Radiator Glossary of Terms
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Octane Number
The rating number that indicates a gasoline's ability to resist knocking.
Odometer
Instrument that measures and records the number of kilometers or miles a vehicle travels.
Offset
The position of the backside of the wheel center section with respect to the centerline of the rim. If the center is closer to the back of the wheel, is has "negative" offset. If the center is closer to the outside face or front of the wheel, it has "posi
Ohm (Q)
The unit used to measure the resistance of conductor-to-electrical flow. One ohm is the amount of resistance that limits current flow to one ampere in a circuit with one volt of pressure.
Ohmmeter
An instrument used for measuring the resistance, in ohms, in an electrical circuit.
Oil
The substance used to lubricate and cool an engine's moving parts and reduce rust and corrosion.
Off-Center Steering
A condition where the steering wheel is not centered or is crooked when traveling straight ahead. The condition contributes to toe wear because anytime the wheels are steered off dead center, they toe out slightly which increases side slippage and scrubbi
Oil Consumption
All engines use a small amount of oil over time. It gets past the piston rings and valve guide seals and is burned in the combustion chamber. A small amount escapes through the PCV system and a few drops usually managed to seep through a gasket or seal. T
Oil Cooler
A heat exchanger for cooling oil. Most automatic transmissions are equipped with an oil cooler that's located inside the radiator. Since the radiator usually runs close to 200 degrees, the amount of "cooling" this kind of setup provides is questionable. A
Oil Drain Plug
A plug securing the drain hole of the oil pan.
Oil Filter
The filter used to strain oil moving through the engine. Oil filters should be replaced when the oil is changed.
Oil Gauge
The dashboard gauge that indicates oil pressure. When this gauge lights up, it indicates a very low level of oil in the engine, which should immediately be replenished.
Oil Pan
A removable pan bolted to the bottom of the crankcase, on the bottom of which is the oil drain plug.
Oil Pressure
The amount of pressure created in an engine's oil system by the oil pump. A certain amount of oil pressure is needed to circulate oil throughout the engine and to maintain adequate lubrication. Low oil pressure or loss of pressure is dangerous because it
Oil Pump
The pump that circulates the oil. The inlet to the pump is fitted with a fine-meshed screen to strain the oil before it enters the pump.
Open Loop
A mode of operation in a computerized engine management system that occurs after a cold start. During open loop, the computer provides a fixed air/fuel ratio that is richer than normal to improve cold derivability until the engine warms up. See also Close
Open PCV System
System where contaminated air from the crankcase is returned to the carburetor or intake manifold after which they are consumed in the combustion process.
Orifice Tube
A metering device located just ahead of the evaporator on the high-pressure side of an A/C system that restricts the flow of refrigerant into the evaporator. A small hole (the orifice) allows only a certain amount of refrigerant to pass through the device
Original Equipment
Industry term for a component supplied with a new car or as an official replacement part. Known as OEM or "Original Equipment Manufacturer" parts, they are not necessarily produced by the carmaker in question.
Outer Or Outboard CV Joint
The CV joints closest the wheel in a front- or rear-wheel drive vehicle.
Output Shaft
The shaft that transmits torque from a device, such as a transmission.
Over Inflation
The condition that exists when a tire is inflated beyond the pressure corresponding to the actual load or beyond the vehicle manufacturer's recommendation.
Overall Diameter
The diameter of an unloaded, inflated tire measured from the crown on one side to the crown on the opposite side. The free radius equals one-half the overall diameter. Sometimes called the outside diameter.
Overdrive
A gear assembly that produces more shaft revolutions than that transmitted to it.
Overhead Camshaft (OHC)
An engine configuration in which the camshaft is mounted on top of the cylinder head and operates the valve either directly or by means of rocker arms.
Overhead Valve (OHV)
An engine configuration in which all of the valves are located in the cylinder head and the camshaft is located in the cylinder block. The camshaft operates the valves via lifters and pushrods.
Overheating
When the temperature of the coolant exceeds the normal operating temperature range of the engine, it is said to be overheating. A number of things can cause this to happen. Idling for long periods of time in traffic during hot weather can cause overheatin
Overload Shocks
A type of shock absorber that's equipped with a helper spring to keep the suspension from sagging when a vehicle is heavily loaded.
Oversteer
The situation that occurs in cornering when the rear of a vehicle tends to skid before the front.
Oxidation
Any reaction in which a chemical joins with oxygen, as rusting or combustion.
Oxidation Catalyst
A two-way catalytic converter or the chamber in a three-way converter that oxidized unburned hydrocarbons (HC) and carbon monoxide (CO) to reduce pollution.
Oxidation Inhibitors
Petroleum-product additives that increase a product's resistance to oxidation and lengthen service life.
Oxides Of Nitrogen (NoX)
Chemical compounds of nitrogen produced as a byproduct of combustion. They combine with hydrocarbons to produce smog.
Oxidize
The result of surface metal combining with oxygen in the air to produce rust.
Oxygen
A gaseous element given the chemical symbol O, and occurring as O2, which makes up approximately 20% of the earth's atmosphere. Necessary for combustion, and measured by an exhaust analyzer to identify lean fuel mixtures.
Oxygen Sensor
Used with the feedback system to sense the presence of oxygen in the exhaust gas and signal the computer that can reference the voltage signal to an air/fuel ratio.
Ozone
A molecular form of oxygen that contains three atoms of oxygen instead of the normal two. It is formed naturally by sunlight and electrical discharge. It has a pungent odor and a strong oxidizing effect. Ozone is broken down by natural chemical reactions,
Ozone Depletion
Destruction of ozone in the ozone layer attributed to the presence of chlorine from manmade CFCs and other forces. The layer is thinning because ozone is being destroyed at a faster rate than it is being regenerated by natural forces.
Ozone Layer
A region in the stratosphere 12 to 35 miles up where the air is very cold and thin, and ozone is found in high concentrations. The ozone layer is continually replenished by solar radiation and screens out about 95 to 99% of the sun's ultraviolet radiation

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