NSSCO Motor Mechanics 6189 key terms

The measured height of the rubber blocks on a tyre from the bottom of the grooves to the surface. It indicates how much grip remains before the tyre must be replaced.

The height of a tyre's sidewall expressed as a percentage of its section width. A 60-series tyre has sidewall height equal to 60 percent of the tyre width.

Adjustment of the angles at which the wheels sit relative to the vehicle and road, mainly camber, caster, toe and steering axis inclination. Correct alignment prevents uneven tyre wear and pulling.

Even distribution of mass around a wheel and tyre so it rotates without vibration. Balance weights are added to correct heavy spots detected on a balancing machine.

The system of springs, dampers, arms and linkages that connects the wheels to the chassis. It absorbs road shocks, keeps the tyres in contact with the road and supports the vehicle weight.

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

A sequence of four piston strokes, induction, compression, power and exhaust, producing one power stroke per two crankshaft revolutions. Each stroke is controlled by valve timing and ignition or injection.

A cycle in which one power stroke occurs every crankshaft revolution, with induction, compression, power and exhaust completed in two piston strokes. Scavenging is usually assisted by the crankcase or a blower.

The ratio of the total cylinder volume when the piston is at bottom dead centre to the volume remaining above it at top dead centre. It affects power, efficiency and fuel octane requirements.

The exact position of the piston at the highest point of its travel in the cylinder. Timing marks and ignition or injection timing are referenced to this position.

The position of the piston at the lowest point of its travel in the cylinder, where it reverses direction. Stroke length is measured between top and bottom dead centre.

The distance the piston travels from top dead centre to bottom dead centre. Together with bore diameter it determines engine swept volume.

The volume displaced by one piston moving through one full stroke. Engine capacity is the swept volume of one cylinder multiplied by the number of cylinders.

The small gap between the valve stem tip and the rocker or cam follower, measured with a feeler gauge. It allows for thermal expansion and must match the manufacturer's specification.

The precise points, measured in crankshaft degrees, at which the intake and exhaust valves open and close relative to piston position. It is set by camshaft design and timing marks.

The opening and closing of transfer, intake and exhaust ports by the moving piston in a two-stroke engine. It is fixed by port position rather than by a camshaft.

A wrench that measures and limits the tightening force applied to a fastener so it reaches a specified torque. Correct use prevents stripped threads and distortion of components.

The manufacturer-specified turning force, usually in newton metres, applied to a fastener. Settings may be given as a staged or angular sequence for critical joints such as cylinder heads.

A set of thin metal blades of marked thickness used to measure small gaps. Typical uses include checking valve clearance and spark plug gaps.

A precision measuring instrument with a calibrated screw that measures small dimensions such as shaft diameter or brake disc thickness to hundredths of a millimetre. It is read on a barrel and thimble scale.

A measuring tool with sliding jaws and a vernier scale that reads outside, inside and depth dimensions to fractions of a millimetre. It is checked against zero before use.

An instrument with a spring-loaded plunger and a graduated dial used to measure small movements or runout. It is commonly used to check shaft bend and brake disc runout.

The removable part bolted to the top of the cylinder block that closes the cylinders and carries the combustion chambers, valve gear and ports. It is usually tightened in a staged sequence to a specified torque.

A split metal ring fitted in grooves on the piston. Compression rings seal combustion gases and oil control rings scrape excess oil from the bore.

The small gap between a rotating shaft journal and its bearing shell, filled by a lubricating oil film. It is measured with plastigauge or a micrometer and must be within specification.

The force with which the lubrication system pushes oil through galleries and bearings. It is generated by the oil pump and indicated on a dashboard gauge or warning lamp.

A temperature-controlled valve in the cooling system that stays closed while the engine warms and then opens to allow coolant to flow through the radiator. It keeps the engine at its design temperature.

The fluid, usually water mixed with antifreeze and corrosion inhibitors, circulated through the engine and radiator to carry away heat. Its mixture strength is checked with a hydrometer.

The amount of charge a battery can deliver, usually stated in ampere-hours at a specified discharge rate, plus a cold cranking current rating. It determines starting ability and electrical reserve.

A belt-driven generator that produces alternating current, rectified to direct current, to charge the battery and run the vehicle's electrical loads while the engine runs.

An electric motor with a pinion that engages the flywheel ring gear to crank the engine until it fires. It draws a very high current from the battery through a solenoid switch.

A friction coupling between the engine and gearbox that can be engaged or released by the driver. It allows smooth take-up of drive and disconnection during gear changes.

The number of teeth on a driven gear divided by the number of teeth on the driving gear. It determines the trade-off between output speed and torque.

A gear assembly in the final drive that divides engine torque between the two driven wheels while allowing them to turn at different speeds during cornering.

The temporary loss of braking force caused by overheating of brake linings or fluid. It may result from prolonged heavy braking and is reduced by cooling and correct adjustment.

A hydraulic liquid, usually glycol-based, that transmits pedal force to the wheel brakes. It absorbs moisture over time and must be replaced at the specified interval.

A safety procedure in which an energy source is isolated, locked and tagged before maintenance begins. It prevents accidental starting or release of stored energy while work is in progress.

Clothing and equipment worn to reduce injury risk, such as safety glasses, gloves, hearing protection and safety footwear. It is the last line of defence after engineering controls.

A rigid support placed under a vehicle to hold it safely after it is raised with a jack. It must never be replaced by the jack alone when working beneath a vehicle.