There are 2 hub thread diameters for bmx. The smaller of the 2 (don't remember the numbers...damn brain) is exclusive to BMX hubs and freewheels go down to 12t. The standard thread...1.37X24 is what you will find on flip-flop hubs. The smallest I have seen is 16 but I think you can get a ****mano in 15t. Thats the smallest you will get on a standard hub thread.