In a downhole motor bearing section having a cylindrical mandrel rotatably disposed within a cylindrical housing, a bi-directional thrust bearing is disposed within a four-shouldered annular containment chamber formed partially into the inner cylindrical wall of the housing and partially into the outer cylindrical wall of the mandrel. Under on-bottom loading conditions, thrust loads are transferred through the thrust bearing by the upper shoulder of the housing and the lower shoulder of the mandrel. The radial widths of these two shoulders are both symmetrical about the centerline of the thrust bearing such that on-bottom thrust loads are transferred non-eccentrically to the thrust bearing, thereby avoiding twisting loads on the thrust bearing races, increasing the effective on-bottom load capacity of the thrust bearing and extending its service life.