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
API 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
BDC Bottom Dead Center-the lowest point in a cylinder that the piston goes
BHP Brake Horsepower. The British measurement for engine power. Also expressed in Kilowatts (KW)
Big-end The larger end of the connecting rod that attaches the piston to the crankshaft. Usually located at the bottom of the engine
Bore Diameter of the cylinder
Bottom-end Term used when referring to the bottom of the engine, usually the crankcase and crank components
BTDC 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
Camshaft 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
Carburettor 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
CDI Capacitor Discharge Ignition-Form of mechanical electronic breaker used to control the spark of a/the spark plugs
Compression 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
Crankcase A structurally strong chamber usually at the bottom of the engine that contains the crankshaft
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
Cylinder 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
Displacement 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
Drivebelt A toothed belt used to transmit movement from the end of the crankshaft to the rear wheel
Driveshaft 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
Dry-sump A 4 stroke engine lubrication system where the oil is located in a separate oil tank rather than the more commonly used oil sump
ECU Electronic Control Unit-A computer which controls engine electronic operations such as ignition timing or an anti-lock braking system (ABS)
EMS Engine Management System-A computer controlled system which manages the fuel injection and the ignition systems
EP 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
Flywheel A rotating disc or mass usually on the end of the crankshaft used to smooth out power impulses and to assist clutch engagement
Friction Resistance between two bodies or objects moving while in contact with each other
Gasket A thin layer installed between 2 metal surfaces to ensure a good seal. Usually made from cork, cardboard, asbestos or soft metal
Grease 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
JASO Quality standard for 2 stroke oils
Lambda sensor Sensor fitted to exhaust systems to measure the oxygen content of exhaust gases
Lubricant 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
Mainshaft 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
Petroil Lubrication mixture for 2 stroke engines. Where the petrol and oil is mixed before going into the engine
Piston 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
Premix 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
Pump Component used to propel oil, coolants or fuel through a circuit, tube or pipe at high pressure
Rebore 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
SAE Society of Automotive Engineers – SAE numbers are used for classifying lubricating oils into viscosity (thickness) ranges at certain temperatures
Scavenge To clear away, particularly exhaust gas from a cylinder or oil from a dry sump
Seizure 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
Stroke 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)
Sump A well, hollow or reservoir for excess fluid. A detachable chamber at the bottom of a 4 stroke engine that contains the oil
TDC 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
Valve A device through which the flow of liquid, gas or vacuum may be stopped, started or regulated
Viscosity 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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