Shimano Splined ES50/51 Bottom Bracket is for use with Shimano Octalink chainsets. It has an oversize BB spindle - large 22 mm diameter hollow pipe billet spindle reduces weight, increases strength and rigidity.
Octalink splined axle/crank arm 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 design allows easy installation without adjustment.