I am buying a new bottom bracket for my bmx but i need to find out how do i know measure it.
E-mail the company that made both your cranks and your frame. They will tell you exactly what you need.
It would really help to know the brand, model, and year of your bike. It isn't very often that a BMX needs a new bottom bracket. Do you mean that you want the spindle length? Just take the old one out and it should be labeled. If you are not sure whether you need standard American or mid version or perhaps some other one, take the bike to a shop. If it is an American version you really won't need a new bottom bracket- just replacing the bearings is fine.
