ABS - Abbreviation for Anti-lock Braking System. Uses sensors at each wheel to sense when the wheels are about to lock, and releases the brakes to prevent locking. This process occurs many times per second, and allows the driver to maintain steering control when braking hard.
Accelerator pump - A device attached to many carburettors which provides a spurt of extra fuel to the carburettor fuel/air mixture when the accelerator pedal is suddenly pressed down.
Additives - Compounds which are added to petrol and oil to improve their quality and performance.
Advance and retard - A system for altering the ignition timing.
AF - An abbreviation of 'Across Flats', the way in which many nuts, bolts and spanners are identified. AF is usually preceded by an Imperial unit of measurement - e.g. 1/2 in AF. Unless otherwise stated, all metric measurements are assumed to be AF, so the abbreviation is not normally used for metric nuts, bolts and spanners.
Air cooling - Alternative method of engine cooling in which no water is used. An engine-driven fan forces air at high speed over the surfaces of the engine.
ALB - Abbreviation for Anti-Lock Braking System. See 'ABS'.
Alternator - A device for converting rotating mechanical energy into electrical energy. In modern cars, it has superseded the dynamo for charging the battery because of its much greater efficiency.
Ammeter - A device for measuring electrical current - the current supplied to the battery by the alternator, or drawn from the battery by the car's electrical systems.
Antifreeze - A chemical mixed with the water in the cooling system to lower the temperature at which the coolant freezes, and in modern cars to prevent corrosion of the metal in the cooling system.
Anti-roll bar - A metal bar mounted transversely across the car, connecting the two sides of the suspension, which counteracts the natural tendency for the car to lean when cornering.
Aquaplaning - A word used to describe the action of a tyre skating across the surface of water.
Automatic transmission - A type of gearbox which selects the correct gear ratio automatically according to engine speed and load.
Axle - Spindle on which a wheel revolves.

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Balljoint - A ball-and-socket type joint used in steering and suspension systems, which allows relative movement in more than one plane.
Battery condition indicator - A device for measuring electrical voltage (a voltmeter) connected via the ignition switch to the car battery. Unlike an ammeter, it gives an indication that a battery is close to failing. Also, most 'maintenance-free' batteries have a battery condition indicator fitted to their casing, which consists of a small disc which changes its colour when the battery is close to failure and requires renewal.
Bearing - Metal or other hard wearing surface against which another part moves, and which is designed to reduce friction and wear (bearings are usually lubricated).
Bendix drive - A device on some types of starter motor, which allows the motor to drive the engine for starting, then disengages when the engine starts to run.
BHP - see Horsepower.
Big end - The end of a connecting rod, which is attached to the crankshaft. It incorporates a bearing and transmits the linear movement of the connecting rod to the crankshaft.
Bleed nipple (or valve) - A hollow screw, which allows air or fluid to be bled out of a system when it is loosened.
Brake caliper - The part of a disc brake system, which houses the brake pads and the hydraulically operated pistons.
Brake disc - A rotating disc, coupled to a roadwheel, which is clamped between hydraulically operated friction pads in a disc brake system.
Brake fade - A temporary loss of braking efficiency due to overheating of the brake friction material.
Brake pad - The part of a disc brake system, which consists of the friction material and a metal backing plate.
Brake shoe - The part of a drum brake system, which consists of the friction material and a curved metal former.
Breather - A device, which allows fresh air into a system or allows, contaminated air out.
Bucket tappet - A bucket shaped component used in some engines to transfer the rotary movement of the camshaft to the up-and down movement required for valve operation.
Bump stop - A hard piece of rubber used in many suspension systems to prevent the moving parts from contacting the body during violent suspension movements.

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Camber angle - The angle at which the front wheels are set from the vertical, when viewed from the front of the car. Positive camber is the amount in degrees, which the wheels are tilted out at the top.
Cam follower - A piece of metal used to transfer the rotary movement of the camshaft to the up-and-down movement required for valve operation.
Camshaft - A rotating shaft driven from the crankshaft with lobes or cams used to operate the engine valves via the valve gear.
Carbon leads - ignition HT leads incorporating carbon (black fibres) which eliminates the need for separate radio and TV suppressors.
Carburettor - A device that is used to mix air and fuel in the proportions required for burning by the engine under all conditions of engine running.
Castor angle - The angle between the front wheels pivot points and a vertical line when viewed from the side of the car. Positive castor is when the axis is inclined rearwards.
Catalytic converter - A device incorporated in the exhaust system which speeds up the natural decomposition of the exhaust gases, and reduces the amount of harmful gases released into the atmosphere. Cars fitted with catalytic converters must be operated on unleaded petrol, as leaded petrol will destroy the catalyst.
Centrifugal advance - System of ignition advance and retard incorporated in many distributors in which weights rotating on a shaft alter the ignition timing according to engine speed.
Choke - This has two common meanings. It is used to describe the device which shuts off some of the air in a carburettor during cold starting (in order to provide extra fuel), and it may be manually or automatically operated. It's also used as a general term to describe a carburettor throttle bore.
Clutch - A friction device, which allows two rotating components to be coupled together smoothly, without the need for either rotating component to stop moving.
Coil spring - A spiral coil of spring steel used in many suspension systems.
Combustion chamber - Shaped area in the cylinder head into which the fuel/air mixture is compressed by the piston and where the spark from the -spark plug ignites the mixture.
Compression ratio (CR) - A term used to describe the amount by which the fuel/air mixture is compressed as a piston moves from the bottom to the top of its travel, and expressed as a number. For example an 8.5:1 compression ratio means that the volume of fuel/air mixture above the piston when the piston is at the bottom of its stroke is 8.5 times that when the piston is at the top of its stroke.
Compression tester - A special type of pressure gauge which can be screwed into a spark plug hole, which measures the pressure in the cylinder when the engine is turning but not firing. This gives an indication of engine wear or possible leaks.
Condenser (capacitor) - A device in a contact breaker point distributor, which stores electrical energy and prevents excessive sparking at the contact breaker points.
Connecting rod ('con-rod') - Metal rod in the engine connecting a piston to the crankshaft.
Constant velocity (CV) joint - A joint used in driveshafts, where the instantaneous speed of the input shaft is exactly the same as the instantaneous speed of the output shaft at any angle of rotation. This does not occur in ordinary universal joints.
Contact breaker points - A device in the distributor, which consists of two electrical points (or contacts), and a cam, which opens and closes them to operate the HT electrical circuit, which provides the spark at the spark plugs.
Crankcase - The area of the cylinder block below the pistons, which houses the crankshaft.
Crankshaft - A cranked shaft that is driven by the pistons and provides the engine output to the transmission.
Crossflow cylinder head - A cylinder head in which the inlet and exhaust valves and manifolds are on opposite sides.
Crossply tyre - A tyre whose construction is such that the weave of the fabric material layers is running diagonally in alternately opposite directions to a line around the circumference of the tyre.
Cubic capacity - The total volume within the cylinders of an engine which is swept by the movement of the pistons.
CVH - A term applied by the Ford Motor Company to their overhead camshaft engines which incorporate a hemispherical combustion chamber. CVH means Compound Valve angle, Hemispherical combustion chamber.
Cylinder - Close fitting metal tube in which a piston slides. In the case of an engine, the cylinders may be bored directly into the cylinder block, or on some engines, cylinder liners are used which rest in the cylinder block and can be replaced when worn with matching pistons to avoid the requirement for reboring the cylinder block.
Cylinder block -The main engine casting which contains the cylinders, crankshaft and pistons.
Cylinder head - The casting at the top of the engine, which contains the valves and associated operating components.

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Damper - See shock absorber.
Dashpot - An oil-filled cylinder and piston used as a damping device in SU and Zenith/Stromberg CD type carburettors.
Dead axle (beam axle) - The simplest form of axle, consisting of a horizontal member attached to the car underbody by springs. This arrangement is used for the rear axle on some front-wheel-drive cars.
Decarbonising ('decoking') - Removal of all carbon deposits from the combustion chambers and the tops of the pistons and cylinders in an engine.
De Dion axle - A rear axle consisting of a cranked tube attached to the wheel hubs, with a separately mounted differential gear and driveshafts. Suspension is normally through coil springs between the wheel hubs and car underbody.
Derv - Abbreviation for Diesel-Engined Road Vehicle. A term often used to refer to Diesel fuel.
Diaphragm - A flexible membrane used in some components such as fuel pumps. The diaphragm spring used on clutches is similar but is made from spring steel.
Diesel engine - An engine that relies on the heat generated when compressing air to ignite the fuel, and which therefore doesn't need an ignition system. Diesel engines have much higher compression ratios than petrol engines, normally around 20:1.
Differential - A system of gears (generally known as a crownwheel and pinion) which allows the torque provided by the engine to be applied to both driving wheels. The differential divides the torque proportionally between the driving wheels to allow one wheel to turn faster than the other, for example during cornering.
DIN -This stands for Deutsche Industrie Norm (German Industry Standard), which provides international standards for measuring engine power, torque, etc.
Disc brake - A brake assembly where a rotating disc is clamped between hydraulically operated friction pads.
Distributor - A device used to distribute the HT current to the individual spark plugs. The distributor may also contain the advance and retard mechanism. On some older cars, the distributor also contains the contact breaker points assembly.
Distributor cap - Plastic cap which fits on top of the distributor and contains electrodes, in which the rotor arm rotates to distribute the HT spark voltage to the correct spark plug.
DOHC - Abbreviation for Double Overhead Camshaft (see 'Twin-cam').
Doughnut - A term used to describe the flexible rubber coupling used in some driveshafts.
Driveshaft - Term usually used to describe the shaft (normally incorporating universal or constant velocity joints), that transmits drive from a differential to one wheel. More commonly found in front-wheel-drive cars.
Drive train - A collective term used to describe the clutch/gearbox/transmission and the other components used to transmit drive to the wheels.
Drum brake - A brake assembly with friction linings on 'shoes' running inside a cylindrical drum attached to the wheel.
Dual circuit brakes - A hydraulic braking system consisting of two separate fluid circuits, so that if one circuit becomes inoperative, braking power is still available from the other circuit.
Dwell angle - A measurement that corresponds to the number of degrees of distributor shaft rotation during which the contact breaker points are closed during the ignition cycle of one cylinder. The angle is altered by adjusting the contact breaker points gap.


Earth strap - A flexible electrical connection between the battery and a car earth point, or between the engine/gearbox and the car body to provide a return current path flow to the battery.
EFI - Abbreviation for Electronic Fuel Injection.
Electrode - An electrical terminal, e.g. in a spark plug or distributor cap.
Electrolyte - A current-conducting solution inside the battery (consisting of water and sulphuric acid in the case of a car battery).
Electronic ignition - An ignition system incorporating electronic components in place of contact breaker points, which can produce a much higher spark voltage than a contact breaker system, and is less affected by worn components.
Emission control - The reduction or prevention of the release into the atmosphere of poisonous fumes and gases from the engine and fuel system of a car. Required to different degrees by the laws of some countries, and achieved by engine design and the use of special devices and systems.
Epicyclic gears (planetary gears) - A gear system used in many automatic transmissions where there is a central 'sun' gear around which smaller 'planet' gears rotate.
Exhaust gas analyser - An instrument used to measure the amount of pollutants (mainly carbon monoxide) in a car's exhaust gases.
Expansion tank - A container used in many cooling systems to collect the overflow from the car's cooling system as the coolant heats up and expands.

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Filter - A device for removing foreign particles from air, fuel or oil.
Final drive - A collective term (often expressed as a gear ratio) for the crownwheel and pinion (see Differential).
Flat engine - Form of engine design in which the cylinders are opposed horizontally, usually with an equal number on each side of the central crankshaft.
Float chamber - The part of a carburettor, which contains a float and needle valve for controlling the fuel level in the reservoir.
Flywheel - A heavy rotating metal disc attached to the crankshaft and used to smooth out the pulsing from the pistons.
Four stroke (cycle) - A term used to describe the four operating strokes of a piston in a conventional car engine. These are (1) Induction - drawing the air/fuel mixture into the engine as the piston moves down- (2) Compression - of the fuel/air mixture as the piston rises; (3) Power stroke - where the piston is forced down after the fuel/air mixture has been ignited by the spark plug, and (4) Exhaust stroke - where the piston rises and pushes the burnt gases out of the cylinder. During these operations, the inlet and exhaust valves are opened and closed at the correct moment to allow the fuel/air mixture in, the exhaust gases out, or to provide a gas-tight compression chamber.
Fuel injection - A method of injecting fuel into an engine. Used in Diesel engines and also on some petrol engines in place of a carburettor.
Fuel injector - Device used on fuel injection engines to inject fuel directly or indirectly into the combustion chamber. Some fuel injection Systems use a single fuel injector, while some systems use one fuel injector for each cylinder of the engine.



Gasket - Compressible material used between two surfaces to provide a leakproof joint.
Gearbox - A group of gears and shafts installed in a housing, positioned between the clutch and the differential, and used to keep the engine within its safe operating speed range as the speed of the car changes.



Half-shaft - A driveshaft used to transmit the drive from the differential to one of the rear wheels.
Hardy-Spicer joint (Hooke's or Cardan joint) - See Universal joint.
Helical gears - Gears in which the teeth are it at an angle across the circumference of the gear to give a smoother mesh between gears and quieter running.
Horsepower - A measurement of power. Brake Horsepower (BHP) is a measure of the power required to stop a moving body.
HT - Abbreviation of High Tension (meaning high voltage) used to describe the spark plug voltage in an ignition system.
Hub carrier - A component usually found at each front corner of a car which carries the wheel and brake assembly, and to which the suspension and steering components are attached.
Hydraulic - A term used to describe the operation of a system by means of fluid pressure.
Hypoid gear - A gear with curved teeth, which transmits drive through a right angle, where the centreline of the drive gear is offset from the centreline of the driven gear. The meshing action of hypoid gears allows a larger and therefore stronger drive gear, and the meshing noise is reduced in comparison with conventional gears.



Independent suspension - A suspension system where movement of one wheel has no effect on the movement of the other, e.g. independent front suspension.
Ignition coil - An electrical coil, which forms part of the ignition system and which, generates the HT voltage.
Ignition system - The electrical system which provides the spark to ignite the air/fuel mixture in the engine. Normally the system consists of the battery, ignition coil, distributor, ignition switch, spark plugs and wiring.
Ignition timing - The time in the cylinder firing cycle at which the ignition spark (provided by the spark plug) occurs. The spark timing is normally a few degrees of crankshaft rotation before the piston reaches the top of its stroke, and is expressed as a number of degrees before top-dead-centre (BTDC).
Inertia reel - Automatic type of seat belt mechanism which allows the wearer to move freely in normal use, but which locks on sensing either sudden deceleration or a sudden movement of the wearer.
In-line engine - An engine in which the cylinders are positioned in one row as opposed to being in a flat or vee configuration.

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Jet - A calibrated nozzle or orifice in a carburettor through which fuel is drawn for mixing with air.
Jump leads - Heavy electric cables fitted with clips to enable a car's battery to be connected to another battery for emergency starting.



Kerb weight - The weight of a car, unladen but ready to be driven, i.e. with enough fuel, oil, etc, to travel an arbitrary distance.
Kickdown - A device used on automatic transmissions which allows a lower gear to be selected for improved acceleration by fully depressing the accelerator.
Kingpin - A device which allows the front wheel of a car to swivel about a near vertical axis.
Knocking - See 'Pinking'.



Laminated windscreen - A windscreen which has a thin plastic layer sandwiched between two layers of toughened glass. It will not shatter or craze when hit.
Lead-free petrol - Contains no lead. It has no lead added during manufacture, and the natural lead content is refined out. This type of petrol is not currently available for general use in the UK, and should not be confused with unleaded petrol.
Leaded petrol - Normal 4-star petrol. Has a low amount of lead added during manufacture, in addition to the natural lead found in crude oil.
Leading shoe - A drum brake shoe of which the leading end (the one moved by the operating pistons) is reached first by a given point on the drum during normal forward rotation. A simple drum brake will have one leading and one trailing (the opposite) shoe.
Leaf spring - A spring commonly used on cars with a live axle, consisting of several long curved steel plates clamped together.
Limited slip differential - A type of differential, which prevents one wheel from standing, still while the other wheel spins excessively. Often used on high-performance cars.
Live axle - An axle through which power is transmitted to the rear wheels.
Loom - A complete car wiring system or section of a wiring system consisting of all the wires of correct length, etc, to wire up the various circuits.
LT - Abbreviation of Low Tension (meaning low voltage) used to describe battery voltage in the ignition system.



MacPherson strut - An independent front suspension system where the swivelling, springing and shock absorbing action of the wheels is dealt with by a single assembly.
Manifold - A device used for ducting the air/fuel mixture to the engine (inlet manifold), or the exhaust gases from the engine (exhaust manifold).
Master cylinder - A cylinder containing a piston and hydraulic fluid, directly coupled to a foot pedal (e.g. brake or clutch master cylinder). Used for transmitting pressure to the brake or clutch operating mechanism.
Metallic paint - Paint finish incorporating minute particles of metal to give added lustre to the colour.
Multigrade - Lubricating oil whose viscosity covers that of several single grade oils, making it suitable for use over a wider range of operating conditions.

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Needle bearing - Type of bearing in which needle or cone-shaped rollers are used around the circumference to reduce friction.
Needle valve - A component of the carburetor, which restricts the flow of fuel or fuel/air mixture according to the position of the valve in an orifice or jet.
Negative earth - Electrical system (almost universally adopted) in which the negative terminal of the car battery is connected to the car body. The polarity of all the electrical equipment is determined by this.



Octane rating - A scale rating for grading petrol.
OHC (overhead cam) - Describes an engine in which the camshaft is situated above the Cylinder head, and operates the valve gear directly.
OHV (overhead valve) - Describes an engine, which has its valves in the cylinder head, but with the valve gear, operated by pushrods from a camshaft situated lower in the engine.
Oil cooler - Small radiator fitted in the oil circuit and positioned in a cooling airflow to cool the oil. Used mainly on high-performance engines.
Overdrive - A device coupled to a car's gearbox which raises the output gear ratio above the normal 1: 1 of top gear.
Oversteer - A tendency for a car to turn more tightly into a corner than intended.



PCV (Positive Crankcase Ventilation) - A system which allows fumes and vapours which build up in the crankcase to be drawn into the engine for burning,
Pinion - A gear with a small number of teeth which meshes with one having a larger number of teeth.
Pinking - A metallic noise from the engine often caused by the ignition timing being too far advanced. The noise is the result of pressure waves, which cause the cylinder walls to vibrate when the ignited fuel/air mixture is compressed.
Piston - Cylindrical component that slides in 3 closely-fitting metal tube or cylinder and transmits pressure. The pistons in an engine, for example, compress the fuel/air mixture, transmit the power to the crankshaft, and push the burnt gases out through the exhaust valves.
Piston ring - Hardened metal ring, which is a spring, fit in a groove running round the piston to ensure a gas-tight seal between the piston and cylinder wall.
Positive earth - The opposite of negative earth.
Power steering - A steering system that uses hydraulic fluid pressure (provided by an engine-driven pump) to reduce the effort required to steer the car.
Pre-ignition - See 'Pinking'.
Propeller shaft - The shaft, which transmits the drive from the gearbox to the rear axle in a front-engined rear-wheel-drive car.
Pushrod - A rod that is moved up and down by the rotary motion of the camshaft and operates the rocker arms in an OHV engine.

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Quarter light - A triangular window mounted in front or behind the main front or rear windows, usually in the front door, or behind the rear door.
Quartz-halogen bulb - A bulb with a quartz envelope (instead of glass), filled with a halogen gas. Gives a brighter, more even spread of light than an ordinary bulb.



Rack and pinion - Simplest form of steering mechanism, which uses a pinion, gear to move a toothed rack.
Radial ply tyre - A tyre in which the fabric material plies are arranged laterally, at right angles to the circumference.
Radiator - Cooling device through which the engine coolant is passed, situated in an airflow and consisting of a system of fine tubes and fins for rapid heat dissipation.
Radius arms (rods) - Locating arms sometimes used with a live axle to positively locate it in the fore-and-aft direction.
Rebore - The process of enlarging the cylinder bores to a very accurately specified measurement in order to fit new pistons to overcome wear in the engine. Not normally necessary unless the engine has covered a very high mileage.
Recirculating ball steering - A derivative of worm and nut steering, where the steering shaft motion is transmitted to the steering linkage by balls running in the groove of a worm gear.
Rev counter - See Tachometer.
Rocker arm - A lever that rocks on a central pivot, with one end moved up and down by the camshaft, and the other end operating an inlet or exhaust valve.
Rotary engine - See 'Wankel engine'.
Rotor arm - A rotating arm in the distributor, which distributes the HT spark voltage to the correct spark plug.
Running on - A tendency for an engine to keep on running after the ignition has been switched off. Often caused by a badly maintained engine or the use of an incorrect grade of fuel.



SAE - Society of Automotive Engineers (of America). Lays down international standards for the classification of engine performance and many other specifications, but is most commonly used to classify oils.
Safety rim - A special wheel rim shape which prevents a deflated tyre from rolling off the wheel.
Sealed beam - A sealed headlamp unit where the filament is an integral part and cannot be renewed separately. Semi-trailing arm - A common form of independent rear suspension.
Servo - A device for increasing the normal effort applied to a control.
Shock absorber - A device for damping out the up-and-down movement of the suspension when the car hits a bump in the road.
Spark plug - A device with two electrodes insulated from each other by a ceramic material, which screws into an engine combustion chamber. When the HT voltage is applied to the plug terminal, a spark jumps across the electrodes and ignites the fuel/air mixture.
Squab - Another name for a seat cushion.
Steel-braced tyre - Tyre in which extra plies containing steel cords are incorporated with the fabric plies to give added strength.
Steering arm (knuckle) - A short arm on the front hub carrier to which the steering linkage connects.
Steering gear - A general term used to describe the steering components, usually refers to a steering rack-and-pinion assembly.
Steering rack - See Rack and pinion.
Stroboscopic light - A light switched on and off by the engine ignition system which is used for checking the ignition timing when the engine is running.
Stroke - The total distance travelled by a single piston in its cylinder.
Stub axle - A short axle that carries one wheel.
Subframe - A small frame that is mounted on the car's body, and carries the suspension and/or the drivetrain assemblies.
Sump - The main reservoir for the engine oil.
Supercharger - A device which uses an engine-driven turbine (usually driven by a belt or gears from the crankshaft) to drive a compressor which forces air into the engine, providing increased fuel/air mixture flow, and therefore increased engine efficiency. Sometimes used on high-performance engines.
Suppressor - A device that is used to reduce or eliminate electrical interference caused by the ignition system or other electrical components.
Suspension - A general term used to describe the components which suspend the car body on its wheels.
Swing axle - A suspension arm, which is pivoted near the front-to-rear centreline of the car, and which, allows the wheel to swing vertically about that pivot point.
Synchromesh - A device in a gearbox, which synchronises the speed of one gear, shaft with another to produce smooth, noiseless engagement of the gears.

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Tachometer - Also known as a rev counter, indicates engine speed in revolutions per minute (rpm).
Tappet - A term often used to refer to the component which transmits the rotary camshaft movement to the up-and-down movement required for valve operation.
Thermostat - A device which is sensitive to changes in engine coolant temperature, and opens up an additional path for coolant to flow through the radiator (to increase the cooling) when the engine has warmed up.
Tie-rod - A rod which connects the steering arms to the steering gear.
Timing belt - Fabric or rubber belt engaging on sprocket wheels and driving the camshaft from the crankshaft.
Timing chain - Metal flexible link chain engaging on sprocket wheels and driving the camshaft from the crankshaft.
Timing marks - Marks normally found on the crankshaft pulley or the flywheel and used for setting the ignition firing point with respect to a particular piston.
Toe-in/toe-out - The amount by which the front wheels point inwards or outwards from the straight-ahead position when steering straight ahead.
Top Dead Centre (TDC) - The point at which a piston is at the top of its stroke.
Torque - The turning force generated by a rotating component.
Torque converter - A coupling where the driving torque is transmitted through oil. At low speeds there is very little transfer of torque from the input to the output. As the speed of the input shaft increases, the direction of fluid flow within a system of vanes changes, and torque from the input impeller is transferred to the output turbine. The higher the input speed, the closer the output speed approaches it, until they are virtually the same.
Torsion bar - A metal bar, which twists about its own axis, and is used in some suspension systems.
Toughened windscreen - A windscreen which when hit, will shatter in a particular way to produce blunt-edged fragments or will craze over but remain intact. A zone toughened windscreen has a zone in front of the driver which crazes into larger parts to reduce the loss of visibility which occurs when toughened windscreens break, but is otherwise similar.
Track rod - See Tie-rod.
Trailing arm - A form of independent suspension where the wheel is attached to a swinging arm, and is mounted to the rear of the arm pivot.
Transaxle - A combined gearbox/axle assembly from which two driveshafts transmit the drive to the wheels.
Transmission - A general term used to describe some or all of the drivetrain components excluding the engine, most commonly used to describe automatic gearboxes.
Turbocharger - A device which uses a turbine driven by the engine exhaust gases to drive a compressor which forces air into the engine, providing increased fuel/air mixture flow, and therefore increased engine efficiency. Commonly used on high-performance engines.
Twin-cam - Abbreviation for twin overhead camshafts (see 'OHC'). Used on engines with a crossflow cylinder head, usually with one camshaft operating the inlet valves and the other operating the exhaust valves. Gives improved engine efficiency due to improved fuel/air mixture and exhaust gas flow in the combustion chambers.
Two stroke (cycle) - A common term used to describe the operation of an engine where each downward piston stroke is a power stroke. The fuel/air mixture is directed to the crankcase where it's compressed by the descending piston and pumped into the combustion chamber. As the piston rises, the mixture is compressed and ignited, which forces the piston down. The burnt gases flow from the exhaust port, but the piston is now compressing another fuel/air mixture charge in the crankcase which repeats the cycle.



Understeer - A tendency for a car to go straight on when turned into a corner.
Universal joint - A joint that can swivel in any direction whilst at the same time transmitting torque. This type of joint is commonly used in propeller shafts and some driveshafts, but is not suitable for some applications because the input and output shaft speeds are not the same at all positions of angular rotation. The type in common use is known as a HardySpicer, Hooke's or Cardan joint.
Unleaded petrol - Has no lead added during manufacture, but still has the natural lead content of crude oil. Generally available in the UK, most modern cars can use this type of petrol, but seek advice first, as engine adjustments may be required. Engine damage can occur if unleaded petrol is used incorrectly. Not to be confused with lead-free petrol which is not currently available in the UK.
Unsprung weight - The part of the car, which is not supported by the springs.



Vacuum advance - System of ignition advance and retard used in some distributors where the vacuum in the engine inlet manifold is used to act or a diaphragm which alters the ignition timing as the vacuum changes due to the throttle position.
Valve - A device, which opens or closes to allow or stop gas or fluid flow.
Valve gear - A general term used for the components which are acted on by the camshaft in order to operate the valves. 16-valve - Term used to describe a four-cylinder engine with four valves per cylinder (usually two inlet valves and two exhaust valves). Gives improved engine efficiency due to improved fuel/air mixture and exhaust gas flow in the combustion chambers.
Vee engine - An engine design in which the cylinders are set in two banks forming a 'V' when viewed from one end. A V8 for example consists of two rows of four cylinders each.
Venturi - A streamlined restriction in the carburettor throttle bore which causes a low pressure to occur; this sucks fuel into the air stream to form a vapour suitable for combustion.
Viscosity - A term used to describe the resistance of a fluid to flow. When associated with lubricating oil, it's given an SAE number, 10 being a very light oil and 140 being a very heavy oil.
Voltage regulator - A device that regulates the alternator output to a predetermined level. On most alternators the voltage regulator is an integral part of the alternator, and regulates the charging current as well as the voltage.

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Wankel engine - A rotary engine which has a triangular shaped rotor which performs the function of the pistons in a conventional engine, and rotates in a housing shaped approximately like a broad-waisted figure of eight. Very few cars use this type of engine.
Wheel balancing - Adding small weights to the rim of a wheel so that there are no out-ofbalance forces when the wheel rotates.
Wishbone - An 'A'-shaped suspension component, pivoted at the base of the 'A' and carrying a wheel at the apex. Normally mounted close to the horizontal.
Worm and nut steering - A steering system where the lower end of the steering column has a coarse screw thread on which a nut runs, The nut is attached to a spindle which carries the drop arm which, in turn, moves the steering linkage.