Install the new outer-wheel bearing into the end of the disc-brake rotor, then add the stock retaining washer. Thread on the original castle nut and tighten it until it's snug. Afterward, back off the castle nut slightly while slowly rotating the rotor on the spindle. Begin tightening the castle nut (while spinning the rotor) just until you begin to feel drag on the rotor. Stop tightening and add a new cotter retaining key.