This version of the American Classic bottom bracket uses a new Stainless Steel bearing that is significantly more durable than the previous version. The lightest just became even more durable. American Classic's ISIS bottom bracket is light because of the unique split ring design (patent applied). Bill Shook engineered a split aluminum ring that fits on front of a small shoulder on the cro-moly axle and behind each spline. The purpose of the split ring is to support the bearing and keep it in place. The bottom bracket axle is a more uniform tubular shape to better distribute the stress loads applied when riding. The American Classic ISIS bottom bracket does not need a large and heavy shoulder as seen on our competitor's ISIS bottom brackets to keep the bearings in place. Only 140 grams!