This Shimano ES25 Octalink Bottom Bracket is a sealed cartridge with oversized ball bearings that uses Shimano's oversized Octalink spindle design. It's large 22mm diameter hollow cold forged chrome-moly pipe billet spindle reduces weight & increases strength & rigidity. With the increased surface area of the spindle, givers the interface between the crank arm & the axle a better more rigid fit then a square taped spindle. Being a sealed system the dirt, mud & water is kept at bay using rubber seals that keep it maintenance free.