NAVTTC Auto Mechanic key terms

The ratio of the total cylinder volume when the piston is at bottom dead centre to the volume remaining above the piston at top dead centre. It indicates how much the air-fuel charge is squeezed before ignition.

The small gap between the valve stem tip and the rocker arm or cam follower, specified by the manufacturer. Correct clearance ensures valves close fully when the engine is hot and expands.

The exact points, measured in crankshaft degrees, at which the intake and exhaust valves open and close relative to piston position. Incorrect timing causes poor power, rough idle and possible valve damage.

A toothed belt that drives the camshaft at exactly half crankshaft speed. It must be replaced at the specified interval because failure usually bends valves on interference engines.

Alignment marks on the crankshaft pulley, camshaft sprocket and engine casing used to set correct valve and ignition timing during assembly. They must be matched precisely before the belt or chain is tensioned.

A diagnostic test in which compressed air is fed into a cylinder at top dead centre to measure the percentage leaking past the rings, valves or head gasket, locating the source by listening for escaping air.

A sealing layer between the cylinder head and engine block that keeps combustion gases, coolant and oil in their separate passages. Failure causes overheating, white exhaust smoke or oil and coolant mixing.

The sequence in which the engine's cylinders deliver their power strokes, for example 1-3-4-2 on a four-cylinder engine. It determines spark plug wiring and injection sequence.

A condition where engine speed rises without a matching increase in road speed because the friction disc cannot transmit full torque. Causes include a worn disc, weak pressure plate or oil contamination.

The small clearance between the release bearing and the pressure plate fingers, or at the pedal, that ensures the clutch is fully engaged when released. Too little causes slip; too much causes poor disengagement.

The bearing that presses on the pressure plate fingers when the clutch pedal is depressed, disengaging the drive. It is a common source of noise when worn or dry.

A joint in a front-wheel-drive driveshaft that transfers power smoothly through changing angles. A torn boot lets dirt in and grease out, producing a clicking noise on turns.

A cross-and-yoke joint in a propeller shaft that allows power transmission through changing angles. Wear shows as vibration or clunking during acceleration and gear changes.

A gear set that lets the driven wheels turn at different speeds while cornering. It also provides the final drive reduction between the propeller shaft and the axle shafts.

Loss of braking force caused by heat build-up in the friction material or brake fluid, often after repeated hard stops. Symptoms include a soft or long pedal and reduced stopping power.

The process of removing air from the hydraulic brake system by opening each bleed nipple in sequence while an assistant holds and releases the pedal, then topping up with fresh fluid.

The temperature at which brake fluid vaporises and creates compressible gas in the hydraulic system. Moisture absorption lowers this value, so fluid should be replaced at the specified interval.

Adjustment of camber, caster and toe so the wheels sit in the correct relationship to the road and each other. Incorrect alignment causes uneven tyre wear and pulling to one side.

The inward or outward tilt of the top of the wheel from vertical when viewed from the front. Excessive camber wears one tyre edge and affects cornering grip.

The forward or backward tilt of the steering axis as seen from the side. Positive caster improves straight-line stability and self-centring of the steering wheel.

The setting where the front edges of the tyres are closer together than the rear edges. Correct toe prevents feathering wear and keeps the vehicle tracking straight.

The remaining capacity in a battery expressed as a percentage, estimated from open-circuit voltage after the surface charge is removed. It guides whether to recharge or replace the battery.

A measurement of voltage lost across a conductor, switch or connection while current flows. Excessive drop indicates high resistance such as corroded terminals or a damaged cable.

Current drawn from the battery when the ignition is off and modules have gone to sleep. Excessive drain flattens the battery overnight and is traced with an ammeter in series.

The distance between the centre and ground electrodes of a spark plug, set to the manufacturer's specification. Too wide a gap strains the ignition coil; too narrow gives a weak spark.

A transformer that steps up battery voltage to the high voltage needed to jump the spark plug gap. A failed coil causes misfire, rough running or a no-start condition.

An electrically operated valve that sprays a measured quantity of fuel into the intake port or cylinder. Clogged or leaking injectors cause misfire, poor economy and rich running.

A check of the fuel delivery system using a gauge connected to the test port, comparing running and residual pressure against specification. It isolates pump, regulator or filter faults.

A component that traps dirt and rust particles before fuel reaches the injectors or carburettor. A blocked filter restricts flow and causes loss of power under load.

A temperature-operated valve that stays closed while the engine warms up and then opens to send coolant to the radiator. A stuck-open unit causes slow warm-up and poor heater performance.

The proportion of antifreeze to water in the cooling system, typically around fifty percent. Correct concentration prevents freezing, boiling and corrosion while protecting the water pump.

A radiator or expansion tank cap containing a spring valve that raises the boiling point of coolant and releases excess pressure. A faulty cap causes overheating and coolant loss.

A number such as 10W-40 describing how easily oil flows at cold and hot temperatures. Using the wrong grade affects lubrication, fuel economy and cold starting.

A dashboard lamp that illuminates when engine oil pressure falls below a safe level. If it stays on while running, the engine must be stopped immediately to prevent bearing damage.

A canister or cartridge element that removes metal particles and sludge from engine oil before it reaches the bearings. It must be replaced at each oil change.

A wrench that clicks or signals when a preset tightening torque is reached, ensuring fasteners are tightened to specification. Over-torquing stretches bolts and distorts components.

The specified order, often spiral from the centre outwards, in which bolts such as cylinder head bolts are tightened. Following it prevents gasket leaks and warping.

A set of thin metal blades of known thickness used to measure small clearances such as valve lash, spark plug gap or brake pad clearance. Blades are selected until one fits with slight drag.

A safety procedure in which energy sources are isolated, locked and tagged before maintenance begins. It prevents accidental starting or release of stored energy that could injure a technician.

A rigid support placed under the vehicle chassis after lifting with a jack, so the vehicle cannot fall. Never work under a vehicle supported only by a hydraulic jack.

Safety items worn by technicians such as goggles, gloves, safety footwear and hearing protection. Correct use reduces injury from chemicals, sharp edges, noise and flying debris.