The Stovebolt Site Forums The Shop Area Engine Shop odd? rod bearing wear question

saw some rod bearings today that had almost identical wear patterns, 2/3, copper colored, on the top bearings only. the cap bearings looked normal 2/3, but no copper. the mains weren't available for comparison. what might have caused it? more slam on the down thrust?

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

said in another way; there's more force on compression and combustion on the top bearing, than there is on intake and exhaust on the lower bearing? so, since the vehicle has to overcome inertia every time it starts to move, stop and go traffic, the load is on the upper bearing. vs a vehicle that runs more often at highway speed?

That, too! Getting up to speed, or pulling a heavy load puts more stress on the bearings than light-throttle cruise does.
Jerry

Rod bearings are actually lubricated by a hydroplaning process which keeps a film of oil between the crank and the bearing shells. During periods of lugging an engine this hydroplaning effect is broken down, and the film of oil breaks down and of course the upper shell shows the most wear. An engine that has remarkably more wear on the upper shell has been subjected to a lot of lugging as compared to bearings showing normal wear. It was nothing unusual for old timers to keep a vehicle in high gear until it started to buck, or they shifted right before they thought the motor was going to die.