Small Engines
Lubrication Systems
Principles of Lubrication
Lubrication is the process of reducing friction between sliding surfaces by introducing a slippery or smooth
substance between them. Oil is the most important lubricant for small engine use, because it is often the only
lubrication the engine needs.
Friction
Friction is the resistance to motion created when one dry surface rubs against another. Even highly polished
metal surfaces have microscopic irregularities that will create a great deal of friction if rubbed together.
 Friction will cause wear and generate heat.
 Friction can be reduced, but not entirely eliminated.
Lubricating Oil
Modern motor oil is a highly specialized product, which has been developed by engineers and chemists to
perform many essential functions in an engine. The thin film of oil between two close fitting parts may only be
a few molecules thick, but this is enough to prevent two metal surfaces from actually touching. Molecules of
oil roll over one another, acting like microscopic ball bearings between surfaces. In order to operate
efficiently, engines depend on motor oil to lubricate, clean, cool, and seal engine parts.
Oil Permits Easy Starting
If oil is too thick it will create drag between moving parts and the engine will not crank fast enough to start
quickly and keep running. Oil must be fluid enough to flow quickly to reduce friction, but also thick enough to
provide protection when the engine reaches normal operating temperatures.
Oil Cleans Engine Parts
Additives are blended into modern motor oils to keep engine parts clean by suspending fine particles of
contaminates produced by combustion until they can be trapped by the oil filter.
Oil Cools Engine Parts
The crankshaft, piston, camshaft, and some other internal engine parts are directly dependent on motor oil for
cooling. Each of these parts has a temperature limit that cannot be exceeded. Oil absorbs heat from these
components and transfers it back to the crankcase.
Oil Seals Engine Parts
The surfaces of piston rings, ring grooves, and cylinder walls have microscopic peaks and valleys. Therefore,
the rings cannot completely prevent high combustion and compression pressures from escaping into the low
pressure area of the crankcase. This results in a loss of engine power and efficiency. Motor oil fills in the
peaks and valleys between surfaces and helps to seal the ring surfaces and cylinder walls.
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Oil Selection
Oil as it comes from the ground is called crude oil. It is then refined and blended into various fuels or oils. The
oil selected for use in a small engine is critical to the life the engine. The oil recommended for an engine may
be shown on the engine or in the owner’s manual. Specifications for engine oils are given in two categories:
 SAE Viscosity
SAE - Society of Automotive Engineers
 API Engine Oil Classification
API - American Petroleum Institute.
Viscosity
Viscosity is a measure of the oil’s thickness and resistance to flow. An oil with high viscosity is thick and flows
slowly. The SAE has established a viscosity range classification system for engine lubricating oils. Thick, slowflowing oils have high numbers. Thin free-flowing oils have low numbers. Keep in mind that thick oil in low
temperatures may make an engine difficult to start. Single grade motor oils commonly used today are SAE
5W, 10W, 15W, 20W, 20, 30, 40, and 50. Oils which meet two or more SAE grades are known as multigrade or
multiviscosity oils. Most oils today are multiviscosity. Common multiviscosity oils are SAE 5W-30 and SAE
10W-30. API uses letters for oil classification such as SJ (for cars, vans, and light-duty trucks).
Multiviscosity Example:
SAE 5W-30
5 is the grade at low temperature.
W is for “Winter”
30 is the grade at operating temperature.
Changing Oil
 Oil should be checked every 5 hours of operation.
 Oil should be changed every 50 hours of operation.
 Engine oil should be drained with the engine at or near operating temperature.
 When you pour fresh oil into the crankcase, it's a golden or amber color. Gradually, the heat, dirt
particles and agitated air in the crankcase cause the oil to darken. Dark oil is not only dirty; it has also
lost much of its ability to lubricate, clean, cool, and seal engine components.
 To inspect the oil level, remove the dipstick and wipe it with a clean cloth. To ensure an accurate
reading, reinsert the dipstick completely. Then, remove it again and check the oil level.
 Avoid overfilling the crankcase. Excess oil can create air bubbles, which will reduce lubrication. Also,
excess oil can burn in the cylinder producing black smoke and leaving carbon deposits.
Types of Lubrication Systems
Small four stroke engines generally use a splash lubrication system. The splash
lubrication system features an oil dipper arm on the connecting rod cap. The dipper
is designed to pick up oil from the crankcase on every revolution of the crankshaft,
splashing oil on the moving parts as it is carried around by the crank throw.
In larger engines a pump drives a pressurized lubrication system.
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