Shimano es51 octalink bottom bracket splined bottom bracket for use with shimano octalink chainsets features oversize bb spindle large 22mm diameter hollow pipe billet spindle reduces weight increases strength and rigidity octalink splined axlecrankarm interface results in more rigid crank assembly compared to square tapered spindle es51 has anti corrosion treatment inside hollow spindle sealed cartridge assembly maintenance free sealed cartridge keeps out dirt mud and water cartridge d