Lubrication is used to:


prevent contact between metal surfaces

reduce friction, wear and heat

remove foreign agents (metal shavings etc) which might otherwise damage parts


Components of a lubricating system for an inboard marine engine include:


oil pump


suction strainer




temperature gauge

pressure gauge

relief valves


















Lubrication System

(Courtesy: Caterpillar)



Oil is drawn from the sump through a suction strainer by the oil pump which is driven by either the camshaft or crankshaft. The oil is pumped to the engine circuit. A pressure relief valve is fitted, which will open when the pressure in the circuit exceeds the recommended pressure. This can happen when oil is cold. As the oil heats up, the pressure reduces and the valve closes to maintain recommended pressure.


Oil at the recommended pressure is pumped to the cooler which maintains a constant temperature by using sea water to draw out heat. Oil then passes through a filter to remove small foreign particles before it enters the engine. Some filters are fitted with relief valves which open to allow oil into the engine if the filter becomes blocked. Most filters are of a disposable type and must be replaced at regular intervals as specified by the manufacturer.















Cut Away of a Caterpillar Oil Cooler


Some oil cooling units are directly housed with the engine heat exchanger.


Temperature and pressure gauges are fitted between the oil cooler and the engine for monitoring purposes.


Oil moves into the engine through a pipe which runs the length of the engine with distribution pipes to each main bearing. Alternatively, oil might be fed to the main bearings by galleries cast in the block. This lubricates the main bearings and the oil then flows through to lubricate the bottom or big end journal. Oil then flows up a hole inside the connecting rod to lubricate the gudgeon pin or little end at the piston.


Excess oil from here helps lubricate the piston rings. Oil and vapour from the sides of the revolving bottom end bearing are thrown onto the cylinder liner and onto the underside of the piston lubricating the gudgeon pin and piston rings. In smaller engines, there is no hole drilled in the connecting rod and relies on the oil thrown from the bottom end bearing.


Camshafts, driving gears and valve gears are lubricated in a similar manner i.e. galleries and pipes to maintain lubrication at all metal to metal contact parts.


Some manufacturers cause the oil to flow from an open pipe onto the underside of the piston as an additional means of piston cooling.


The oil return to the sump is by gravity through drain holes and open galleries.

Ranger Hope 2005 (contains reworked material courtesy of ANTA)