it uses the same size bearings as an american BB but the bearing fit dirctly into the BB shell w/o the cups. You can buy american, mid, euro, spanish, etc type of bottom brackets, but Im not sure if the only difference between the american and mid BB's is that one uses bearing cups while the other doesn't. Most new bmx frames(for street/dirt/park not race) come with mid BB shells. I found this:
http://www.danscomp.com/serve.php?se...bb_diagram.htm