ITI/NCVT Mechanic Motorcycle (AITT Theory) key terms

Gear worn to reduce injury risk while working, such as safety goggles, gloves, steel-toe boots, ear plugs and a dust mask. It supplements, but never replaces, safe tools, guards and correct work procedures.

Physical barriers, interlocks or covers fitted around rotating or moving parts of equipment so that hands and clothing cannot reach the danger zone. Guards must remain in place while the machine is running and be checked before each shift.

A procedure for isolating energy sources before servicing equipment: the supply is switched off, locked by the person doing the work, and tagged to warn others. It prevents accidental start-up or release of stored energy such as compressed air or springs.

The grouping of fires by fuel type: Class A solid combustibles, Class B flammable liquids, Class C gases, Class D metals and electrical fires. The extinguisher chosen must match the class, and electrical fires need a non-conducting agent such as CO2 or dry powder.

A document supplied with a hazardous chemical that lists its hazards, safe handling, required protective equipment, first-aid measures and spill response. Workers should read it before opening or using an unfamiliar product such as solvent, paint or battery acid.

A precision instrument with a calibrated screw and spindle used to measure external dimensions to 0.01 mm. The reading combines the main barrel scale with the thimble scale, and the ratchet stop prevents over-tightening that would distort the reading.

A measuring tool with a fixed main scale and a sliding vernier scale, used for outside, inside and depth measurements. The vernier division reading is added to the main scale value, giving resolution typically of 0.02 mm.

A reading shown by an instrument when it should read zero, caused by wear, dirt or misalignment. It must be noted and added to or subtracted from all measurements, and the tool should be cleaned and rechecked before use.

A set of thin steel blades of known thickness used to check small gaps, such as valve clearance, spark plug gap or bearing play. The correct clearance is the thickest blade that enters the gap with slight drag.

A wrench that indicates or limits the tightening force applied to a fastener, so bolts are tightened to the manufacturer's specified value. It should be set to the required figure, applied smoothly and returned to its lowest setting after use.

A gauge with a spring-loaded plunger and a needle dial, used to measure small linear movement or runout. Mounted on a magnetic base, it checks shaft bend, disc warp and gearbox shaft end float.

An iron-carbon alloy containing mainly carbon as the alloying element, typically up to about 2 percent carbon. Low-carbon grades are ductile and weldable, while higher-carbon grades harden on heat treatment and are used for springs and cutting tools.

A metal made mostly of aluminium with additions such as silicon, magnesium or copper to improve strength and casting quality. Its low weight makes it common for engine cases, cylinder heads, wheel rims and frame components.

Controlled heating and cooling of a metal to change its properties. Hardening followed by tempering increases strength while reducing brittleness, annealing softens metal for forming, and normalising refines grain structure after welding or casting.

The distance between corresponding points on adjacent thread crests, expressed in millimetres for metric threads or as threads per inch for inch threads. Matching pitch and diameter is essential when replacing a bolt or repairing a tapped hole.

An assembly in which the shaft is always smaller than the hole, leaving a gap so the parts move or slide freely. It is used for parts such as pistons in cylinders and shafts in plain bearings where lubrication must circulate.

An assembly in which the shaft is slightly larger than the hole so the parts grip when pressed or shrunk together. It transmits torque without a key, as with valve seats and bearing races in housings, but requires careful heating or pressing.

A component that supports a rotating or sliding shaft while reducing friction. Rolling bearings use balls or rollers in races, while plain bearings rely on an oil film between surfaces; both need correct fit, lubrication and alignment to survive.

The operating cycle in which the piston makes four strokes (intake, compression, power and exhaust) and the crankshaft turns twice per combustion event. Valves control gas flow, and camshaft speed is half crankshaft speed.

A cycle in which each crankshaft revolution produces one power stroke, with intake, transfer, compression and exhaust occurring through ports as the piston moves. It gives high power for its weight but needs crankcase sealing and usually oil mixed with fuel.

The ratio of cylinder volume above the piston at bottom dead centre to the volume at top dead centre. A higher ratio raises power and thermal efficiency but demands higher-octane fuel to avoid detonation.

The small gap between the valve stem tip and the rocker arm or cam follower, allowing for thermal expansion. Too little clearance holds valves open and causes compression loss; too much causes noise and reduced lift.

A rating of a petrol's resistance to knocking, with higher numbers resisting detonation better in high-compression engines. Using a grade below the manufacturer's recommendation can cause pinging and engine damage.

Abnormal combustion in which the fuel-air charge ignites explosively ahead of the flame front, producing a sharp metallic knock. It is caused by over-advanced ignition timing, low-octane fuel, overheating or excessive compression, and can damage pistons.

The moment, measured in degrees before top dead centre, at which the spark plug fires. Correct timing gives peak cylinder pressure just after top dead centre; too advanced causes detonation and too retarded causes overheating and power loss.

A number describing how quickly a plug sheds heat to the cylinder head. A hot plug runs hotter and resists fouling in low-speed use, while a cold plug transfers heat faster for sustained high-speed running; the wrong range can foul or pre-ignite.

A classification such as 10W-40 describing how thick oil is when cold (W number) and at operating temperature. The owner's manual specifies the grade, and using too thick or thin an oil affects starting, lubrication and fuel economy.

A pump, usually gear or rotor type, that circulates lubricating oil from the sump to bearings and the cylinder head under pressure. It is driven from the crankshaft or camshaft, and low pressure indicates wear, blockage or low oil level.

The liquid-cooling circuit in which coolant absorbs engine heat and releases it through the radiator, assisted by a thermostat, water pump and fan. Coolant must be the correct antifreeze mixture, and pressure caps raise the boiling point.

A condition in which the clutch fails to transmit full engine torque, so engine speed rises without a matching increase in road speed. Causes include worn friction plates, weak springs, incorrect free play or oil contamination of the plates.

The small amount of lever or pedal travel before the clutch begins to disengage, needed so the release mechanism does not stay loaded. Too little free play causes slip and wear; too much prevents full disengagement and causes difficult gear selection.

The up-and-down movement measured at the midpoint of a drive chain, specified by the manufacturer. Correct slack allows suspension travel without overloading the chain and sprockets, while an over-tight chain damages bearings and an over-loose one can derail.

A front suspension unit in which sliding tubes move inside fixed tubes, with a spring and damper controlling movement. Fork oil level, seal condition and alignment affect steering feel, and unequal oil levels cause handling imbalance.

The amount by which a rotating wheel deviates from a true circle (radial runout) or from its correct side-to-side plane (lateral runout). It is measured with a dial indicator and corrected by spoke adjustment or rim replacement.

Adding small weights to a wheel rim so its mass is distributed evenly about the axle, preventing vibration at speed. Static balancing corrects a heavy spot, while dynamic balancing also corrects couple imbalance across the rim width.

Loss of braking force when heat builds up in the brake, either from pads overheating or from vapour forming in the hydraulic fluid. It is managed by using the correct fluid, bleeding the system and letting brakes cool between hard applications.

Removing air from a hydraulic brake circuit by pumping fluid through the system and out at a bleed nipple until no bubbles appear. Air is compressible and makes the lever feel spongy, so bleeding restores a firm brake and must be done with the correct fluid.

The height of rubber between the tread grooves and the road contact surface, which drains water and maintains grip. Wear bars and legal minimum limits indicate replacement, and uneven wear points to incorrect pressure, alignment or suspension faults.

A rechargeable battery in which lead plates and sulphuric acid electrolyte produce a nominal 12 volts from six cells. It must be kept charged, topped up with distilled water if serviceable, and never shorted, as it can release explosive hydrogen.

The alternator or magneto and rectifier/regulator that convert engine rotation into direct current to run the electrical load and recharge the battery. A measured charging voltage outside the specified range indicates a faulty regulator, stator or wiring.

The presence of a complete, low-resistance path through a circuit, checked with a multimeter set to resistance or continuity. An open circuit shows infinite resistance, while high resistance at a connector causes dim lights or slow cranking.

A code stored by the electronic control unit when a sensor or circuit reads outside its expected range. Reading codes with a scanner or flash pattern is the starting point for diagnosis, but the code identifies a circuit, not always the failed part.

A check in which a gauge is fitted in place of the spark plug and the engine is cranked with the throttle open. Low readings indicate leaking valves, worn rings or a blown head gasket, and a wet test helps separate ring wear from valve leakage.

The manufacturer's table of service tasks at specified time or distance intervals, covering oil changes, filter cleaning, fastener checks and adjustments. Following it preserves warranty and safety, and each task should be recorded in the service history.