Automotive Glossary

Automotive Glossary

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Intolerance of race. A person with prejudiced belief that one race is superior (better) than another.


Gear teeth cut on a straight bar or rod.

Rack and Pinion

Type of steering system gearing.


Outwards from the centre and at right angles to the axis.

Radial (Tire)

Tread supported by crown plies and casing plies laid radially.

Radial (Tyre)

Tread supported by crown plies and casing plies laid radially.

Radial Force

A force that acts outwards from a centre point.

Radial Groove

A groove running radially, or radiating from the centre and around a component.

Radial Play

Free play at right angles to the axis.

Radial Runout

Out of true movement at the periphery of a rotating circular component.


Usually refers to a pump where a number of pistons are used that act outwards from the centre.


The emission and diffusion of rays.


An apparatus for radiating or diffusing heat.

Radiator Cap

Cap used to pressurise cooling system.


The straight line distance from the centre of a circle to any fixed point on the circumference.

Radius Arm

Suspension link with a fixed inner bearing and support for the stub axle assembly at the outer end.


(1) Gear selector component carrying the selector forks in a manual transmission. (2) Pressurised petrol supply tube for all fuel injectors in a port electronic fuel injection system.


Random Access Memory. Dynamic memory in computers which holds data being worked on. This data is not permanent and will be lost when power is switched off.

Ram Air Effect

Forced movement of air due to the movement of the vehicle.

Ram Cylinder

Hydraulic cylinder that applies force to a rod. Often part of a steering system.

Ram Pressure

Pressure caused by air of fluid movement.


The difference between the highest and lowest numbers.

Range Change

(1) High and low gear ratios for an off-road or sport utility vehicle. (2) Secondary gearing in a large goods vehicle transmission gearbox that couples with the primary gears to double or treble the overall gear ratios.

Rare earth magnets

These magnets are made from substances which hold a more dense magnetic flux than regular magnets. This means more power for the weight. Rare earth magnets are always preferable although not necessary as they can be expensive.


A mechanism consisting of a toothed wheel, the ratchet, and a locking or actuating pawl. Used to either prevent the pawl lever or the wheel from rotating in one direct only or in some mechanisms to drive the wheel forward.


The numerical relationship between two or more components transmitting a force or movement.

Ravigneaux Planetary Gears

An epicyclic gear set with two sun wheels and planetary gears on a common annulus.


Term used to describe the action when the face of a valve or other component is machined.


To respond.


An equal and opposite force.

Reaction Member

The held component of an epicyclic gear train that produces the specific gear ratio when held.


Part of a fluid flywheel that reacts to the movement of fluid.


Describes paint which has been properly mixed with all necessary components and is ready to apply to the substrate. A variation is 'Ready-for-Use' (RFU) which refers to material that is purchased ready to apply straight from the container.


Accurate cutting of a hole using a reamer.


Recutting of a cylinder bore to an oversize dimension to remove wear and or damage. The cylinder is then fitted with oversize pistons.


Suspension movement following compression of the spring when the energy of impact is released.


Device used to receive or pick up radio signals or similar.


Part of an air conditioning system in the high pressure side between the condenser and expansion valve that acts as a reservoir and as a filter and drier for the refrigerant.


An undercut area in a component usually machined to give access for a fixing or another component.


Putting back the electricity taken from a battery.


The value of something when divided into one. The reciprocal of R is 1/R.


Moving backwards and forwards.

Recirculating Ball Steering

A steering gearbox using the nut and worm principle. It has a series of ball bearings forming the thread of the worm, which gives a very smooth operation.


The application of fresh paint material over a recently painted material.

Recombinant Battery

A type of battery design which has slightly more negative plate material than positive plate material allowing the oxygen released by the positive plates, near to the fully charged position, to combine with the negative plate so that no gases are discharged.


An electrical device that converts alternating current AC to direct current DC.


To lower the viscosity of a paint by the addition of solvent or thinner.


A solvent used to reduce or thin paints to sprayable viscosity.

Reduction Gear

A gear (or more often pair of gears) used to reduce the speed of a shaft.


In the context of systems or message transmission, redundant means extra information on say another channel that may not be used except for checking.

Reed Relay

Thin steel contacts that are joined when they become magnetic. The magnetism is created by a small coil, usually in series with a bulb.

Reed Switch

Thin steel contacts that are joined when they become magnetic.

Reference Voltage

A voltage value used in an electronic control module that is conducted to a sensor and against which the return voltage is measured in order to determine the signal value.


Repair of an OEM or previously painted substrate.


Body or surface reflecting rays, particles, objects. Legal requirement on rear of vehicles.


Deflection at a certain angle when light enters obliquely from another medium of different density (light lens).


The gas in an air conditioning system. It is condensed to a liquid by pressurisation and the heat in the refrigerant is taken in or passed out during the changes of state from liquid to gas and from gas to liquid. Two refrigerants are used for vehicle air conditioning. These are R12 and R134a. These must not be mixed or allowed to escape to the atmosphere.

Regenerative braking

A regenerative brake is an energy recovery mechanism that reduces vehicle speed by converting some of its kinetic energy and/or potential energy (due to elevation) into a useful form of energy instead of dissipating it as heat as in a conventional brake.


An electronic device for controlling the output voltage from a generator.

Regulator Terminals

Terminals on the regulator that connect to, for example, a warning light, field windings, sensing supply and ignition. Some also output signals to the engine management ECU.


To make information sink in.

Relative Density

The ratio of the density of one substance to the density of water. Used to measure battery acid strength. Specific gravity is a similar but older term.


Electromagnetic switch used in electric circuits to reduce load on smaller switches.

Relay Coil

The winding in a relay that when power is supplied to it, causes magnetism, which in turn makes the relay contacts close.


Electromagnetic switches used in electric circuits.

Release Bearing

Shaft-mounted thrust bearing that transmits the action of the clutch pedal to disengage the clutch.

Release Fingers

The fingers in a circular section of the diaphragm on a clutch that move when the release bearing is operated.

Release Fork

The actuating lever that moves the release bearing of a clutch.

Release Levers

See Release Fork.

Release Mechanism

Components joining the clutch pedal to the pressure plate of the transmission clutch.

Release Shaft

Pivot for the clutch release lever.

Relevant Information

The suitability of information based upon your needs.

Relief Valve

See Pressure Relief Valve.


A measure of resistance to magnetic forces.


Remote adjustment for CO.

Remote Starter

A switch, usually connected directly to the solenoid terminal of the starter motor, to allow remote operation during repair or testing.

Renewable resource

A natural resource is a renewable resource if it is replaced by natural processes at a rate comparable or faster than its rate of consumption by humans. Solar radiation, tides, winds and hydroelectricity are perpetual resources that are in no danger of a lack of long-term availability.

Replacement manual pages

Replacement pages are normally sent out under a Special Notice. They must be inserted into the manual immediately. Superseded pages should be removed.


To study something thoroughly.

Reserve Capacity

The time for which a battery will produce 25A at 25C.


A container holding a supply of liquid, gas, air or vacuum in a system.

Residual Magnetism

The magnetism held in an electromagnetic pole after the electrical current has been turned off. Natural magnetism in a magnet.

Residual Pressure

The pressure held in a hydraulic line after a valve has closed and is used to reduce the pressure build up time when next applied.


A property of material that has the ability to absorb energy from sudden loading by extension and resistance to fracture from stress loadings.


A solid or semisolid material, usually polymeric, which deposits a film and is the actual film-forming ingredient in paint. Solutions of polymers are often called resins, but the term actually applies only to the film forming solids, not to the solution.


(1) Electrical resistance, the Ohm, is a measure of the opposition to electrical current flow in a conductor. (2) A force that acts in opposition to another.

Resistance Test

A measurement of resistance using an ohmmeter.


Electrical device which has resistance to the passage of current.

Resistor Pack

Set of dropping resistors used as part of a heater motor circuit to set different speeds.


The number of pixels per square inch shown on the computer screen. The greater the resolution the better the picture.

Resonant Frequency

Vibration at the natural frequency of a component.

Resonant Noise

A noise that results from vibration at the natural frequency of a component.


Component of the exhaust system to reduce noise.


Sources of information, expertise and knowledge.


A device worn over the face to filter particles and fumes out of the air being breathed. Modern 2K paints require the use of air-fed respirators which provide an efficiently filtered supply of air at a positive pressure around a half-or full-face mask worn by the painter.


A narrowing of a passage used to control flow of a gas or liquid.

Resultant Force

A force acting in one direction as result of other forces acting on an object from other directions.


A slow-evaporating solvent added to a paint to prolong the drying time. Typically used to reduce orange peel or blushing.


The fitting of a system after the vehicle has left the factory or later in its life.


Retrofitting refers to the act of adding new technology or features to older systems.


Pipe or similar where, for example, fuel is returned to the tank.

Return Spring

Simple spring to move an item back into its rest position.


A tachometer that indicated revolutions per second.

Reverse Bias

An electrical term applied to semiconductors and transistors which normally allow the flow of current in one direction only but will allow current to flow in the opposite direction under specific conditions.

Reverse Switch

A switch operated by a gearbox component that illuminates the reverse light.

Reversing Circuit

A circuit using changeover switches or relays that reverses the voltage supply to a motor.


To look over and study information again.


To review information in order to remind yourself of its content.


Radio frequency interference. Electromagnetic radiation from an electrical source.


Reason for Rejection


Right-Hand Drive


A type of variable resistor having only two terminals.

Rich Fuel/Air Mixture

A fuel/air mixture with a ratio greater than the stoichiometric ratio of 14.7:1. A mixture strength with an excess of fuel.


Outside edge, periphery. Part of wheel carrying the tyre.

Ring Gear (Starter)

A large diameter gear wheel mounted on the engine flywheel or torque converter flex plate and used to engage with the starter motor drive pinion to start the engine.

Ring Groove

Slot cut round the piston for the piston rings to sit in.

Ring Land

Seating area for piston rings.


Rubbish In Rubbish Out. In relation to the Internet.

Road Adhesion

Grip between the tyres and the road surface.


A component such as a lever to operate a valve due to action of a cam.

Rocker Arm

Arm that works on a rocker shaft to open a valve on an overhead valve engine.

Rocker Shaft

A shaft, usually at the top of an overhead valve engine, on which the rockers pivot.

Rocker Switch

A switch that has two or three positions. Often used with the centre as an off position and then with a voltage reversal process as it is rocked in either direction.


Vehicle body movement from side to side.

Roller Bearing

A type of bearing consisting of a series of parallel rollers held in a cage between inner and outer bearing tracks.

Roller Pump

A type of fuel pump that uses a rotor and a series of parallel rollers in an elliptical casing to produce the pumping and carrying chambers of the pump. Usually electrically driven and used on many fuel injection systems.

Roller-Cell Pump

A type of fuel pump that uses a rotor and a series of parallel rollers in an elliptical casing to produce the pumping and carrying chambers of the pump. Usually electrically driven and used on many fuel injection systems.

Rolling Resistance

The frictional value for the circumferential surface of a rolling component in contact with the track or road on which it runs. An example is the rolling resistance of a road tyre on various road surfaces.

Rolling Road

Rollers set into the ground on which the wheels of a vehicle can be run. The system can then be used to check performance under driving conditions.


Read Only Memory in a computer, which is the permanent memory that is retained even when the power is switched off.


Rated Octane Number


Additional structural panel of vehicle body.

Roots Supercharger

Type of mechanical pressure charger using two twin or triple lobed rollers to form an air pump in the inlet tract of an engine.

Rotary Air Valve

A valve that uses a shutter to cover a hole depending on the amount of rotation. Usually used to refer to an auxiliary air valve on an injection system.

Rotary Control Valve

Power steering control valve coupled to the steering column, which operates by inner and outer components coming together to open valve orifices to allow hydraulic fluid flow.

Rotary Movement

Circular motion. Movement provided by the basic gear wheel or crank.

Rotary Oil Pump

A positive displacement oil pump that uses two rotors inside a housing. There is an eccentrically driven rotor with eternal lobes that mesh with the internal lobes of a driven rotor. The external rotor has one more lobe than the internal rotor and this forms the pumping and carrying chambers of the pump.

Rotary Pump

Type of diesel fuel injection pump.

Rotoflex Joint

A propeller shaft universal joint using a sleeved hexagonal rubber ring between alternate three pointed couplings on each of the two connecting shafts.


A component that rotates! Such as a brake disc, alternator rotor or oil pump rotor.

Rotor Windings

Copper windings that create the magnetic field on the alternator rotor.

Rounding Off

Express as a round number (i.e., 4.7 rounded off becomes 5)


Revolutions per minute.


Repetitive Strain Injury. This type of injury occurs from repeated physical movements. It can be caused by bad typing technique, bad posture and lack of adequate rest and breaks. RSI is common in the wrists.


Room temperature vulcanizing. A type of form-in-place gasket material.


The result of sanding or buffing through one layer of paint and into the substrate below. A rub-through is common from clearcoat to basecoat or from primer to metal.

Rubber Mountings

Hard rubber connection units used to insulate one component from another to reduce vibrations and noise.

Rubbing Compound

An abrasive that smoothes and polishes the paint film. Also known as polishing compound.


A measure of the out-of-true state on the face or faces of rotating components such as brake rotors and the clutch face on engine flywheels.


An engine intake tube joining the plenum chamber to the inlet port in the cylinder head.

Running Clearance

A small gap between moving components that allows movement and lubrication.

Running On

The term applied to an engine that continues to run after the ignition has been switched off. Also run on and dieseling.


A measure of the out-of-true state on the face or faces of rotating components such as brake rotors and the clutch face on engine flywheels.


The corrosion product that forms on a steel surface when it is exposed to oxygen and water. Also known as oxidation.


Rear Wheel Drive.

Rzeppa Joint

A type of constant velocity joint used on vehicle transmission drive shafts. It has 6 steel balls in a cage between inner and outer elliptical housings.

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