Base is another way of stating english threading; it comes from British Standard cycle. English threading on road bikes mean a 68mm wide, 34mm inside diameter shell with reverse threading on the drive side. It's the most common threaded Standard employed today. This kit works with 24mm diameter thru-axle cranks, found on Shimano, FSA, rotor, among others. Choose between two bearings for the cups. Standard bearings have low Drag thanks to hardened-steel races and our ceramic BALLS. Coated bearings have those same balls installed on coated, hardened-steel races for longer life and less bearing drag.