That's normal wear for an engine that does a lot of hard pulling- - - - -lugged down, or close to it, at or near the torque peak of the engine. The load goes on the upper bearing through the power stroke, and there's little, if any thrust load on the lower bearing during the intake stroke. An engine that runs at, or near top RPM will wear the top and bottom bearings about equally because of the load of pulling the piston down the cylinder from its high centrifugal force upper limit.
Jerry
