The present invention provides a lubrication system for small internal combustion engines. In a first aspect, a helical oil pump includes a helical insert which is disposed within a bore in the engine camshaft and is rotationally fixed with respect to the crankcase. During running of the engine, rotation of the camshaft draws oil from the oil sump upwardly through the bore in the camshaft and around the helical insert to provide lubrication oil directly to the upper camshaft bearing and to the upper crankshaft bearing. In another aspect, an impeller oil pump includes a pump chamber defined between the camshaft gear and a pump body which is rotationally fixed with respect to the crankcase and in communication with the oil sump. An impeller assembly mounted on the camshaft is disposed within the pump chamber, and draws oil from the oil sump into the pump chamber. The oil is forced outwardly of the pump chamber through an opening in the camshaft gear in the form of a stream of pressurized oil which is directed toward different locations within the crankcase as the camshaft gear rotates with the camshaft.