NABTEB Motor Vehicle Mechanics key terms

Clothing and gear such as overalls, goggles, gloves, safety boots and hearing protection worn while working on vehicles, reducing the risk of injury from fuels, batteries, sharp edges, hot parts and falling objects.

The three conditions - fuel, oxygen and heat - that must all be present for a fire to burn. Removing any one of them puts the fire out, which explains why different extinguisher types suit different fires.

A rigid, height-adjustable support placed under the chassis or axle after a vehicle is jacked up, so the weight is held mechanically while a technician works underneath.

A wrench that signals, usually by a click, when a set turning force is reached. It is used to tighten fasteners to the manufacturer's specified torque so joints are neither loose nor overstressed.

A measuring instrument with a sliding jaw and graduated scales that reads outside, inside and depth dimensions to about 0.02 mm, used to check shaft diameters, bore wear and component thickness.

A set of thin metal blades of known thickness used to measure small gaps, such as valve clearances, spark plug gaps and piston ring end gaps, by finding the blade that just fits.

A pressure gauge screwed into a spark plug or injector hole that records the maximum pressure built up during cranking, revealing whether rings, valves or the head gasket are sealing properly.

Bore is the internal diameter of a cylinder; stroke is the distance the piston travels from top dead centre to bottom dead centre. Together they fix the swept volume and character of an engine.

The ratio of the cylinder volume above the piston at bottom dead centre to the volume remaining at top dead centre. Higher ratios improve efficiency but require higher-octane fuel to avoid knocking.

The small gap between the valve stem tip and the rocker or cam follower. It allows valves to close fully when the engine is hot; too little can burn valves, too much causes noise and reduced lift.

The clearance between the two ends of a piston ring when it is squared in its bore. It allows for heat expansion and must be checked with a feeler gauge before assembly.

A prescribed order, usually spiralling outward from the centre, in which head bolts are tightened in stages so clamping load spreads evenly and the head and gasket are not distorted.

A thin plastic strip laid across a bearing shell and journal. After the cap is torqued, the flattened width of the strip is compared with a printed scale to show the oil clearance.

The small axial movement permitted between a crankshaft and its thrust bearing. Excessive or insufficient float points to worn, missing or wrongly fitted thrust washers.

A narrowed throat in a carburettor or throttle body where air speeds up and its pressure drops, so fuel is drawn out through a jet and mixed into the airstream.

The chemically ideal mass ratio of air to fuel for complete combustion, about 14.7:1 for petrol. Richer or leaner mixtures alter power, fuel economy and exhaust emissions.

A system in which a pump supplies pressurised fuel to injectors opened by electrical signals, so the quantity and timing of fuel delivery are controlled precisely by the engine management unit.

The point, expressed in degrees of crankshaft rotation before or after top dead centre, at which the spark occurs. Wrong timing causes knocking, hard starting, overheating or lost power.

The number of degrees of distributor rotation during which the contact breaker points stay closed, controlling how long the ignition coil charges before each spark is produced.

A rating of how quickly a plug transfers heat away from its tip. A hot plug runs hotter to burn off deposits; a cold plug conducts heat away faster for sustained high-load running.

An electronic module that reads sensors for airflow, oxygen, temperature and throttle position, then calculates injector and ignition outputs so the engine runs efficiently and within emission limits.

The small clearance at the pedal or release mechanism before the release bearing touches the pressure plate. It ensures the clutch engages fully and does not slip as it wears.

A cone-and-hub device inside a manual gearbox that friction-matches the speed of a gear to its shaft before engagement, allowing smooth gear selection without crashing the teeth.

The ratio of crown wheel teeth to pinion teeth in the differential. It multiplies engine torque reaching the wheels and fixes road speed for a given engine speed.

A cross-and-yoke coupling that lets a propeller shaft transmit drive through a changing angle as the suspension moves, keeping the gearbox connected to the final drive.

A fluid coupling used in automatic transmissions that multiplies engine torque at low road speed and allows the vehicle to stand still in gear without a clutch pedal.

The inward or outward tilt of a wheel from the vertical when viewed from the front of the vehicle. It influences tyre wear, grip and cornering behaviour.

The forward or backward lean of the steering axis from vertical, seen from the side. Positive caster improves straight-line stability and helps the steering self-centre after a turn.

The amount by which the front edges of two wheels point towards each other relative to the rear edges. Incorrect toe scrubs tyres rapidly and makes steering feel vague.

Even distribution of mass around a wheel and tyre so it does not vibrate at speed. It is corrected by spinning the assembly on a balancing machine and fitting weights at the rim.

Loss of braking force when the brakes overheat, either because friction material loses grip or because brake fluid vaporises. Better cooling, suitable linings and fresh fluid reduce it.

Removing trapped air from the hydraulic brake system by opening the bleed nipples and pumping fluid through until a clear, bubble-free stream appears, restoring a firm pedal.

Wheel speed sensors and a hydraulic modulator that release and reapply brake pressure many times a second to prevent wheel lock, so the driver keeps steering control during heavy braking.

The density of battery acid compared with water, measured with a hydrometer to estimate the state of charge of a lead-acid battery; it falls as the battery discharges.

A belt-driven generator with a built-in rectifier that charges the battery and powers the vehicle's electrical loads while the engine runs, replacing the older dynamo.

A heavy-duty relay that switches the large current from the battery to the starter motor and also thrusts the drive pinion into mesh with the flywheel ring gear.

A numbered code stored by an electronic control unit when it detects an out-of-range sensor signal or a circuit fault, read with a scan tool to guide systematic fault finding.

A test in which compressed air is fed into a cylinder held at top dead centre; air escaping at the dipstick tube, exhaust, intake or radiator identifies worn rings, leaking valves or a failed head gasket.

Scheduled servicing such as oil and filter changes, fluid level checks and inspection of belts, brakes and tyres, carried out at set intervals so faults are avoided rather than repaired afterwards.

Driving the vehicle under varied conditions after work is done, to confirm the repair, listen for noises, feel for vibration or pulling and check that the reported fault has gone.