Ok, so I'm planning to build up a new dirt jumper/urban bike sometime soon (tax refund time). Right now I'm not quite sure on a frame, but the Transition Trail or Park looks pretty bombin'. For the time being I'm going to transfer over most of the parts off my STP.

I'm kinda curious about BMX cranks and BBs. Having never ridden or wrenched on BMX bikes in the past, they are completely new to me. It seems that some manufacturers are including them stock on DJ bikes. I see alot of BMX cranks are 4130, so I'd assume they're stronger, but what about the BB. From what I can tell, you have to put a spacer in the BB shell to insure proper bearing preload. What if the shell isn't exactly 68 or 73mm wide though (ie. 67.6mm)? Would the bearings be improperly preloaded?

Does anyone here run a BMX setup on their mountain bike? How's it working out for ya?
Thanks.