A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
O
O Rings: Circular rubber rings used to seal pipe and other connections.
O2 Sensor: Oxygen sensor, lambda EGO or HEGO sensor to provide exhaust oxygen content signal to the ECM.
OBD: On board diagnostic. A connector for diagnostic analysis of a vehicle's electronic systems.
OC: Oxidation Catalyst.
Octane Rating: The anti-knock property of petrol fuel. The octane rating or number is a comparison against two standard fuels, heptane and iso-octane having values of zero and 100 respectively. Most road vehicles use petrol with octane ratings between 93 and 98.
Odometer: Instrument that displays distance travelled by vehicle.
OEM: Original equipment manufacturer.
Offset: Any point that is not on the axis of shaft or component.
OHC: Overhead cam - type of valve operating gear. Camshaft in cylinder head above the valves.
Ohm: Electrical unit of resistance.
Ohm's Law: The electrical law, named after its originator, that shows the relationship between electrical pressure in volts, electrical current in amps and the electrical resistance in Ohms is proportional. Volts is equal to amps multiplied by Ohms.
Ohmmeter: A meter for measuring Ohms, the standard unit for electrical resistance.
OHV: Overhead valve - type of valve operating gear. Camshaft in cylinder block connected to valves by push rods and rockers.
Oil Cooler: A small heat exchanged (radiator), which has oil pumped through so that a flow of air will cool it.
Oil Dilution: The thinning of engine oil by fuel flowing down the cylinder walls into the oil pan.
Oil Drillings: Holes through various engine components, such as the crank, to allow oil to flow.
Oil Pan: Sump that holds oil for lubricating the engine.
Oil Pump: An mechanically driven pump used to supply oil pressure.
Oil Seal: A seal that prevents oil loss. Usually described as a radial lip seal but other types are used.
Oil Separator: Plates or baffles used to prevent oil being mixed with air or drawn through air pipes.
Oil Strainer: The mesh filter on the end of an oil pick up pipe.
On Board Diagnostics (OBD): See OBD
One-way Clutch: An automatic mechanical clutch that permits drive from one shaft to another in one direction only. A drive transmitted to the outer hub of the clutch bypasses the inner hub and the clutch free wheels. This type of clutch allows drive to be transmitted by two alternative routes and at different rotational speeds.
Opacimeter: Smoke meter used to measure the amount of smoke in diesel engine exhaust gases.
Opacity: A state of being opaque, not transmitting, reflecting or allow light to pass through.
Open Circuit: Applied to an electrical circuit when the switch is 'off' or the circuit is not continuous.
Open Period: (1) The number of degrees of distributor rotation during which the primary circuit remains open. (2) The number of degrees of crankshaft rotation during which an inlet or exhaust valve remains open.
Open-loop: An electronic control system using a comparison of sensor signals and a pre-programmed map.
Operational Safety: Safety aspects that are operational during use of a vehicle.
Optical Gauge: A measuring instrument that uses light/reflection to align steering (for example).
Orifice: A hole, usually with a precise drilling diameter to control a fluid flow or spray.
Oscillate: To move or swing from one place to another (eg. side to side).
Oscillator: An electronic circuit that produces as signal of a set frequency.
Oscilloscope: Test equipment with a screen that can display electrical signals.
Otto Cycle: An alternative term for the four stroke engine cycle using the name of its inventor.
Out-of-round
: A term used to describe a component that should be perfectly round or cylindrical but is not. Usually caused by wear but can be from faulty machining.
Outer Races: The outside rings of ball or roller bearings.
Outlet Ports: Used to describe any port that allows a gas or liquid to leave. Exhaust ports are a good example.
Output: Power or data that comes out from a mechanical unit or electronic control module. Usually adapted or modifed by the unit or module to suit a particular purpose.
Output Circuit: The battery charging circuit for the electrical current from a generator.
Output Shaft: Shaft that outputs drive from a gearbox to a propshaft or final drive.
Overall Ratio: The term used to describe the output ratio of a series of mechanical components such as gears or levers.
Overdrive Gear: A gear drive that increases the rotational speed of the output shaft above that of the input shaft.
Overdrive Unit: A separate unit that at one time was fitted on the output shaft of a gearbox. It produces a higher gear ratio.
Overlap (Valve): The term used to describe the angle of crankshaft rotation when the inlet and exhaust valves are both open.
Overrun: Movement of vehicle when engine is not under power. Engine braking.
Overrun Clutch: (1) A clutch on a starter drive pinion that prevents the engine from driving the starter motor after the engine has started. (2) A one-way clutch in an automatic transmission.
Oversteer: Steering condition of a turning vehicle when the vehicle attempts to turn by a greater degree than the steered position taken by the driver. A difficult driving condition requiring opposite lock to control.
Oxidation: The chemical reaction of oxygen with another element that forms oxides that can be either harmless (carbon dioxide) or harmful (rust or ferrous oxide).