On crosstrainers, the timing belt helps keep the rocker motion smooth. Requires two: one for the right rocker assembly drive and one for the left rocker assembly drive. OEM #0K69-01041-0002 works on most LifeFitness Crosstrainers. See also Rocker Casting with Pulley Kit, Right (LXR448) and Rocker Casting with Pulley Kit, Left (LXR451).