- 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.