Motorcycle engine and engine oil terminology

A list of common phrases and terms that are ued with 2 stroke and 4 stroke motorbike engines and motorcycle oils

Term Meaning
Air/fuel ratio

The proportions that air and fuel are mixed together in the carburettor to form a combustible gas


American Petroleum Institute – Body responsible for testing and classifying oil

API Grade

Grade given by the American Petroleum Institute to determine the type of use an oil is suitable for


Bottom Dead Center-the lowest point in a cylinder that the piston goes


Brake Horsepower. The British measurement for engine power. Also expressed in Kilowatts (KW)


The larger end of the connecting rod that attaches the piston to the crankshaft. Usually located at the bottom of the engine


Diameter of the cylinder


Term used when referring to the bottom of the engine, usually the crankcase and crank components


Before Top Dead Center-The spark plug usually ignites the fuel/air mix slightly before the piston reach’s its highest point, this position is known as Before Top Dead Center (BTDC)

Cam chain

Chain connected to the end of the crank shaft at the bottom of the engine and the cam shafts at the top of the engine


Shaft located at the top of the engine, driven by a chain, belt or gear mechanism. The camshaft(s) have a series of egg shaped ‘lobes’ which, as the shaft rotates, open and close the inlet and exhaust valves


Usually located outside the engine, close to the top of the engine cylinder head. The carburettor mixes air and fuel to form a combustible gas which is then burnt in the engine cylinder

Catalytic converter

A device used is exhaust systems to remove or convert some of the more harmful components and pollutants in the exhaust gases. Often contains volcanic rock which reacts with exhaust gas components


Capacitor Discharge Ignition-Form of mechanical electronic breaker used to control the spark of a/the spark plugs


As the piston travels up to the top of the cylinder head, it ‘compresses’ the fuel/air mixture from the carburettor in the top of the cylinder head, making the fuel/air mix ready for ignighting by the spark plug. This is known as ‘Compression’.

Connecting-rod (Con-rod)

The rod in an engine that connects the crankshaft to the piston


A structurally strong chamber usually at the bottom of the engine that contains the crankshaft


A shaft, usually made from hardened steel, which is joined by the con-rod to the pistons, which is used to drive the motorcycle. Usually runs across the bottom of the engine and is connected to the drive chain at one end and the clutch at the other


A normally circular cavity used to house the piston

Cylinder head

Component used to block off one of the open ends (usually the top) of the cylinder

Detonation or pinking

Explosion of the fuel/air mix in the cylinder instead of the usual controlled burning. Can cause a ‘tinkling’ noise under an open throttle and can be highly destructive to an engine


The amount of volume displaced by the piston as it moves from the lowest point in the cylinder to the highest point.

Double-overhead camshaft (DOHC)

An engine that uses 2 overhead camshafts, 1 for the inlet valves and 1 for the exhaust valves


A toothed belt used to transmit movement from the end of the crankshaft to the rear wheel


Any shaft used to transmit movement in the engine. Most common application is used to transmit power from the end of the crankshaft to the rear wheel


A 4 stroke engine lubrication system where the oil is located in a separate oil tank rather than the more commonly used oil sump


Electronic Control Unit-A computer which controls engine electronic operations such as ignition timing or an anti-lock braking system (ABS)


Engine Management System-A computer controlled system which manages the fuel injection and the ignition systems


Extreme Pressure-Oil type used

Expansion chamber

Section of a 2 stroke motorcycle or scooters exhaust system designed to improve engine efficiency and boost power

Final drive

Component that transfers drive from the transmission (gearbox) to the rear wheel. Usually in the form of a chain, drive shaft or belt

Firing order

Order in which the cylinders fire or deliver their power/ignition stroke


A rotating disc or mass usually on the end of the crankshaft used to smooth out power impulses and to assist clutch engagement


Resistance between two bodies or objects moving while in contact with each other


A thin layer installed between 2 metal surfaces to ensure a good seal. Usually made from cork, cardboard, asbestos or soft metal


A mixture of a metallic soap and a lubricating oil

Gudgeon pin

Pin, usually made from hardened steel, that connects the small end of the con rod to the piston. Often referred to as a piston pin

Ignition timing

The moment at which the spark plug fires. Often controlled by points (in older motorbikes and scooters), CDI or ECU


Quality standard for 2 stroke oils

Lambda sensor

Sensor fitted to exhaust systems to measure the oxygen content of exhaust gases


A substance, usually an oil, that operates between 2 rubbing surfaces to reduce friction

Main bearing

The principle bearing(s) that carry a component. Usually refers to the crankshaft bearings in a motorcycle engine


Principle shaft, usually the crankshaft in a motorcycle engine

Monograde oil

An oil that has a very narrow viscosity or thickness range. e.g. SAE40

Multigrade oil

An oil that has a wide viscosity or thickness range. e.g. 10w40

Oil cooling

The use of oil to assist in the cooling of hot parts of a motorcycle engine of other moving component

Oil injection

A system where 2 stroke oil is pump fed into the engine in accordance with the throttle position

Oil pump

A mechanically-driven device for distributing or pumping oil around a 4 stroke engine and into a 2 stroke engine

Otto cycle

Another name for the 4 stroke cycle

Overhead valve (OHV)

Valves in the top of a 4 stroke engine that are operated by pushrods

Overhead cam (OHC)

A system where a shaft in the top of the engine, driven by a chain, belt or gear from the crankshaft, is used to open the valves

Oversize (OS)

Term used for a piston fitted to an engine after a cylinder rebore


Lubrication mixture for 2 stroke engines. Where the petrol and oil is mixed before going into the engine


A moving plunger inside a cylinder. Intended to seal the cylinder and accept or deliver thrust

Power band

The band of RPM in which the engine produces the most useful amounts of power


2 stroke engine lubrication system, used especially in older, racing and competition motorcross motorcycles where the 2 stroke oil and petrol is ‘premixed’ in the fuel tank


Component used to propel oil, coolants or fuel through a circuit, tube or pipe at high pressure


Removing of the worn or damaged interior surface of a cylinder to create a new working surface. As the cylinder becomes bigger or ‘oversized’, larger or ‘oversized’ pistons and piston rings need to be used

Reed valves

A valve that functions similar to a ‘reed in the wind’. Pressure causes the valve to flap open or closed. Used mainly in 2 stroke engines due to their ability to work well at high speeds

Rotary valve

A type of valve used in 2 and 4 stroke engines which, as it rotates, opens and closes gas passageways. Usually a disc, conical or cylindrical in shape and normally found on inlet systems


Society of Automotive Engineers – SAE numbers are used for classifying lubricating oils into viscosity (thickness) ranges at certain temperatures


To clear away, particularly exhaust gas from a cylinder or oil from a dry sump


More commonly referred to as ‘F**K IT!! I’ve seized my bl**dy engine! The binding together of 2 moving parts, usually through pressure, temperature or lack of lubrication

Single overhead camshaft (SOHC)

An engine that uses a single camshaft to operate both the inlet and exhaust valves

Small end

The smaller end of the connecting rod to which the piston is usually attached

Spark plug

Device used to ignite the fuel/air mixture in a cylinder. The spark is created by ‘arcing’ an electric current between 2 electrodes


Either the up or down movement of the piston from the top to the bottom or bottom to top of the cylinder (So the piston going from the bottom of the cylinder to the top would be 1 stroke, from the top back to the bottom would be another)


A well, hollow or reservoir for excess fluid. A detachable chamber at the bottom of a 4 stroke engine that contains the oil


Top Dead Center-the highest point in a cylinder that the piston goes

Top end

A general description for an engines cylinder block, head and valve components

Total loss

A system of lubrication in which the oil is lost or used up after it’s been used to lubricate moving surfaces. E.g. A 2 stroke engine uses a ‘total loss’ system as the 2 stroke oil is burnt with the fuel and air mixture

Transfer port

Port (or passageway), in a 2 stroke engine, which transfers the fuel, air and oil mixture from the crankcase at the bottom of the engine to the cylinder at the top of the engine


A device through which the flow of liquid, gas or vacuum may be stopped, started or regulated


The thickness of a liquid (or it’s resistance to flow). Often indicated by an SAE number, the higher the number, the thicker (or more viscous) a liquid is

Wet Sump

Commonly used 4 stroke engine lubrication system in which the oil is carried in a pan at the bottom of the engine


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