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A backfire occurs when an unburnt fuel/air mixture passes into the exhaust system where it is ignited, rapidly burning out through the exhaust.


The amount of play or clearance between two parts. In the case of gears, it refers to how much one gear can be moved back and forth without moving the gear into which it is meshed.


A rigid metal plate, fixed to the axle, on which the wheel cylinders, brake shoes and other brake parts are mounted.

Back pressure (Exhaust)

Any pressure holding back the flow of the gases in the exhaust system. Pressure exerted in the exhaust system in reverse direction.


1. An obstruction (e.g., plate, vane, wall) in a tank or container used to slow down or divert the flow of gases, liquids, sound, etc.

2. A wall or partition to impede or divide a fluid flow in a tank to reduce sloshing of liquid.

3. Plates fitted between cylinders of air-cooled engines to assist cooling.


The trademark for a synthetic thermosetting plastic resin used in electrical parts because it is a good insulator. The name comes from its inventor, L. H. Baekeland, 1863-1944.

Baulk Ring

A mechanism which helps ensure that gear changes are smooth and quiet by preventing the gears from being engaged until both components coming into contact are travelling at the same speed. The synchro cone is the component which actually speeds up or slows down the gears before they are engaged while the baulk ring prevents the engagement taking place until the speeds match. See also Blocker ring.

Ball bearing

An anti-friction bearing consisting of an inner and outer hardened steel race separated by a series of hardened steel balls, which are sometimes retained in a cage.

Ball check valve

A valve assembly which permits flow of fluid in one direction only.

Ball Joint

A flexible joint using a ball and socket type of construction, used in steering and suspension setups, comonly track and tie rods. Their flexibility helps to compensate for the changes in the suspension and steering when turning or hitting a bump on the road. There are usually upper and lower ball joints attached to the upper and lower A-arms. Some ball joints have a grease nipple to allow periodic lubrication.

Band Brake

A flexible band wrapped partially around the periphery of a wheel or drum. One end is anchored, and the braking force is applied to the other.

Banjo Axle

One of the commonest form of rear-axle casing in which the provision of the differential casing in the centre produces a resemblance to a banjo with two necks.

Banjo Union

A type of hydraulic fitting, shaped like a banjo, through which a hollow bolt passes, allowing fluid transfer from a hydraulic line to a hydraulic component.


An electro-chemical device with one or more cells for producing direct-current electricity by converting chemical energy to electrical energy. The typical automotive battery is a lead-acid type, supplying the source of power for cranking the engine and also provides the necessary electrical energy for the ignition/fuel system.

B.D.C. - Bottom Dead Centre

The lowest point of the piston and connecting rod travel in a cylinder. In a horizontally opposed engine. Opposite to Top Dead Centre (TDC)


The portion of a tire which fits onto the rim of the wheel. On a tubeless tyre, the contact of the bead with the rim seals the air into the tire. Bead heel, bead sole, and bead toe form a foot-like shape.

Beam Axle

A rigid or dead axle which supports the non-driven wheels.


1. The area of a unit in which the contacting surface of a revolving part rests in order to minimise wear and friction between two surfaces.

2. An anti-friction reducing device that is usually found between two moving parts. The shell bearings found between the connecting rod and the crankshaft are lubricated and cushioned with oil, gearbox bearings are lubricated with gearbox oil and wheel bearings must be packed with grease at regular intervals. Some bearings are adjustable.

Bearing Cap

A rigid, semicircular part which encloses and holds the outer shell of a shell bearing.

Bearing Preload

On adjustable bearing setups, the amount of static pressure exerted on a bearing or a set of bearings. The preload is usually adjusted by a threaded collar or shims. The bearings are generally classified in 3 states of adjustment.

1. Unloaded - Free to rotate with measurable end float.

2. Loaded - Free to rotate with no end float.

3. Preloaded - Measurable resistance to rotation with no end float.

Bearing Race

1. In ball or roller bearings, it is one of the two steel rings on either side of the ball or roller.

2. The inner or outer ring that provides the smooth, hard contact surface for the balls or rollers in a bearing.

Bevel Gear

1. A gear shaped like the wide end (frustum) of a cone, used to transmit motion through an angle. They are found in differential and final drives.

2. A system of toothed wheels connecting shafts whose axes are at an angle to one another but in the same plane.

Before Bottom Dead Centre (B.B.D.C.)

As the crankshaft rotates, it brings the piston down to a place just before it reaches Bottom Dead Centre.

Before Top Dead Centre (B.T.D.C.)

As the crankshaft rotates, it pushes the piston up to a place just before it reaches Top Dead Centre.

Bell Housing

Sometimes called clutch housing. An extended metal covering from the gearbox, around the flywheel and clutch (of a manual transmission) or torque converter assembly (of an automatic transmission).


A sealed, accordion-type chamber which expands and contracts in accordance with the movement of parts to which is attached. The primary purpose of bellows is to keep contaminants such as dust, dirt and water out, while keeping lubricants in. Most commonly found on modern cars in the steering rack.

Belly pan

A plate of sheet metal or plastic/composite attached to the bottom of a vehicle to protect the engine and transmission components, as well as improving aerodynamics.


A circular band used to transmit rotational movement and power from one pulley to another. For example, an auxiliary belt to transmit power from the crankshaft to the alternator. Also a toothed belt to transmit power from the crankshaft to the camshaft while maintaining timing. See: Cam belt

Belt tensioner

A device consisting of an idler pulley, usually located between the driving and driven pulleys. It can be adjusted to increase the tension on the belt, either automatically by way of a spring or manually tensioned during installation or adjustment.

Bevel Gear

A gear shaped like the wide end of a Cone, used to transmit drive through an angle. They are commonly found in differentials and final drives.

Big End

The end of the connecting rod which is secured to the crankshaft.

Big End Bearing

A bearing (generally a shell type) fitted to the end of the connecting rod that is secured to the crankshaft.


A twelve sided figure. In tools and nuts & bolts, a star like fitment of the tool or fixing.


Two types of metal bonded into a strip. Each type of metal has different thermal expansion characteristics, making the strip deform in accordance with different temperatures. Often found in thermal trips and automatic chokes.

Black Smoke

Incompletely burned fuel in the exhaust system indicating that the fuel/air mixture is too rich.


The rubbing of brake shoes against the drum or brake pads against the disc/rotor.

Blade Impeller

A rotating part of a centrifugal pump that has blades or vanes.

Blanking Piece

A flat piece of metal which closes off a tube. Also called a blanking plate

Blanking Plate

A flat piece of metal which closes off a tube. Also called a blanking piece.

Bleed / Bleeding

to remove air bubbles from a system. i.e. Braking, Fuel or Cooling system


The part of the engine that contains the cylinders.

Bleed Nipple

A hollow screw used to open a bleed valve to allow fluid and air bubbles from a system. For example, the brake system during a bleeding process after repairs have been carried out.

Blocker ring

A mechanism which helps ensure that gear changes are smooth and quiet by preventing the gears from being engaged until both components coming into contact are travelling at the same speed. The synchro cone is the component which actually speeds up or slows down the gears before they are engaged while the baulk ring prevents the engagement taking place until the speeds match. See also Baulk ring

Blow back

The return, at low speeds, of some of the induced mixture through the carburettor of a petrol engine, due to the late closing of the intake valve during compression, or by worn or sticking valves.


A mixture of fuel/air that finds its way past the piston rings into the crank case. This can be seen by the engine "breathing" through the oil filler and dipstick hole.


  1. Also called a supercharger or turbocharger. This is a pump that forces air into the cylinders at higher than atmospheric pressure. The increased pressure forces a larger volume of air into the cylinders than what would be drawn in by normal aspiration. In this way the engine can burn more fuel and in turn produce more power. 

  2. The electrically driven fan that circulates air inside the  cabin of a vehicle.

Blow-off valve

A one-way valve that opens to atmosphere above a certain set pressure, to relieve excessive pressure. This is often used with a turbocharger to limit Boost pressure to the engine. Also known as a Pressure-relief valve.

See also Wastegate

Blue smoke

The colour of the exhaust, indicating that oil is escaping into the combustion chamber and being burned. The most likely cause of this being worn piston rings and valve stem seals.


A protective rubberised cover protecting an assembly from the ingress of water and other contaminants. e.g. CV joint boot, Ball joint boot, mater cylinder etc.

Boost (Pressure)

The amount of positive pressure created in an intake system above normal atmospheric pressure, most commonly by a turbocharger or supercharger.

Body roll

The tilting motion of a vehicle when it goes around a corner.

Bottom Dead Centre (BDC)

The lowest point of the piston and connecting rod travel in a cylinder. The opposite end of Top Dead Centre.

In a horizontally opposed engine, it is often called inner dead centre and outer dead centre.

Bottle Jack

A hydraulic lifting device made in the shape of a bottle.


The area/diameter of a cylinder hole in which a piston moves.

Bowden cable

A wire control cable within a metal or rubber sheath that is used for controlling a clutch, choke, or accelerator.

Bottoming out

When something reaches the end of its travel. i.e. when the suspension reaches the end of its travel when the vehicle goes over a bump.

Bottom end

All the moving parts withing the crank case, including bearings, crankshaft etc.

Boxer Engine

A horizontally opposed engine.


A protective cover mounted to the front of a vehicle to protect the finish from stones. Usually made of leather or other synthetic material.

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supporting stays, rods and ties used to strengthen a structure.

Braided Hose

A rubber type hose covered in braided wire or a woven material.

Often used in high performance brake hoses.

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