XD-15 Ceramic bearings provide superior smoothness, reduced weight and extended bearing life A CN (tight clearance) upper pulley bearing maximizes shifting accuracy while a C4 (loose clearance) lower pulley bearing promotes unconstrained chainline movement AB0030: 12t pulley set with 4mm upper & 5mm lower boltholes is designed as direct fit upgrade for SRAM 12 speed road and gravel derailleurs AB0031: The 11t upper and twelve tooth lower size and profile with 5mm boltholes are designed as an upgrade for most 11 speed Shimano GRX derailleurs Lifetime warranty Third party online sales strictly prohibited by manufacturer