Box Two™ 24mm External bottom brackets feature cold-forged, CNC-machined alloy cups, dual-labyrinth bearing covers, and precision sealed bearings. They are the perfect match to Box Three™, Four ™ cranks, and the ultimate upgrade to the budget BBs sold with most cranksets. Box Two™, 24mm External Bottom Brackets, are designed to fit all 24-millimeter crank spindles and frames with 68 or 73 x BC 1.37-inch bottom bracket shells.