Cylinder bore lubrication with residual oil

A lubrication system for an internal combustion engine provides drain openings at locations within individual crankcase portions where residual oil collects during normal operation. The drain openings are connected to orifices formed through cylinder walls of other crankcase portions of the engine. Conduits connect preselected drain openings to associated orifice check valves to pump the residual oil from the collecting regions to the cylinder walls of other crankcase portions using differential pressures that occur naturally between the associated crankcase portions. The drain openings located at the collecting regions can be located directly under primary oil entry points of the crankcase portions, but this relationship is not absolutely necessary as long as the drain openings are placed at the collecting regions or, alternatively stated, as long as the drain openings are placed at locations where oil will tend to flow from the points of primary oil entry into the crankcase portions.