WBCHSE HS Vocational Automobile key terms

A calibrated tool that tightens a fastener to a set twisting force, then signals when that value is reached. It prevents over- or under-tightening of critical parts such as cylinder head bolts.

A precision measuring instrument with a screw-driven spindle that reads small thicknesses or diameters, usually to 0.01 mm. Used to check wear on journals, pistons and brake discs.

A sliding measuring tool with a main scale and a short vernier scale, able to read outside, inside and depth dimensions to about 0.02 mm. Common for quick workshop checks.

A set of thin steel blades of marked thickness used to measure small gaps. Typical workshop uses include valve clearance, spark plug gap and brake pad clearance checks.

The operating sequence of most vehicle engines: intake, compression, power and exhaust, each needing two piston strokes, so one power stroke per two crankshaft revolutions.

The ratio of cylinder volume with the piston at bottom dead centre to the volume at top dead centre. Higher values raise thermal efficiency but demand higher-octane fuel in petrol engines.

Abnormally rapid, uncontrolled burning of the fuel-air charge that produces a metallic knocking sound and can damage pistons. Causes include low-octane fuel, excessive advance and overheating.

The small gap left between the valve stem and its rocker or cam follower in a mechanical valve train. It allows for thermal expansion and is checked with a feeler gauge when cold or hot as specified.

The exact crankshaft positions at which the intake and exhaust valves open and close, often expressed relative to top dead centre. Incorrect timing, such as a slipped belt, upsets engine performance.

A prescribed order, usually spiral from the centre outwards, in which head bolts are tightened in stages to a torque figure. It distributes clamping load evenly and prevents gasket leaks or head distortion.

A sealing layer clamped between the cylinder block and head. It contains combustion pressure, coolant and oil, and its failure shows as white smoke, overheating or mixing of fluids.

Loss of cylinder diameter and roundness caused by piston ring friction, heat and poor lubrication, often producing a taper or ridge. It is measured with a bore gauge or micrometer and may need reboring.

The clearance between the two ends of a ring when it sits squarely in its cylinder. Too small a gap causes ring breakage as it heats; too large a gap allows blow-by and oil consumption.

Combustion gases that escape past worn piston rings or cylinder walls into the crankcase. It raises crankcase pressure, contaminates oil and often shows as smoke from the breather.

A condition where the clutch cannot transmit full engine torque, so engine speed rises without matching vehicle speed. Worn friction lining, weak springs or oil contamination are usual causes.

The small movement of the clutch pedal before the release bearing touches the pressure plate. Correct play prevents slip from a preloaded bearing and ensures full disengagement.

A stepped-ratio transmission in which the driver selects gear pairs through a selector mechanism. Sliding-mesh and constant-mesh types differ in how gears engage, with synchromesh used to match speeds.

A cone-and-sleeve device that frictionally equalises the speed of a gear and its shaft before dog teeth engage. It allows smooth gear changes without double-declutching.

The ratio between the crown wheel and pinion in the differential, which multiplies gearbox output speed reduction. A numerically higher ratio improves acceleration but lowers top speed and raises fuel use.

A gear set that lets the driven wheels turn at different speeds while cornering, while still sharing engine torque. Open, limited-slip and locking types behave differently when one wheel loses grip.

Adjustment of camber, caster, toe and steering axis inclination so wheels sit correctly relative to the road and each other. Wrong settings cause uneven tyre wear and pulling to one side.

The tilt of a wheel's top away from or towards the vehicle centreline when viewed from the front. Excessive camber wears one edge of the tyre and affects cornering grip.

The amount by which the front edges of a pair of wheels are closer together than their rear edges. It counteracts the outward splay produced as wheels roll, and is set with the vehicle laden.

A hydraulic damper that converts suspension spring oscillation into heat, limiting body bounce after a bump. A worn unit causes continued bouncing and poor tyre contact.

Loss of braking force after repeated or prolonged use, caused by heat building up in friction material or fluid. It recovers as parts cool, and is reduced by better ventilation and fluid choice.

The temperature at which brake fluid forms vapour in the hydraulic circuit. Vapour compresses, giving a spongy or failed pedal; absorbed moisture lowers this point over time.

Removing trapped air from the hydraulic brake circuit by opening each bleeder screw while pedal pressure forces fluid through. Air removal restores a firm pedal.

An electronic system that rapidly releases and reapplies brake pressure at individual wheels when sensors detect lock-up. It keeps wheels rolling so steering control is retained under heavy braking.

The density of electrolyte compared with water, indicating state of charge in a lead-acid battery. A hydrometer reading near 1.265 suggests a fully charged cell at normal temperature.

A bulb-and-float instrument that draws electrolyte from a battery cell and shows its specific gravity on a scale. It cannot be used on sealed maintenance-free batteries.

A belt-driven generator that produces alternating current, rectifies it to direct current and charges the battery while running. Its regulator holds output voltage within a safe range.

Measuring the potential difference across a cable or connection while current flows, to find excess resistance. High readings indicate corroded terminals, undersized cables or a poor earth.

The crankshaft position at which the spark occurs, usually stated in degrees before top dead centre. Too much advance causes detonation; too little causes overheating and poor power.

An electronic self-check that monitors sensors and actuators and stores fault codes for a scan tool. It also turns on the malfunction lamp when emissions-related faults persist.

A coded number stored by the engine or chassis controller identifying a monitored circuit or component fault. The code points to a system, but further testing confirms the actual cause.

Cranking the engine with a pressure gauge screwed into each spark plug or injector hole to measure sealing. Low or uneven readings reveal worn rings, leaking valves or a failed head gasket.

A planned list of inspections, fluid changes and replacements set by time or distance. Following it avoids breakdowns and keeps warranty and statutory fitness checks valid.

A toothed belt that drives the camshaft at exactly half crankshaft speed. It must be replaced at the maker's interval, because failure causes valve and piston contact.

A safety procedure that isolates energy sources and attaches a warning tag before repair, ensuring a machine cannot start unexpectedly. It is applied to lifts, compressors and battery circuits.

Wearable protection such as goggles, gloves, safety shoes and ear defenders chosen for the task. It is the last line of defence, used after guarding and safe work practices.

The set of legal duties and workplace practices that protect people from injury and ill health. In a workshop it covers fire safety, chemicals, ventilation, housekeeping and safe lifting.

A person who spots a market need, organises money, materials and people, and bears the risk of a new venture. In the automotive trade this may mean a repair shop or parts business.

The money tied up in day-to-day operations, calculated roughly as current assets minus current liabilities. A workshop needs enough to pay for parts, wages and rent before customers pay.

The sales level at which total revenue exactly equals total cost, so profit is zero. Above it each extra job contributes towards profit; below it the business runs at a loss.