The Mechanical Glossary should help explain every confusing term, abbreviation or phrase you may come across during your mechanical career. If you have any suggestions for any jargon we've missed then let us know!.
  • ABS (Anti-lock Brake System)

    A system, usually electronically controlled, that senses incipient wheel lockup during braking and relieves hydraulic pressure at wheels that are about to skid.

  • Acceleration Skid Control (ASC)

    Acts to minimise the effect of wheelspin on slippery surfaces and the wheels would otherwise spin excessively. It helps powerful cars to accelerate quickly, but is also useful for helping heavy vehicles to pull away more smoothly on slippery surfaces.

  • Adaptive Cruise Control

    Maintains a pre-set speed without the driver having to use the accelerator, like normal cruise control, but can slow down when traffic gets in the way and speed up again when it’s clear.

  • Air Bag

    An inflatable bag hidden in the steering wheel (driver’s side) or the dash or glove box (passenger side). In a head-on collision, the bags inflate, preventing the driver and front passenger from being thrown forward into the steering wheel or windscreen.

  • Air Cleaner

    A metal or plastic housing, containing a filter element, which removes dust and dirt from the air being drawn into the engine.

  • Air Conditioning (A/C)

    The removal of heat from indoor air for thermal comfort.

  • Air Filter Element

    The actual filter in an air cleaner system, usually manufactured from pleated paper and requiring renewal at regular intervals.

  • All-Wheel Drive (AWD)

    Improves tyre grip on particularly slippery or irregular surfaces by powering all four wheels. The term is often used interchangeably with four-wheel drive (4WD), although all-wheel drive more often relates to permanently active systems as opposed to those that can be switched to two wheel drive.

  • Allen Key

    A hexagonal wrench which fits into a recessed hexagonal hole.

  • Alligator Clip

    A long-nosed spring-loaded metal clip with meshing teeth. Used to make temporary electrical connections.

  • Alternator

    A component in the electrical system which converts mechanical energy from a drivebelt into electrical energy to charge the battery and to operate the starting system, ignition system and electrical accessories.

  • Ampere (amp)

    A unit of measurement for the flow of electric current. One amp is the amount of current produced by one volt acting through a resistance of one ohm.

  • Anaerobic Sealer

    A substance used to prevent bolts and screws from loosening. Anaerobic means that it does not require oxygen for activation. The Loctite brand is widely used.

  • Anti-Seize Compound

    A coating that reduces the risk of seizing on fasteners that are subjected to high temperatures, such as exhaust manifold bolts and nuts.

  • Antifreeze

    A substance (usually ethylene glycol) mixed with water, and added to a vehicle’s cooling system, to prevent freezing of the coolant in winter. Antifreeze also contains chemicals to inhibit corrosion and the formation of rust and other deposits that would tend to clog the radiator and coolant passages and reduce cooling efficiency.

  • Asbestos

    A natural fibrous mineral with great heat resistance, commonly used in the composition of brake friction materials. Asbestos is a health hazard and the dust created by brake systems should never be inhaled or ingested.

  • Automatic Clutch System

    A system in which there is a conventional manual gearlever, but no clutch pedal. The clutch operates automatically when the driver moves the gear lever. Easier to drive than a full manual set-up, but does not offer automatic gear changing, unlike a semi-auto.

  • Automatic Gearbox

    A system capable of changing gears according to vehicle speed, without input from the driver. There is no clutch pedal so automatics are easier to use, especially in heavy traffic. The downside is a slight reduction in fuel efficiency and power delivery. Most systems enable the driver to take some control of the gearchanging when needed, for situations such as towing or winter driving, while the most sophisticated can offer the driver full control of the gears.

  • Automatic Wipers

    Automatic wipers detect rainfall on the windscreen and activate the wipers. The speed of the wipers depends on how heavy the rain is. These wipers can usually be manually operated as well. Also known as Intelligent wipers.

  • Axle

    A shaft on which a wheel revolves, or which revolves with a wheel. Also, a solid beam that connects the two wheels at one end of the vehicle. An axle which also transmits power to the wheels is known as a live axle.

  • Axleshaft

    A single rotating shaft, on either side of the differential, which delivers power from the final drive assembly to the drive wheels. Also called a driveshaft or a halfshaft.

  • Ball Bearing

    An anti-friction bearing consisting of a hardened inner and outer race with hardened steel balls between two races.

  • Bearing

    The curved surface on a shaft or in a bore, or the part assembled into either, that permits relative motion between them with minimum wear and friction.

  • Bi-Fuel

    A Car capable of using two different types of fuel for power. They typically use petrol and LPG or bioethanol. The specifics can vary from model to model, but the fuels are either mixed together in the same tank or the bi-fuels use completely separate systems.

  • Big-End Bearing

    The bearing in the end of the connecting rod that’s attached to the crankshaft.

  • Bleed Nipple

    A valve on a brake wheel cylinder, caliper or other hydraulic component that is opened to purge the hydraulic system of air. Also called a Bleed Screw.

  • Bleed Screw

    See Bleed Nipple.

  • Blind Spot Information System

    Using cameras to monitor the area to the rear and side of the car that fall outside the coverage of the rear-view and wing mirrors, and are thus out of view from the driver. When an object enters this area a light comes on in the appropriate wing mirror to warn the driver, reducing the chances of pulling out into the path of another vehicle.

  • Brake Bleeding

    Procedure for removing air from lines of a hydraulic brake system.

  • Brake Disc

    The component of a disc brake that rotates with the wheels.

  • Brake Drum

    The component of a drum brake that rotates with the wheels.

  • Brake Horsepower (BHP)

    An agreed unit of measurement used by manufacturers to indicate the power of a car’s engine. The higher the BHP figure, the more powerful the engine. Although the terms BHP, HP and PS are often used interchangeably, it is worth noting that these are calculated slightly differently.

  • Brake Linings

    The friction material which contacts the brake disc or drum to retard the vehicle’s speed. The linings are bonded or riveted to the brake pads or shoes.

  • Brake Pads

    The replaceable friction pads that pinch the brake disc when the brakes are applied. Brake pads consist of a friction material bonded or riveted to a rigid backing plate.

  • Brake Shoe

    The crescent-shaped carrier to which the brake linings are mounted and which forces the lining against the rotating drum during braking.

  • Braked Towing Weight

    Refers to how heavy a trailer a car that it pulls can be. The higher the weight, the greater power needed to tow it. The figure also varies between the type of trailer: the figure for unbraked is lower than braked.

  • Breaker Bar

    A long socket wrench handle providing greater leverage.

  • Bulkhead

    The insulated partition between the engine and the passenger compartment.

  • Caliper

    The non-rotating part of a disc-brake assembly that straddles the disc and carries the brake pads. The caliper also contains the hydraulic components that cause the pads to pinch the disc when the brakes are applied. A caliper is also a measuring tool that can be set to measure inside or outside dimensions of an object.

  • Cam Follower

    See Tappet.

  • Camshaft

    A rotating shaft on which a series of cam lobes operate the valve mechanisms. The camshaft may be driven by gears, by sprockets and chain or by sprockets and a belt.

  • Canister

    A container in an evaporative emission control system; contains activated charcoal granules to trap vapours from the fuel system.

  • Carbon Fibre

    A glossy, man-made material which appears almost black, but when light strikes it from different angles, the woven layers it is constructed from reveal themselves with different shades. Its attractive, hi-tech appearance makes it popular for decorative purposes, but its low weight and high strength mean that it is an excellent building material and this was its original intended purpose. Carbon fibre is expensive. As a result, imitation carbon fibre is often used on cheaper applications to give a similar look.

  • Carburettor

    A device which mixes fuel with air in the proper proportions to provide a desired power output from a spark ignition internal combustion engine.

  • Castellated

    Resembling the parapets along the top of a castle wall. For example, a castellated balljoint stud nut.

  • Castor

    In wheel alignment, the backward or forward tilt of the steering axis. Castor is positive when the steering axis is inclined rearward at the top.

  • Catalytic Converter

    A silencer-like device in the exhaust system which converts certain pollutants in the exhaust gases into less harmful substances.

  • Chassis

    The supportive structure of the car that bears the wheels, engine and bodywork. It also refers to the suspension components.

  • Circlip

    A ring-shaped clip used to prevent endwise movement of cylindrical parts and shafts. An internal circlip is installed in a groove in a housing; an external circlip fits into a groove on the outside of a cylindrical piece such as a shaft.

  • Clearance

    The amount of space between two parts. For example, between a piston and a cylinder, between a bearing and a journal, etc.

  • Climate Control

    The climate control function refers to the car’s integrated heating, ventilation and air conditioning system. It’s more complex than air-conditioning alone, allowing occupants to specify an exact temperature which the system will then create and maintain.

  • CO2 Emission Figure

    The level of carbon dioxide released via the car’s exhaust and is an indicator of environmental friendliness. This figure is growing in importance in the UK as vehicles’ road tax bands are calculated accordingly. The lower the figure, the cheaper the car is to tax. The unit of measurement for this figure is calculated in g/km.

  • Coil Spring

    A spiral of elastic steel found in various sizes throughout a vehicle, for example as a springing medium in the suspension and in the valve train.

  • Compression

    Reduction in volume, and increase in pressure and temperature, of a gas, caused by squeezing it into a smaller space.

  • Compression Ratio

    The relationship between cylinder volume when the piston is at top dead centre and cylinder volume when the piston is at bottom dead centre.

  • Constant Velocity (CV) Joint

    A type of universal joint that cancels out vibrations caused by driving power being transmitted through an angle.

  • Continuously Variable Transmission (CVT)

    A CVT usually has no clutch pedal and can be driven like any other automatic, but uses a distinctly different design to that of a normal auto or manual gearbox. It stands for continuously variable transmission, because instead of fixed gear ratios, the ratio between engine and wheel speed can be varied infinitely (within limits) allowing the engine to keep to its most efficient speed.

  • Core Plug

    A disc or cup-shaped metal device inserted in a hole in a casting through which core was removed when the casting was formed. Also known as a freeze plug or expansion plug.

  • Cornering Headlamps

    Cornering Headlamps create a lit area in front of the car that moves with the steering wheel, improving visibility when cornering.

  • Crankcase

    The lower part of the engine block in which the crankshaft rotates.

  • Crankshaft

    The main rotating member, or shaft, running the length of the crankcase, with offset “throws” to which the connecting rods are attached.

  • Crocodile Clip

    See Alligator Clip.

  • Crumple Zones

    Crumples zones are energy absorbing safety features designed to help minimise passenger injury in the event of an accident. Rather than strengthening areas of the car’s body, crumple zones are specifically designed to scrunch up and soften the blow. This absorbs energy, cushioning occupants within the vehicle.

  • Dashboard

    The dashboard, or dash, is the horizontal control panel spanning the width of the cabin beneath the windscreen. As well as housing the steering wheel and other associated driving functions, the dash usually also features an integrated centre console which controls many other key features, such as the ventilation and entertainment systems.

  • Daytime Runnning Lights

    Rather than just being headlights that remain on during daylight hours, Daytime Running Lights feature a specific light unit, occasionally a bright strip of LED lights. It can be justified as a safety feature as the car stands out more, a benefit that is easier to appreciate in winter months and poorer weather conditions.

  • Deadlocks

    Semi-automatic security feature preventing the doors from being opened unintentionally. Predominantly designed to guard against theft, the doors cannot be opened until the deadlocks are deactivated.

  • Diagnostic Code

    Code numbers obtained by accessing the diagnostic mode of an engine management computer. This code can be used to determine the area in the system where a malfunction may be located.

  • Differential

    When a car goes around a corner, the wheels on the outside of the curve have to travel further and so must turn quicker than those on the inside. A differential allows this to happen while still allowing engine power to be fed to each wheel. Four-wheel drive vehicles have a differential between the the two front wheels, the two back wheels and one between the front and rear axles so that engine power can be varied between the front and rear wheels.

  • Double-overhead cam (DOHC)

    An engine that uses two overhead camshafts, usually one for the intake valves and one for the exhaust valves.

  • Drivebelt(s)

    The belt(s) used to drive accessories such as the alternator, water pump, power steering pump, air conditioning compressor, etc. off the crankshaft pulley.

  • Driver & Vehicle Agency (DVA)

    Northern Ireland’s equivalent of the DVLA.

  • Driver and Vehicle Licensing Agency (DVLA)

    Organisation of UK Government responsible for maintaining a database of drivers and a database of vehicles in Great Britain

  • Driveshaft

    Any shaft used to transmit motion. Commonly used when referring to the axleshafts on a front wheel drive vehicle.

  • Drum Brake

    A type of brake using a drumshaped metal cylinder attached to the inner surface of the wheel. When the brake pedal is pressed, curved brake shoes with friction linings press against the inside of the drum to slow or stop the vehicle.

  • Dual-Clutch Semi-Automatic Gearbox

    A form of semi-automatic gearbox, which can change gear automatically or manually, via a gearlever or steering wheel-mounted paddles without the need for a clutch pedal. The system features two clutches which allows one gear to be engaged and driving, while the next is ready to engage. This results in near-instantaneous gearchanges, improving acceleration figures.

  • EGR Valve

    A valve used to introduce exhaust gases into the intake air stream.

  • Electronic Brake Assist (EBA)

    A further development of the electronics that enable anti-lock brakes, the EBA system recognises possible emergency situation and applies maximum braking capability, even if the driver fails to apply maximum pressure on the pedal. It minimises stopping distances in an emergency.

  • Electronic Brakeforce Distribution (EBD)

    A further development of anti-lock brakes, EBD can vary the level of braking pressure applied to each individual wheel, taking account of possible variations in road surface grip. For example, in a situation when one wheel is on a slipperier surface than other wheels, anti-lock brakes would prevent all wheels from locking, but this would mean the wheels with more grip wouldn’t be working at their best. EBD treats each wheel individually, helping to reduce stopping distances.

  • Electronic Control Unit (ECU)

    A computer which controls (for instance) ignition and fuel injection systems, or an anti-lock braking system.

  • Electronic Fuel Injection (EFI)

    A computer controlled fuel system that distributes fuel through an injector located in each intake port of the engine.

  • Electronic Stability Control (ESC)

    A sophisticated system that detects if a vehicle starts to lose control, and helps the driver to quickly regain control by automatically braking certain wheels or regulating engine power. The system also provides traction control by to preventing wheels from spinning on slippery surfaces or when the driver applies too much power.

  • Emergency Brake

    A braking system, independent of the main hydraulic system, that can be used to slow or stop the vehicle if the primary brakes fail, or to hold the vehicle stationary even though the brake pedal isn’t depressed. It usually consists of a hand lever that actuates either front or rear brakes mechanically through a series of cables and linkages. Also known as a handbrake or parking brake.

  • Endfloat

    The amount of lengthwise movement between two parts. As applied to a crankshaft, the distance that the crankshaft can move forward and back in the cylinder block.

  • Engine Management System (EMS)

    A computer controlled system which manages the fuel injection and the ignition systems in an integrated fashion.

  • Exhaust Manifold

    A part with several passages through which exhaust gases leave the engine combustion chambers and enter the exhaust pipe.

  • Expansion Plug

    See Core Plug.

  • Fan Clutch

    A viscous (fluid) drive coupling device which permits variable engine fan speeds in relation to engine speeds.

  • Feeler Blade

    A thin strip or blade of hardened steel, ground to an exact thickness, used to check or measure clearances between parts.

  • Firing Order

    The order in which the engine cylinders fire, or deliver their power strokes, beginning with the number one cylinder.

  • Flat-Four

    An engine with four cylinders that are arranged horizontally in two rows of two, turning a central crankshaft. Also known as a Boxer engine.

  • Flywheel

    A heavy spinning wheel in which energy is absorbed and stored by means of momentum. On cars, the flywheel is attached to the crankshaft to smooth out firing impulses.

  • Four-Wheel dDrive (4WD)

    Four-wheel drive (4WD) improves tyre grip on particularly slippery surfaces. This helps the vehicle out of tricky situations as the 4WD system divides power to all four wheels and doubles the propulsion.

  • Free Play

    The amount of travel before any action takes place. The “looseness” in a linkage, or an assembly of parts, between the initial application of force and actual movement. For example, the distance the brake pedal moves before the pistons in the master cylinder are actuated.

  • Freewheeling Doorlocks

    A security feature. The lock-barrel will disengage from the lock mechanism and revolve without unlocking, if anything other than the correct key is used.

  • Freeze Plug

    See Core Plug.

  • Front Wheel Drive (FWD)

    A transmission layout, where the engine powers the car from the front only. This means that the car is dragged from the front, rather than pushed from the back and it affects the way the car handles. Cars with FWD tend to be lighter and, consequently, easier to steer and cheaper to run. As the driveshaft is integrated into the engine at the front, they also allow greater interior space, too. On the other hand, when accelerating hard with FWD cars, the steering wheel can tug left and right and feel unstable or move off-line.

  • Fuse

    An electrical device which protects a circuit against accidental overload. The typical fuse contains a soft piece of metal which is calibrated to melt at a predetermined current flow (expressed as amps) and break the circuit.

  • Fusible Link

    A circuit protection device consisting of a conductor surrounded by heat-resistant insulation. The conductor is smaller than the wire it protects, so it acts as the weakest link in the circuit. Unlike a blown fuse, a failed fusible link must frequently be cut from the wire for replacement.

  • Gap

    The distance the spark must travel in jumping from the centre electrode to the side electrode in a spark plug. Also refers to the spacing between the points in a contact breaker assembly in a conventional pointstype ignition, or to the distance between the reluctor or rotor and the pickup coil in an electronic ignition.

  • Gasket

    Any thin, soft material – usually cork, cardboard, asbestos or soft metal – installed between two metal surfaces to ensure a good seal. For instance, the cylinder head gasket seals the joint between the block and the cylinder head.

  • Gauge

    An instrument panel display used to monitor engine conditions. A gauge with a movable pointer on a dial or a fixed scale is an analogue gauge. A gauge with a numerical readout is called a digital gauge.

  • Grams Per Kilometre (g/km)

    G/km stands for grams per kilometre and refers to the amount of grams of carbon dioxide emitted by a car every kilometre travelled. Increasing in importance, cars’ green credentials and UK road tax bands are primarily calculated using g/km as a unit of measurement.

  • Ground Clearance

    It is important for 4x4s and other off-road vehicles to have sufficient ground clearance when being driven off the beaten track. Irregular or unpredictable terrain can damage the underside of the car if the ground clearance is too low. Many off-roaders are fitted with underbody protection as a result. Alternatively, heavily modified or particularly fast cars often have minimal ground clearance for better aerodynamics.

  • Halfshaft

    A rotating shaft that transmits power from the final drive unit to a drive wheel, usually when referring to a live rear axle.

  • Halogen Headlamps

    Fonventional bulbs giving a bright, white light. Despite the benefits of Xenon headlamps, Halogens are still more common and much cheaper to replace. Xenon headlamps use xenon for the high beam and a weaker halogen bulb for the dipped beam.

  • Handbrake

    See Emergency Brake.

  • Harmonic Balancer

    A device designed to reduce torsion or twisting vibration in the crankshaft. May be incorporated in the crankshaft pulley. Also known as a vibration damper.

  • Head-Up Display (HUD)

    Projects vehicle information, such as speed, navigation instructions and warning lights, on to the windscreen in front of the driver. T his reduces the need for the driver to glance downwards at the speedometer or instrument cluster as the information is displayed in direct vision of the road.

  • High Beam Assist

    Dips the mainbeam headlights when sensors detect the headlights of an oncomming vehicle, then returns to mainbeam when no other vehicles are detected..

  • Hill-Descent Control

    An off-road driving aid activated with the touch of a button, at the top of a hill that is to be descended. The system lets the vehicle make a smooth descent, with the driver controlling tthe steering, but not needing to use the brakes or accelerator.

  • Hill-Start Assist

    A driver-aid that works automatically to make pulling away on a slope easier. With Hill-start assist, the driver can release the handbrake on a slope, and the vehicle will hold it’s position automatically allowing the driver to accelerate cleanly away.

  • Hone

    An abrasive tool for correcting small irregularities or differences in diameter in an engine cylinder, brake cylinder, etc.

  • Hybrid Engine

    A hybrid engine has two different power sources. This usually takes the form of a petrol engine linked to an electric motor so that when you brake or coast, the captured energy charges the battery for the electric motor. At high speeds the car uses the petrol engine, at low speeds it switches to electric. That means you get the more power for your money, as a hybrid uses up much less fuel. Not only that, but hybrids are greener for the planet and, as a result, road tax costs are lower too.

  • Hydraulic Tappet

    A tappet that utilises hydraulic pressure from the engine’s lubrication system to maintain zero clearance (constant contact with both camshaft and valve stem). Automatically adjusts to variation in valve stem length. Hydraulic tappets also reduce valve noise.

  • Ignition Timing

    The moment at which the spark plug fires, usually expressed in the number of crankshaft degrees before the piston reaches the top of its stroke.

  • Immobiliser

    An immobiliser is an electronic anti-theft device which requires the correct key in order for the engine to run. This system prevents the car from being “hot wired” after entry has been achieved.

  • Inlet Manifold

    A tube or housing with passages through which flows the air-fuel mixture (carburettor vehicles and vehicles with throttle body injection) or air only (port fuelinjected vehicles) to the port openings in the cylinder head.

  • Instrument Cluster

    A fancy way of describing the collection of dials displaying important information such as the car’s speed, revs and mileage. It primarily houses the speedometer, tachometer and odometer though this varies from car to car.

  • Insurance Group (IG)

    Ranging from one to 20 (one being the cheapest, 20 the highest), insurance groups are a guide for individuals and insurance companies to indicate the cost of insurance. An expensive, high performance car will be a higher insurance group than a cheaper to repair runabout. Confusingly, a second system has been introduced which ranges from one to 50 in an attempt to cope with increasingly complex and diverse model availability. However, a multitude of other factors affect how much your insurance costs too, such as your age, previous claims and post code as well as the car’s capabilities, security features and engine size. As a rule of thumb, the higher the group, the more expensive the car is to insure.

  • Intelligent Wipers

    See Automatic Wipers.

  • Intercooler

    A radiator that cools the otherwise hot air generated by a turbo- or supercharger before it is fed into the engine. As colder air is denser, it means more air is fed in, creating more power.


    ISOFIX are anchoring points used to securely fasten child seats. It is used across the European car industry and these standards mean that one child seat should be interchangeable to fit most cars. Seats can neatly, quickly and safely secure into anchor points to fasten the child seat down.

  • Jump Start

    Starting the engine of a vehicle with a discharged or weak battery by attaching jump leads from the weak battery to a charged or helper battery.

  • Keyless Entry

    Allows the driver to access the cabin without needing to manually unlock the door. When both the sensors within the car and the key fob are close to each other, they communicate and the doors are opened. When the driver gets in, the key fob is again automatically sensed and the ignition can be started by pressing a button. The driver can have key in their pocket, walk up to the car – which automatically senses the key and opens the doors – get in and drive away without ever needing to get the key out of their pocket.

  • Lambda Sensor

    See Oxygen Sensor.

  • Lashing Points

    Purpose-built, fold-away loops within a luggage compartment for securing items that might otherwise move about when the vehicle is in motion. They can often hold a net in plce, too to stop things rolling around. The best systems, normally found in estate cars, can be positioned to suit.

  • Limited-Slip Differential (LSD)

    Helps prevent wheelspin and is usually found on high performance cars. It is also often used on off-road vehicles to improve grip in slippery conditions. Accelerate hard in a car without a limited slip diff, and there’s a good chance that one wheel will loose grip, spin and waste power. A limited-slip differential transmits power to both wheels.

  • Liquified Petroleum Gas (LPG)

    a mixture of gases which most petrol vehicles can be adapted to run on. Since it is a gas, a second, pressurized fuel tank is required. LPG vehicles can be bought ready converted, or a conversion can be carried out on an existing car. Benefits include a quieter ride and lower running costs. The UK Government has tried to tempt buyers into using LPG with grants, cheaper fuel and lower VED, though it is still fairly uncommon due to high conversion costs and seemingly low demand from buyers.

  • Load Sensing Proportioning Valve (LSPV)

    A brake hydraulic system control valve that works like a proportioning valve, but also takes into consideration the amount of weight carried by the rear axle.

  • Locknut

    A nut used to lock an adjustment nut, or other threaded component, in place. For example, a locknut is employed to keep the adjusting nut on the rocker arm in position.

  • Lockwasher

    A form of washer designed to prevent an attaching nut from working loose.

  • Low-Ratio Gearbox

    A gearbox suitable for normal driving is not ideal for tackling extreme conditions like steep hills or very soft ground when off-roading. Consequently off-road cars are often fitted with a second, lower-ratio gearbox which means better traction. Low ratio is normally engaged by a second lever next to the main gear lever and turns the wheels slower, reducing the risk of losing grip. An increasingly popular alternative to a low-ratio gearbox (which adds considerable wieght), is to use electronics to minimise wheelspin.

  • Lumbar Support

    The lumbar region is another name for our lower back, which is the body area that this feature supports. Drivers can find a comfortable driving position by adjusting a button on the side of the seat. This feature is very common and could be particularly helpful during long journeys and for frequent drivers.

  • MacPherson Strut

    A type of front suspension system devised by Earle MacPherson at Ford of England. In its original form, a simple lateral link with the anti-roll bar creates the lower control arm. A long strut – an integral coil spring and shock absorber – is mounted between the body and the steering knuckle. Many modern so-called MacPherson strut systems use a conventional lower A-arm and don’t rely on the anti-roll bar for location.

  • Magnetic Ride

    An advanced suspension system, with the benefit being that the ride comfort can be adjusted from hard to soft. There are a number of adjustable suspension designs, but magnetic ride differs by using dampers filled with a liquid that changes its state when a magnetic field is applied.

  • Mid-Engined

    A mid-engined car has the engine located between the two axles to give a favourable weight distribution – this set-up is mostly found in sports cars. It helps to make the steering more responsive and the back wheels grip better.

  • Milometer

    See Odometer.

  • Ministry Of Transport (MOT) Test

    An annual test of automobile safety, aspects and exhaust emissions which are applicable to most vehicles over three years old in the United Kingdom if they are used on public roads.

  • Multi-Point Fuel Injection

    Fuel injection squirts a precise, computer-controlled measure of fuel into an engine. Multi-point injection refers to the fact that there are individual injectors for each cylinder. Benefits are low exhaust emissions, low fuel consumption and improved engine performance compared to single point injection or carburettors.

  • Multi-Purpose Vehicle (MPV)

    A large, tall vehicle with often more than five seats.

  • Multilink Suspension

    Suspension that comprises multiple (usually five or more) arms to support each wheel. The multitude of links gives superior control of wheel movements compared to simpler systems, contributing to improved comfort and handling. It is not a low-cost solution so its fitment signifies that the manufacturer has taken ride and handling seriously when developing the car.

  • Multimeter

    An electrical test instrument with the capability to measure voltage, current and resistance.

  • Naturally Aspirated

    A naturally aspirated car engine sucks air in rather than having it forced in under pressure by a turbo or supercharger. A naturally aspirated engine is usually less powerful than an engine of the same size that uses a turbocharger, unless it

  • NOx

    Oxides of Nitrogen. A common toxic pollutant emitted by petrol and diesel engines at higher temperatures.

  • O-Ring

    A type of sealing ring made of a special rubber-like material; in use, the O-ring is compressed into a groove to provide the sealing action.

  • Odometer

    Measures the total distance travelled by the car. Usually located within the instrument cluster, many newer vehicles feature an electronic odometer, though conventionally this is a manual device. Clocking is an associated term, whereby the odometer is illegally tampered with. The clock is rewound to falsify the total figure and make the car appear to be newer than it actually is. This method deceitfully inflates the vehicle’s value. These days this is done with a laptop and software. In the past it was done mechanically – wound back with a drill or a screwdriver. Also known as a milometer.

  • Ohm

    The unit of electrical resistance. One volt applied to a resistance of one ohm will produce a current of one amp.

  • Ohmmeter

    An instrument for measuring electrical resistance.

  • One Touch Down Windows

    One touch down windows offer a quick and easy way to operate the car’s electric windows. Unlike other electric windows which require the button to be pressed until the window is wound all the way down, one touch down windows only require to be pressed once to perform the same function – hence this feature’s name. If the occupant requires the window to be wound down to an intermediate height, the button can be pressed twice in quick succession: once to begin operation and again when the window has reached the desired level.

  • Overhead Cam (OHC) Engine

    An engine with the camshaft(s) located on top of the cylinder head(s).

  • Overhead Valve (OHV) Engine

    An engine with the valves located in the cylinder head, but with the camshaft located in the engine block.

  • Oversteer

    Oversteer is a term related to how well a car handles on corners. If the car requires less steering during cornering than it should, it has cornered too tightly. This is called oversteer, which can cause the car to skid. The opposite to oversteer is called understeer.

  • Oxygen Sensor

    A device installed in the engine exhaust manifold, which senses the oxygen content in the exhaust and converts this information into an electric current. Also called a Lambda sensor.

  • Paddleshift

    Cars which do not have a clutch pedal often feature a gearbox that allows gearschanges to be made by flipping small levers located behind the steering wheel. This allows the driver to change gears without taking their hands off the wheel, in the style of a Formula 1 car although there may also be a conventionally located gearlever too. The name stems from the fact that these are normally of a flat, paddle-like appearance.

  • Panoramic Roof

    A panoramic roof is an extra-large sunroof allowing greater visibility and additional sunlight in. Nonetheless, large areas of glazing can reduce the car’s overall body strength and negatively affect the way it handles.

  • Parking Brake

    See Emergency Brake.

  • Parking Sensor

    Parking sensors assist with low-speed manoeuvres and help reduce the chances of a parking accident. Sensors are concealed within either the rear or both bumpers and automatically measure the distance between the car and the nearest object, and provide an audible and/or visual indication to the driver. More sophisticated systems include a visual representation of the car which demonstrates where the nearest object is, or a parking camera – where the driver sees a live rear-view image as they park.

  • Particulate Trap (DPF)

    The particulate trap reduces the soot deposits typically produced by diesel engines, helping to achieve emmisions targets. A car using a particualte trap will emit no visible smoke. The trap captures the soot particles, until it fills up, then runs a special mode which burns up the contents, emptying the trap. It does not in itself cover CO2 emissions, but helps to make the car cleaner.

  • Phillips Screw

    A type of screw head having a cross instead of a slot for a corresponding type of screwdriver.

  • Plastigage

    A thin strip of plastic thread, available in different sizes, used for measuring clearances. For example, a strip of Plastigage is laid across a bearing journal. The parts are assembled and dismantled; the width of the crushed strip indicates the clearance between journal and bearing.

  • Pollen Filter

    A pollen filter is integrated into the car’s ventilation system and prevents pollen particles from entering the cabin. This feature is particularly useful for hayfever sufferers in the Summer months.

  • Pop-Up Bonnet

    A pop-up bonnet is designed to acivate in a split second if sensors detect a frontal impact, increasing the level of cushioning between any pedestrian which may have been struck and the unyeilding parts of the engine bay.

  • Power Assisted Steering (PAS)

    Power assisted steering (PAS) lightens the car’s steering and makes it easier to drive. This feature makes light work of low speed manoeuvres – particularly parking – and is becoming a standard option on more and more cars.

  • Propeller Shaft

    The long hollow tube with universal joints at both ends that carries power from the transmission to the differential on front-engined rear wheel drive vehicles.

  • Proportioning Valve

    A hydraulic control valve which limits the amount of pressure to the rear brakes during panic stops to prevent wheel lock-up.

  • Quickclear Windscreens

    A quickclear windscreen uses ultra-thin electrical strands to heat and clear the windscreen extremely quickly on frosty mornings. Due to the advanced technology, quickclear windscreens are expensive to replace when chipped or cracked, though this is covered on most insurance policies.

  • Rack-And-Pinion Steering

    A steering system with a pinion gear on the end of the steering shaft that mates with a rack (think of a geared wheel opened up and laid flat). When the steering wheel is turned, the pinion turns, moving the rack to the left or right. This movement is transmitted through the track rods to the steering arms at the wheels.

  • Radiator

    A liquid-to-air heat transfer device designed to reduce the temperature of the coolant in an internal combustion engine cooling system.

  • Rake And Reach Adjustable

    A rake and reach adjustable steering wheel allows the driver to tilt the angle and fine-tune the closeness of the wheel according to their personal needs.

  • Ramp Angle

    The ramp angle gives a measure of the ground clearance a vehicle can tackle without grounding out. Specifically it is the angle between a line drawn from the centrepoint of the underside to the front and rear wheels. A short car with big wheel will likely have a lower ramp angle and therefore better ground clearance than a longer, smaller-wheeled vehicle.

  • Rear Wheel Drive (RWD)

    a transmission layout, where the engine powers the car from the rear only. This means that the car is pushed from the back, rather than dragged from the front and it affects the way the car handles. RWD improves weight distribution and eliminates torque steer, making the car more predictable to handle.

  • Refrigerant

    Any substance used as a heat transfer agent in an air-conditioning system. R-12 has been the principle refrigerant for many years; recently, however, manufacturers have begun using R-134a, a non-CFC substance that is considered less harmful to the ozone in the upper atmosphere.

  • Registered Keeper

    This is the name of the person or company showing on a vehicles V5C (log book).

  • Release Bearing

    See Thrust Bearing.

  • Remote Central Locking

    Remote central locking simultaneously activates the doorlocks, in all doors, at the push of a button. There’s no need to manually insert the key. This allows the driver to activate all locks quickly, safely and from close distance.

  • Rev Limiter

    Electronically prohibits the engine from being driven in too low a gear for a sustained period. This is commonly called red lining. As driving at high revs in the wrong gear for too long can cause engine damage, many modern cars are fitted with a rev limiter as a precautionary measure.

  • Revolutions Per Minute (RPM)

    The number of revolutions per minute (RPM) indicates how hard an engine is working in its current gear. Another term for RPM is revs. The higher the number of revs, the harder the engine is working. If the revs are too high for too long and the engine is in the wrong gear, it can cause serious and costly damage.

  • Rocker Arm

    A lever arm that rocks on a shaft or pivots on a stud. In an overhead valve engine, the rocker arm converts the upward movement of the pushrod into a downward movement to open a valve.

  • Rotor

    In a distributor, the rotating device inside the cap that connects the centre electrode and the outer terminals as it turns, distributing the high voltage from the coil secondary winding to the proper spark plug. Also, that part of an alternator which rotates inside the stator. Also, the rotating assembly of a turbocharger, including the compressor wheel, shaft and turbine wheel.

  • Run-Flat Tyres

    In the event of a puncture, run-flat tyres can continue to be used for a limited time at reduced speed. As the run-flat tyre is made of sterner stuff, it can bear the car’s weight for a while – even when the tyre has lost most of its pressure. Since the tyre does not collapse, it is better able to maintain safe grip levels. A downside is that these are more expensive than traditional tyres.

  • Runout

    The amount of wobble (in-and-out movement) of a gear or wheel as it’s rotated. The amount a shaft rotates “out-of-true.” The out-of-round condition of a rotating part.

  • Sealant

    A liquid or paste used to prevent leakage at a joint. Sometimes used in conjunction with a gasket.

  • Sealed Beam Lamp

    An older headlight design which integrates the reflector, lens and filaments into a hermetically-sealed one-piece unit. When a filament burns out or the lens cracks, the entire unit is simply replaced.

  • Serpentine Drivebelt

    A single, long, wide accessory drivebelt that’s used on some newer vehicles to drive all the accessories, instead of a series of smaller, shorter belts. Serpentine drivebelts are usually tensioned by an automatic tensioner.

  • Shim

    Thin spacer, commonly used to adjust the clearance or relative positions between two parts. For example, shims inserted into or under bucket tappets control valve clearances. Clearance is adjusted by changing the thickness of the shim.

  • Slide Hammer

    A special puller that screws into or hooks onto a component such as a shaft or bearing; a heavy sliding handle on the shaft bottoms against the end of the shaft to knock the component free.

  • SORN (Statutory Off Road Notification)

    If you do not renew your vehicle excise licence (known as a tax disc) then you must declare that the vehicle is off the road with the DVLA, which is now a legal requirement.

  • Speedometer

    The speedometer gauges the speed travelled by the car. It is usually located within the instrument cluster and many newer vehicles feature an electronic speedo – though conventionally this is a mechanical device.

  • Sprocket

    A tooth or projection on the periphery of a wheel, shaped to engage with a chain or drivebelt. Commonly used to refer to the sprocket wheel itself.

  • Starter Inhibitor Switch

    On vehicles with an automatic transmission, a switch that prevents starting if the vehicle is not in Neutral or Park.

  • Strut

    See MacPherson Strut.

  • Tachometer

    Measures how high the engine’s revs are (also known as revolutions per minute or RPM). The device is usually located behind the steering wheel and many newer vehicles feature an electronic tachometer – though conventionally this is a mechanical device. Tachometers display whether the revs are too high by featuring a ‘red line’ segment. If the needle is in this section, it’s time to change gear. As driving at high revs in the wrong gear for a sustained period can cause engine damage, many modern cars now feature a rev limiter, which electronically prohibits such damage from happening.

  • Tappet

    A cylindrical component which transmits motion from the cam to the valve stem, either directly or via a pushrod and rocker arm. Also called a cam follower.

  • Thermostat

    A heat-controlled valve that regulates the flow of coolant between the cylinder block and the radiator, so maintaining optimum engine operating temperature. A thermostat is also used in some air cleaners in which the temperature is regulated.

  • Thrust Bearing

    The bearing in the clutch assembly that is moved in to the release levers by clutch pedal action to disengage the clutch. Also referred to as a release bearing.

  • Timing Belt

    A toothed belt which drives the camshaft. Serious engine damage may result if it breaks in service.

  • Timing Chain

    A chain which drives the Camshaft.

  • Toe-In

    The amount the front wheels are closer together at the front than at the rear. On rear wheel drive vehicles, a slight amount of toe-in is usually specified to keep the front wheels running parallel on the road by offsetting other forces that tend to spread the wheels apart.

  • Toe-Out

    The amount the front wheels are closer together at the rear than at the front. On front wheel drive vehicles, a slight amount of toe-out is usually specified.

  • Tracer

    A stripe of a second colour applied to a wire insulator to distinguish that wire from another one with the same colour insulator.

  • Tune-Up

    A process of accurate and careful adjustments and parts replacement to obtain the best possible engine performance.

  • Turbocharger

    A centrifugal device, driven by exhaust gases, that pressurises the intake air. Normally used to increase the power output from a given engine displacement, but can also be used primarily to reduce exhaust emissions (as on VW’s “Umwelt” Diesel engine)

  • Universal Joint (U-Joint)

    A double-pivoted connection for transmitting power from a driving to a driven shaft through an angle. A U-Joint consists of two Y-shaped yokes and a cross-shaped member called the spider.

  • V5C (Vehicle Registration Certificate)

    This is the log book for a vehicle, also known as a Registration Document.

  • V750 (Certificate Of Entitlement)

    This is the piece of paper officially known as a Certificate Of Entitlement. A peronalised registration mark can be purchased and held on a V750 and then transferred to a vehicle of choice at a later date.

  • V778 (Retention Certificate)

    This is the piece of paper officially known as a Retention Certificate which is proof of a registration mark having been retained from a vehicle, pending assignment.

  • Valve

    A device through which the flow of liquid, gas, vacuum, or loose material in bulk may be started, stopped, or regulated by a movable part that opens, shuts, or partially obstructs one or more ports or passageways. A valve is also the movable part of such a device.

  • Valve Clearance

    The clearance between the valve tip (the end of the valve stem) and the rocker arm or tappet. The valve clearance is measured when the valve is closed.

  • Vernier Caliper

    A precision measuring instrument that measures inside and outside dimensions. Not quite as accurate as a micrometer, but more convenient.

  • Viscosity

    The thickness of a liquid or its resistance to flow.

  • Volt

    A unit for expressing electrical “pressure” in a circuit. One volt that will produce a current of one ampere through a resistance of one ohm.

  • VOSA (Vehicle and Operator Services Agency)

    A non-departmental public body granted Trading Fund status in the United Kingdom sponsored by the Department for Transport of the United Kingdom Government. Most commonly known for operating the MOT Testing Scheme.

  • Welding

    Various processes used to join metal items by heating the areas to be joined to a molten state and fusing them together.

  • Wiring Diagram

    A drawing portraying the components and wires in a vehicle’s electrical system, using standardised symbols.