Box Two External bottom brackets feature cold-forged and 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 Two 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.