Four-stroke lubrication system
1 Oil strainer. 2 Oil pump. 3 Pressure release valve. 4 Oil cooler. 5 Oil filter. 6 Oil gallery. 7 Oil feed to main bearing. 8 Oil feed through crankshaft to big end bearing. 9 Oil feed through connecting rod to small end bush and gudgeon pin. 10 Oil feed to valve gear. 11 Dipstick. 12 Sump pump. 13 Sump drain plug.
Lubricating oil acts as a barrier between moving parts, reducing wear. It is normally distributed around the engine by means of a pump before being returned to the sump. In some slow running engines lubrication is by ’splash’. With this system, as the name implies, oil is thrown to the bearings, bores etc as the engine turns over.
In a pumped system the oil normally flows to the bearings etc through holes either cast or drilled into the engine casing.
The lubricating oil level in the engine should always be kept within the limits marked on the dipstick.
Constant displacement pumps are continuously driven from the engine, their output depending on engine speed. The internal parts of the pump are machined to fine tolerances and use very close fitting parts. Any scoring or wear will harm their efficiency. This will become apparent if the pressure generated at low speed is insufficient to operate the pressure switch or fails to meet the manufacturer’s recommended minimum pressure.
Fitted primarily to protect the pump from sucking in stray nuts and bolts, the strainer is sited low down in the sump and connected to the pump by a steel pipe.
Oil pressure in an engine is determined by the resistance to flow, and limited by a factory-set relief valve. This is a spring-loaded plunger which lifts to allow oil to bypass the system and flow directly from the outlet of the pump to the inlet or to the sump when the setting is reached. If debris jams this relief valve open, oil will bypass continuously and the working pressure will not be reached.
The filter ensures that the oil reaching the moving parts and bearings is clean. The quality of filtration is determined by the engine manufacturer.
Filter cartridges may incorporate reverse flow non-return valves and a bypass in case the filter becomes clogged, set to operate at a specific pressure. Only manufacturer’s recommended spares should be used.
The non-return valve stops oil draining out of the bearings back into the sump when the engine is stopped, and thereby ensures that pressure builds up promptly when the engine is started.
Lubricating oil washes the engine parts and collects debris. Most of this will settle in the sump but fine particles will remain suspended in the oil and pass through the pump to be collected by the filter.
The filter element should be changed whenever the lubricating oil is changed. All engines in current production use spin-on filter cartridges, which can be removed easily with a chain or strap wrench.
It may be possible to undo a cartridge by hand: if a two-handed grip is possible wrap a piece of emery paper around the cartridge to provide a better grip. Another solution is to fit a large hose clip around it and tap round with a hammer.
Fill the new cartridge with fresh oil. Then, making sure that the rubber sealing element is in place, screw on until the sealing ring is compressed, and tighten by hand using the emery paper to provide a better grip.
Before restarting the engine, turn it over by hand or on the starter motor with the fuel pump engine stop valve closed to circulate the new oil around the engine and fill the filter and passages. Check and tighten the filter after running the engine.