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 bolt-holes 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 bolt-holes are designed as an upgrade for most 11 speed Shimano GRX derailleurs. Lifetime warranty.