Abstract: Air-cooled reciprocating engines usually have a lubrication and cooling oil system providing positive oil delivery to a valve operating mechanism above the cylinder heads and a gravity flow return to the crankcase. In many such engines, the flow return channels are bored or cast conduits within the cylinder jacket structure which surrounds the piston guiding cylinder sleeve. These cylinder jackets are assembled to the engine crankcase by several long drawbolts which clamp each jacket between the respective head and the crankcase. Cylinder jacket bases have turned cylindrical surfaces which socket into a corresponding bore in the crankcase against an annular face. The socket relationship between the jacket base and crankcase positionally confines the jacket base literally relative to the crankcase. Locator pins bridging the interface between the jacket and crankcase secure the jacket from rotation within the crankcase socket.