The thrust bearing should first be packed with grease, and then added to the upper side of the spindle. This allows it to carry the weight of the car against the lower side of the upper kingpin boss. Shims are then added to the bottom side of the spindle, against the upper side of the lower kingpin boss. The shims take up any vertical play between the spindle and the upright due to wear or manufacturing tolerances, but not to bear any weight. Early Ford axles have the exact opposite arrangement, with the thrust bearing on the bottom side (hence some confusion).