ITI/NCVT Mechanic Motorcycle (AITT Theory) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Stoichiometric air-fuel ratio

The mass ratio of air to petrol at which complete combustion occurs, about 14.7:1 for petrol; richer mixtures give more power and leaner mixtures give better economy but risk overheating.

Compression ratio

The ratio of the cylinder volume above the piston at bottom dead centre to the volume at top dead centre; higher ratios raise efficiency but require higher-octane fuel to avoid detonation.

Vernier least count

The smallest measurement a vernier caliper can read, equal to one main scale division divided by the number of vernier divisions; a 50-division scale on a 1 mm main scale gives 0.02 mm.

Micrometer least count

The smallest reading of a micrometer, equal to the screw pitch divided by the number of thimble divisions; a 0.5 mm pitch with a 50-division thimble gives 0.01 mm.

Torque formula

Tightening torque equals the nut factor multiplied by the clamp force and the nominal bolt diameter; lubricated threads need less torque for the same preload, so always use the specified value.

Clearance fit

A fit in which the shaft is always smaller than the hole, leaving a gap so the parts move or slide, such as a bearing journal running in its shell.

Four-stroke cycle

The sequence of intake, compression, power and exhaust strokes completed in two crankshaft revolutions, with valves controlling the gas flow in and out of the cylinder.

Two-stroke scavenging

The process in which fresh mixture entering through transfer ports pushes burnt gas out of the exhaust port, relying on crankcase pressure and exhaust pressure waves rather than valves.

Valve clearance

The small gap between the cam or rocker and the valve stem, set cold to the manufacturer's specification; too little causes valve burning and too much causes noise and reduced lift.

Octane number

A rating of a petrol's resistance to knock, compared with iso-octane and heptane; higher compression or forced induction engines require a higher octane fuel to avoid detonation.

Lambda value

The ratio of actual air-fuel ratio to stoichiometric ratio; 1.00 means chemically correct, below 1.00 is rich and above 1.00 is lean, as measured by the oxygen sensor.

Spark plug heat range

A classification of how quickly a plug dissipates heat; a hot plug has a long insulator nose and runs hotter, while a cold plug has a short nose and suits high-load engines.

Wet multiplate clutch

A clutch with alternating friction and steel plates running in engine oil, clamped by springs; the release mechanism lifts the pressure plate to disengage drive to the gearbox.

Overall gear ratio

The product of primary drive ratio, selected gearbox ratio and final drive ratio; it determines how many engine revolutions are needed for one turn of the rear wheel.

Chain slack adjustment

The specified up-and-down movement of the drive chain, usually measured midway between sprockets with the machine at the stated load; too tight damages bearings and too loose risks derailment.

Trail

The horizontal distance between the steering axis intersection with the ground and the tyre contact point; more trail increases straight-line stability and less trail quickens steering response.

Static sag

The difference between the suspension's fully extended length and its length with the machine resting on its wheels; it shows whether spring preload matches the rider and load.

Brake fade

The loss of braking force when heat boils the fluid or overheats the pads, producing a spongy or long lever; it is prevented by correct fluid and pad specification and by cooling.

Tyre aspect ratio

The tyre sidewall height expressed as a percentage of the section width; a 120/70 tyre is 120 mm wide with a sidewall height of 84 mm.

Ohm's law

The relationship that voltage equals current multiplied by resistance; combined with power equal to voltage times current, it lets a technician calculate expected readings in a circuit.

Battery open-circuit voltage

The voltage of a rested lead-acid battery with no load; about 12.6 to 12.8 volts indicates full charge and below about 12.0 volts indicates a discharged battery.

Voltage drop test

A measurement taken across a cable or connection while current flows; a drop above about 0.5 volts on a main cable shows excessive resistance and a poor connection.

Compression test

A test with the throttle open and the engine warm that measures peak cylinder pressure; a low reading or large variation between cylinders indicates ring, valve or head gasket faults.

Leak-down test

A test that pressurises the cylinder at top dead centre and measures leakage; air escaping from the exhaust, intake or crankcase locates the faulty valve or rings.

Plastigauge

A plastic strip placed across a bearing journal and crushed by the assembled cap; its flattened width is compared with a scale to give the bearing clearance.

Ring end gap

The gap between the ends of a piston ring when fitted squarely in the bore; too small a gap closes when hot and can break the ring, so it must be checked and filed.

Hacksaw blade selection

Blade choice by teeth per inch: about 18 for thick sections and soft metals, and 24 to 32 for thin tube and hard metals, with at least three teeth in contact.

Torque wrench types

Click types signal a set torque and must be backed off to zero after use; beam types read deflection directly; neither should be used to loosen tight fasteners.

Case hardening

A heat treatment that adds carbon to the surface of low-carbon steel and hardens it, producing a wear-resistant skin over a tough, shock-resistant core.

DOT brake fluid

A glycol-based hydraulic fluid graded by boiling point; it absorbs water from the air, which lowers the boiling point, so it must be replaced at the specified interval.

Carbon monoxide hazard

An odourless gas in engine exhaust that binds haemoglobin far more strongly than oxygen; engines must never run in an enclosed space without ventilation and monitoring.

5S workplace method

A housekeeping system of sort, set in order, shine, standardise and sustain; it keeps tools and parts controlled and prevents foreign objects entering engines during repair.