The authors have constructed a modified engine with a transparent glass cylinder, for motoring experiments, to observe lubricating oil flow from the sump to the combustion chamber through the clearance between the piston and the cylinder. The modified engine was motored at engine brake condition and the oil flow was filmed by a video camera. The amount of actual oil flow was also measured.The effects of pressure difference between the intake manifold and the crankcase, oil temperature, and oil ring tension on oil flow through the piston ring belt were studied. The results indicate that the main stream of oil flow increases with pressure difference and with oil temperature, and that oil flow is increased when a higher tension oil ring with excessive spacer expander circumference is utilized. Measuring equipment was also developed to examine the deflection of the oil ring underside in the cylinder when the expander circumference was varied. The results indicate that the oil ring deforms at the expander gap, with resultant oil leaks at the expander gap when an oil ring with excessive expander circumference is utilized.It was also found that oil injected from the large end of the connecting rod flows from the inside of the piston through the oil ring groove drain back slots into the oil ring groove. The drain back slots which should lead the oil in the oil ring groove to the inside of the piston, act in the reverse way. Covers were mounted on the inside wall of the piston to prevent this reverse flow. The amount of undesirable oil flow was considerably reduced by utilizing a piston with these covers, as compared to the standard piston.