When packing rings have a slightly smaller bore than the piston rod diameter, the segments contact only at one end. The center portion of each ring segment provides a direct gas passage along the rod surface. This condition is permissible if it is not too severe. During the break-in process, packing will gradually wear to the point of conformation with the general rod surface curvature.
The potential for overheating due to lack of proper lubrication exists for oversized rods for the same reasons stated for undersized rods.