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⢠and Four ⢠cranks, and the ultimate upgrade to the budget BBs sold with most cranksets. Box Twoâ¢, 24mm External Bottom Brackets, are designed to fit all 24-millimetre crank spindles and frames with 68 or 73 x BC 1.37-inch BSA threaded bottom bracket shells.