your drivetrain.