A round ring having a square or round cross section used as a seal, such as at the end of a hydraulic line.
An abbreviation for outside diameter.
O2 Sensor
A term used for oxygen sensor.
An abbreviation for overall height.
An abbreviation for overall length.
An acronym for On-Line Automotive Service Information System.
An abbreviation for overall width.
An abbreviation for on-board diagnostics.
Observed Horsepower
The brake output of an engine as observed on a dynamometer.
Occupational Safety and Health Administration (OSHA)
A branch of the Federal Government that regulates and oversees the occupational environment as related to health and safety.
Occupational Safety Glasses
Protective eye wear designed with special high-impact lenses, frames, and side protection.
A gasoline's ability to resist detonation. The higher the octane number, the greater the fuel's resistance to detonation.
Octane Number
The number representing the average blend of iso-octane with an index of 100, and other hydrocarbons with an index of 0, usually between 87 and 92.
Octane Rating
A term used for octane number.
Octane Requirement
The minimum octane rating of a fuel required to operate an engine without a spark knock.
Odd Fire
A V–6 engine with a 60- or 90-degree block having a conventional crankshaft with two connecting rods attached to each of three journals that produces uneven firing.
A mechanical or electronic counter in the speedometer that indicates trip or total miles accumulated on the vehicle.
An abbreviation for original equipment manufacturer.
Off Highway
A term used for off-road.
Off Pavement
A term used for off-road.
Off the Line
A good start from the starting line in a drag race.
Off-Highway Vehicle (OHV)
A vehicle intended for off-road use.
Any unpaved, rough, or ungraded terrain on which off-road activities take place.
Off-Road Vehicle (ORV)
Any vehicle designed for use in off-road activities.
Off-the-Road Vehicle (ORV)
A term used for off-road vehicle.
The driver's compartment.
Office Manager
An employee or company official whose duties vary from general office procedures to serving as a parts manager and bookkeeper.
An Offenhauser racing engine.
Any product, such as a manifold or valve cover, manufactured by the Offenhauser Equipment Company.
A term used for wheel offset.
A condition when two parts are not directly in line with each other.
Offset Link
A type of chain link that utilizes bent, or offset link plates and is assembled with a pin at one end and a bushing and roller at the other, so as to act as a combination link.
Offset Rod
A rod on which the bran section is not directly centered over the bearing housing.
Offset Section
A factory-assembled section of roller chain, made up of a roller link and an offset link, and used to connect strands of chain having an odd number of pitches.
The side of a vehicle away from the curb.
An S-shaped curve.
An abbreviation for overhead cam or overhead camshaft.
A metric unit of electrical resistance.
Ohm's Law
The law that states that the direct current through an electric circuit is proportional to the voltage applied; given by the equation I = E/R, where I is current, E is electromotive force, and R is resistance.
An analog or digital instrument used to measure electrical resistance in ohms.
An abbreviation for off-highway vehicle.
An abbreviation for overhead valve.
A diesel fuel.
A liquid lubricant used to reduce friction between moving parts.
Oil Bath Filter
An engine air filter that channels the air through an oil bath that traps dust and debris.
Oil Bleed Line
An external line that usually bypasses a metering device to ensure positive oil return to the compressor at all times.
Oil Bleed Passage
Internal orifice that bypasses a metering device to ensure a positive oil return to the compressor.
Oil Burner
A diesel.
An older car or truck that uses excessive oil, generally evident by a smoking exhaust.
Oil Canning
A sheet-metal panel popping from convex to concave and/or vice versa.
Oil Clearance
The small space between the main bearing and crankshaft journal, usually 0.001 to 0.003 inch (0.025 to 0.076 mm), for lubricating oil to circulate.
Oil Control Ring
The bottom piston ring that scrapes the oil from the cylinder wall.
Oil Cooled Piston
A piston that is cooled by a jet of oil sprayed under the dome of some diesel- and endurance-race engines.
Oil Cooler
A device used to cool oil or automatic transmission fluid.
A device used to cool race-car manual transmission and final-drive lubricants.
Oil Dilution
The thinning of oil in the crankcase, usually caused by gasoline seepage past the piston rings from the combustion chamber.
Oil Dipper
A small scoop located at the bottom of the connecting rod in early, low-performance engines to dip into the oil to lubricate the rod bearings and crankshaft.
Oil Filter
A component, located near the oil pump, that removes abrasive particles from the motor oil by a straining process as the oil circulates through the lubrication system.
Oil Gallery
Passages drilled or cast into the cylinder heads, engine block, and crankshaft to receive pressurized oil from the oil pump for distribution throughout the engine.
Oil Injector
A term used for oil-injection cylinder.
Oil Level Indicator
A term used for dipstick.
Oil Pan
A removable part of the engine that contains the oil supply.
Oil Pressure
The pressure, 15 to 75 psi (103 to 517 kPa), developed by the oil pump to force oil through the lubrication system.
Oil Pump
A pump, driven directly or indirectly by the camshaft, that draws oil from the oil pan and forces it, under pressure, through the engine lubrication system.
Oil Pumping
The leakage of oil past the rings and into the combustion chamber.
Oil Reservoir
A shock-absorber section containing an extra fluid supply to meet operational requirements.
The engine-oil pan.
A container for an oil reserve supply.
Oil Ring
Piston ring that scrapes oil from the cylinder wall to control cylinder wall lubrication and prevent excessive oil loss past the piston and into the combustion chamber.
Oil Scraper Ring
A ring used to scrape oil off the cylinder wall that may have been left by the oil control ring.
Oil Seal
A seal around a rotating shaft or other moving part to prevent oil leakage.
Oil Seal and Shield
A pair of devices that are used to prevent or control oil leakage past the valve stem and guide into the combustion chamber ports.
Oil Separator
A device used to separate oil from air, or oil from another liquid.
Oil Slinger
A metal disc that is located between the engine pulley and timing gear to force oil away from the timing-gear cover seal.
Oil Sludge
An accumulation of thickened oil, water, carbon, and dust particles inside an engine.
Oil Squirt Hole
A small hole located near the lower end of the connecting rod, providing a lubrication path to the cylinders and camshaft.
Oil Strainer
A wire-mesh screen placed at the inlet of the oil-pump-pick-up tube to prevent dirt and debris from entering the oil pump.
Oil Sump
A term used for oil pan.
Oil-Fouled Plug
A wet, oily deposit on a spark plug that may be caused by oil leaking past worn piston rings.
Oil-Injection Cylinder
A special calibrated cylinder that may be used to inject a measured amount of refrigeration oil into the system.
Oil-Intake Screen
A strainer located at the lower end of the oil pickup tube in sump to remove large contaminant particles.
Oil-Pan Rail
The flat-flanged portion of the oil pan with many bolt holes used to secure it to the engine.
Oil-Pressure Gauge
An instrument used to display the oil pressure of the engine lubrication system.
Oil-Pressure Indicator
A term used for oil-pressure gauge.
Oil-Ring Expander
A thin metal strip used to maintain a constant pressure on the oil-ring rail against the cylinder wall.
Oil-Ring Rails
Two thin metal rings used to scrape oil from the cylinder wall.
Oil-Ring Separator
A term used for oil-ring expander.
Oil/Air Separator
A device used to prevent the aeration of oil.
The origin of bus, from the French term voiture omnibus, or carriage for all.
On a Pass
Making a good, fast run in drag racing.
On the Bubble
The last position on the grid subject to be bumped if another driver achieves a better qualifying speed.
On the Cam
The operation of an engine at its most efficient rpm.
On the Grid
A starting position for a race. Also known as on the line.
On the Line
Same as on the grid.
On the Piano
A term used when something is misplaced.
On the Wood
An accelerator pedal pressed to the floor.
On-Line Automotive Service Information System (OASIS)
A computerized information system for service technicians.
Refers to paved or smooth-graded surface terrain on which a tractor/trailer will operate; generally considered to be part of the public highway system.
Onboard Computer
A resident computer in a vehicle, such as an automobile.
Onboard Diagnostics (OBD)
A special, standardized diagnostic software and hardware system used to detect performance problems that adversely affect emissions and engine performance.
Onboard Fire-Extinguisher System
A fire-extinguishing system permanently installed in the driver's compartment of a vehicle; required by most race-sanctioning bodies.
One Off
A custom-built vehicle with no plans for mass production.
One-Piece Oil Ring
An oil ring having the expander and rails combined in a single part.
One-Way Clutch
A friction or ratcheting device that permits motion in only one direction.
One-Way Roller Clutch
A term used for one-way clutch or overrunning clutch.
One-Wire System
An electrical system using body and/or chassis metal as a ground circuit.
To operate an engine at idle speed.
Open Circuit
A circuit in which there is a break in continuity.
Open Course
That part of a race track that extends beyond the finish line into a shut-off area.
Open Loop
In engines with a computer and oxygen-sensor control system, a mode of operation during which the computer ignores the signal from the oxygen sensor, typically before the engine reaches normal operating temperature.
Open Structural Member
A flat body panel having an open access from the rear.
Open System
A crankcase, emission-control system having no tube from the crankcase to the air cleaner; drawing air through the oil filter cap only.
Open the Tap
Increase the speed.
Open-Chamber Head
A cylinder head in which the cylinders have no quench area.
Open-End Spring
A coil spring having its end loops apart from the spring coils.
Operational Control Valve
A device used to control the flow of compressed air through the brake system.
Operational Specifications
Specifications used to show how the vehicle operates, such as acceleration, tire inflation, and other general information.
Operational Test
A term used for performance test.
Opposed Engine
An engine with cylinder banks at 180 degrees, such as the Volkswagen flat four.
Opposite Lock
Turning the steering wheel in the opposite direction of a turn to control or correct oversteer.
Optical Horn
A Chrysler term for a flash to pass dimmer switch feature.
Organic Brake Lining
Brake linings that are made of a carbon-based compound combined with non-organic magnesium silicate and/or glass and synthetic fibers, replacing asbestos which has been determined to be hazardous to health and the environment.
Organic Friction Material
A friction material having organic binders substantially formulated with nonmetallic fibers.
A small hole or opening.
Orifice Spark Advance Control (OSAC)
A Chrysler emissions control system which slows vacuum advance of ignition timing by means of an orifice in a component mounted on the air cleaner.
Orifice Spark Advance Control Valve (OSAC Valve)
A device used on some older Chrysler engines to limit oxides of nitrogen formation by delaying the vacuum signal to the distributor advance during idle and part-throttle operation.
Orifice Tube
A term used for expansion tube or fixed-orfice tube.
An abbreviation for off-road vehicle.
An abbreviation for oversize.
An abbreviation for Orifice Spark Advance Control.
OSAC Valve
An abbreviation for Orifice Spark Advance Control Valve.
A device moving back and forth or to and fro, like a clock pendulum.
Oscillating Fifth Wheel
A term used for fully oscillating fifth wheel.
The rotational movement, either in a fore and aft or side-to-side direction around a pivot point, such as in a fifth wheel design in which such articulation is permitted.
Oscillation Damper
A shock absorber may be considered a damper that controls energy stored in the springs under load.
An instrument that produces a visible image of one or more rapidly varying electrical quantities with respect to time or with another electrical quality.
An acronym for Occupational Safety and Health Administration.
Otto Cycle
The basic principle of operation of the four-stroke piston engine: intake, compression, power, and exhaust.
Otto-Cycle Engine
A four-stroke cycle engine.
Out of Phase
A term that relates to a drive shaft having two universal joints that are not in phase with each other, producing a jouncing or jerking motion as the drive shaft turns.
Out of Round
The condition of a circular part when it is not circular, as in slightly oval shaped.
Out of Square
The condition of a square or rectangular part when its vertical sides are not at right angles to its horizontal sides.
Out of the Box
A vehicle, device, or component that is absolutely stock without modification.
Out of the Chute
Off the starting line in a drag race. Also known as out of the gate and out of the hole.
Out of the Gate
Same as out of the chute.
Out of the Hole
Same as out of the chute.
Out to lunch
Anything of no value.
A worthless car.
Outboard Brakes
A brake assembly that is mounted at the outer or wheel end of an axle half shaft.
Outer cam Clutch
A one-way roller clutch which has the cam profile surfaces on the outer element.
Outer Race
The race nearest to the outside of the hub of a roller bearing.
To release gas from a solid, such as plastic, as the result of heat generated during machining or cutting.
Output Driver
An electronic switch in a computer used to turn ON and OFF an actuator in a vehicle.
Output Shaft
The main shaft of a transmission.
Outside Diameter (O.D.)
The major diameter of the outside of a tube or tube-like device.
Outside Snap Ring
A snap ring used on the outside of a shaft or part to hold the assembly in place.
Outside-Vehicle Lubrication Services
Work performed outside the vehicle during a chassis lubrication such as checking tire pressure, lubricating door and trunk hinges and locks, cleaning the windshield, and inspecting the safety-related items.
Out of round or egg-shaped.
An elliptical race track.
Oval Piston
A term used for cam-ground piston.
Oval Port
An exhaust or intake port that is oval in shape.
A term used for bore or overbore.
To enlarge the cylinders of an engine to a size larger than stock diameters.
Overcenter Preload
The adjustment of a steering-gear-sector shaft's resistance to turning.
Overcenter Spring
A spring used on some clutch linkages to reduce the effort required to depress the pedal.
A term used when too much refrigerant or oil is added to the refrigeration system.
A condition where the charging system is supplying too much voltage/current to the battery.
The continual charging of a battery after it has reached its normally charged condition.
The covering bonded to the tensile member of a synchronous belt that protects it from frictional wear if a backside idler is used or if power is transmitted from the backside of the belt.
A transmission having a ratio of less than 1:1 where the output shaft turns at a greater rpm than does the input shaft.
Overdrive Band
A transmission brake device that is engaged in overdrive.
Overdrive Ratio
A ratio identified by the use of a decimal point, such as 0.80, indicating less than one driving input revolution compared to one 1.0 output revolution of a shaft, or 0.85:1.0.
The spilling of the excess of a substance.
To run or spill over the sides of a container.
Overflow Tank
A tank at the top of a radiator to receive heated coolant and vent trapped air due to expansion.
A device found in some fuel tanks to prevent gas escaping due to expansion.
A term used for expansion tank.
To rebuild a device or an assembly, such as an engine or a transmission.
Overhaul and Maintenance Specifications
Specifications used to service vehicle components such as an engine, differential, or transmission.
Overhead Cam (OHC)
A term used for overhead camshaft or overhead camshaft engines.
Overhead Camshaft
A camshaft mounted in the cylinder head.
Overhead Camshaft Engine
An engine in which the camshaft is mounted over the cylinder head.
Overhead Position
The position in which welding is performed from the underside of the joint.
Overhead Valve (OHV)
An I-head arrangement where the valves are located over the piston in the cylinder head.
Overhead-Valve Engine
An engine in which the valves are mounted in the cylinder head over the combustion chamber.
To become excessively hot.
The condition of a tire that is inflated to more than the recommended pressure, decreasing the contact area, increasing the rolling diameter, and stiffening the tire resulting in excessive wear at the center of the tread.
The interval of valve timing when the intake valve starts to open before the exhaust valve is fully closed.
Overlay Cam
A camshaft having a hard face material welded to the nose and flank of the lobes to help decrease wear and increase lift.
Term referring to the application of lubricant amounts in excess of factory recommendations that may overload or damage grease seals.
The pulling of a body member beyond its specifications with the expectation that it will snap back to its intended shape when the pulling effort is discontinued.
To run an engine at excessive rpm.
Overrunning Clutch
A device used when two members are to run freely relative to each other in one direction, but are to lock in the other direction.
Overrunning-Clutch Drive
An overrunning clutch used for the engagement and disengagement of a starting motor.
Oversize (OS)
A part that is larger than the original to make up for wear and machining.
Oversize Valve Stem
A valve having a stem diameter that is larger than the stem diameter of the original valve. It is used to fit a worn valve guide that is reamed oversize.
A paint spray mist that drifts onto a surface where it is not wanted.
The overlap of new paint over old paint.
A cylinder with a bore greater than its stroke.
Staging a drag racer ahead of the usual staging position.
A condition in cornering when the rear wheels of a race car tend to break loose and slide outward.
The combination of a substance with oxygen forming an oxide, such as rust.
Oxidation Catalyst
A two-way catalytic converter which promotes the oxidation of HC and CO in an engine's exhaust stream, as distinguished from a three-way or reduction catalyst.
Oxidation Inhibitor
An additive to reduce chemicals in gasoline that react to oxygen.
A compound formed when a substance combines with oxygen.
Oxides of Nitrogen (NOX)
Harmful, gaseous emissions of an engine composed of compounds of nitrogen and varying amounts of oxygen which are formed at the highest temperatures of combustion.
To form an oxide.
A material that causes oxidation.
An additive that increases the oxygen content of an air/fuel mixture.
Oxidizing Agent
The same as oxidizer.
Oxidizing Flame
An oxyfuel gas flame in which there is an excess of oxygen, resulting in metal vaporization.
Oxyacetylene Welding
An oxyfuel gas welding process that uses acetylene as the fuel gas.
Oxyfuel Gas Welding
A group of welding processes, with or without a filler metal, that produces a merger of work pieces by heating them with an oxyfuel gas flame.
A colorless, gaseous, tasteless, element (O) that makes up 21% of the atmosphere.
Oxygen Cutting
The process of cutting metal at a high temperature with the chemical reaction of oxygen.
Oxygen Sensor
A device found in the exhaust manifold, which generates a small voltage dependent on the amount of oxygen present in the exhaust stream, used as a signal to the engine-control computer to determine the amount of fuel necessary to maintain a proper air/fuel ratio.
A term used for oxidizer.
A molecule of oxygen, an unstable pale-blue gas (O3), which is formed by exposure of O2 to an electrical discharge. It has a penetrating or pungent odor and a strong oxidizing effect.® ©2001- 2020  AutoZone, Inc. All Rights Reserved.