Common BMX bottom bracket sizes are US BB (51mm), , Spanish BB (37mm) , Euro BB (35mm Threaded) or China BB (31mm) and roughly describe the outer diameter. In addition, you should know the diameter of your crank spindle. Common BMX crank spindle sizes are 19mm, 22mm or 24mm . To find out what bottom bracket size you have, you can simply measure the diameter of the bottom bracket tube on your BMX frame and crank spindle.