The Scramjet uses just one ceramic ball in each bearing. The super hard ceramic ball pulverizes any grit that enters into thru the seals and prevent the races from getting pitted by debris. The steel balls offer strength (though they are not as hard as ceramic, they are less brittle and have a higher load rating). IRD offers a 5 year warranty on the bearings. Fits English threaded BB shells (68 to 100mm wide) and 24mm axle cranks.