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Factors involved in piston lubrication have been studied on a motored small bore engine equipped with a glass cylinder. The resultant picture of oil and piston ring movement reflects the primary effects of inertia and shearing forces. At the beginning of the upstroke what appears to be cavitation of the oil was observed. Near top dead center oil droplets are projected into the combustion space under influence of these forces and of the top ring movement. Because of the effect of combustion chamber pressure on top ring movement it is concluded that spark timing or other factors which affect this pressure will influence the passage of oil across the ring belt, and that the major part of the oil consumption takes place on the exhaust stroke. Loss of oil control at high speeds appears at least in part due to the increasing magnitude of the inertia forces acting on the top ring.