Cert III Motorcycle Mech key terms

A power cycle where each cylinder produces one power stroke for every revolution of the crankshaft, with intake, compression, combustion and exhaust occurring as the piston passes through transfer and exhaust ports.

A power cycle where the piston makes four separate strokes (intake, compression, power, exhaust) and the crankshaft turns twice to complete one cycle, with valves controlling gas flow.

A passage in the cylinder wall of a two-stroke engine that carries the fuel-air mixture from the crankcase into the combustion chamber as the descending piston uncovers it.

A flexible petal of spring steel or composite material that opens and closes with pressure changes to control mixture flow into the crankcase or inlet tract of a two-stroke engine.

The pressure build-up inside a two-stroke engine's sealed crankcase as the descending piston compresses the trapped mixture, forcing it through the transfer ports into the cylinder.

A lip-type or labyrinth seal that keeps oil and air from leaking into or out of the crankcase, maintaining the pressure balance needed for two-stroke scavenging and lubrication.

The small measured gap between the tip of a valve stem and the rocker arm or cam follower, allowing for thermal expansion so the valve still seats fully when hot.

A rotating shaft with lobes that push valves or followers open at precise times, controlling valve lift and duration in a four-stroke engine.

The precise points in crankshaft rotation at which the intake and exhaust valves open and close, often expressed in degrees and checked against manufacturer specifications.

A spring or hydraulic device that keeps the camshaft drive chain at correct tension, preventing timing drift, noise and possible valve-to-piston contact.

The ratio of the cylinder's total volume at bottom dead centre to its volume at top dead centre, indicating how much the mixture is squeezed before ignition.

The exact position of the piston at the highest point of its travel in the cylinder, used as a reference point for ignition and valve timing.

The point in crankshaft rotation, usually measured in degrees before top dead centre, at which the spark plug fires to start combustion.

A rating of how quickly a spark plug sheds heat to the cylinder head, with hotter plugs suited to low-load engines and colder plugs to high-performance engines.

A device that mixes fuel with air in the correct proportion for combustion by using venturi vacuum to draw fuel through jets into the intake airstream.

The measured level at which the carburettor float shuts off the fuel inlet needle, controlling fuel level in the bowl and therefore mixture strength across the rev range.

A tapered needle attached to the carburettor slide that moves through the needle jet to regulate fuel flow in the mid-throttle range.

The proportion by mass of air to fuel delivered to the engine, with roughly 14.7:1 being chemically ideal for petrol combustion under light load.

A system that delivers pressurised fuel through an injector, metered electronically by an engine control unit rather than by carburettor jets.

An electronic module that reads sensor inputs such as throttle position, air temperature and oxygen content, then calculates and commands fuel delivery and ignition timing.

A stored alphanumeric code set by the engine control unit when a monitored circuit or sensor reading falls outside its expected range, retrieved with a scan tool.

A test instrument combining voltmeter, ammeter and ohmmeter functions, used to check battery voltage, circuit continuity, resistance and sensor signals.

A measurement of the difference in voltage between two points on a live circuit, revealing resistance in cables, connectors or earths that restricts current flow.

The frame or engine path that completes an electrical circuit back to the battery negative terminal, relying on clean, tight connections to keep resistance low.

A unit that converts alternating current from the alternator into direct current and limits charging voltage to protect the battery and electrical components.

The stationary coil assembly of an alternator that generates alternating current as magnets on the flywheel pass over its windings.

A heavy rotating mass bolted to the crankshaft that smooths power delivery and usually carries magnets for the charging and ignition systems.

The rotating magnet assembly that induces current in the stator windings, often combined with the flywheel and triggered by a pickup coil for ignition.

The amount of charge a battery can store, expressed in amp-hours or cold cranking amps, indicating how much starting current it can supply.

The density of battery electrolyte compared with pure water, used on serviceable lead-acid batteries to estimate state of charge.

A calibrated tool that releases or signals when a set rotational force is reached, ensuring fasteners are tightened to the manufacturer's specified value.

The tightening force stated by the manufacturer for a given fastener, usually in newton metres, accounting for thread size, material and lubrication.

Restoring damaged internal or external threads using a tap and die set, helically coiled insert or thread insert, so a fastener again holds its rated torque.

A set of thin metal blades of known thickness used to measure small gaps such as valve clearance, spark plug gaps and breaker points.

A precision measuring instrument with a calibrated screw that measures external dimensions to hundredths of a millimetre, used for piston, journal and shim sizes.

A measuring tool that reads the internal diameter of a cylinder or bearing bore, often used with a micrometer to calculate piston-to-bore clearance.

The designed gap between the piston skirt and cylinder wall, allowing for thermal expansion and maintaining the oil film and compression seal.

The clearance between the two ends of a piston ring when fitted squarely in the bore, critical for sealing, oil control and avoiding ring breakage when hot.

The machined groove between the top of a piston and the ring grooves that supports the ring and transfers heat to the cylinder wall.

The amount by which a rotating part such as a disc, shaft or wheel deviates from true rotation, measured with a dial indicator.

Adjustment of wheel position and angle relative to the frame, including toe and camber, so the motorcycle tracks straight and tyres wear evenly.

The measured up-and-down movement in the middle of the drive chain run, set to the manufacturer's specification to avoid excessive load on bearings and sprockets.

Deterioration of drive sprocket teeth into hooks or asymmetric shapes, which accelerates chain wear and should be assessed together with chain condition.

The tendency of brake fluid to absorb moisture from the air over time, lowering its boiling point and increasing the risk of vapour lock under hard braking.

The process of removing air bubbles from the hydraulic brake system by flushing fluid through the calipers until a clear, bubble-free stream emerges.

A loss of braking caused when overheated fluid boils and forms compressible gas in the hydraulic circuit, giving a spongy or ineffective lever.

A specially formulated lubricant mixed with petrol or injected into the intake of a two-stroke engine, burning cleanly while protecting bearings and cylinder walls.

The enclosed space above the piston at top dead centre where the compressed mixture is ignited and burned, shaped to promote efficient flame travel.