Maharashtra HSC Scooter & Motorcycle Servicing (Bifocal) key terms

A wrench that measures and limits the twisting force applied to a threaded fastener, allowing bolts to be tightened to the maker's stated figure without stretching threads or distorting covers.

A set of thin metal blades, each marked with its thickness, used to measure or set narrow gaps such as valve clearance, spark plug gap and piston ring end gap.

A sliding gauge with a fixed main scale and a short vernier scale that reads outside, inside and depth measurements, usually to the nearest 0.02 mm, for quick workshop checks.

An instrument with a spring-loaded plunger and a dial indicator, first set against a micrometer and then lowered into a cylinder to show bore diameter, taper and ovality.

Clothing and equipment worn to reduce injury risk in the workshop, such as safety goggles, gloves, steel-toe shoes and ear protection, selected according to the task being done.

The engine sequence of intake, compression, power and exhaust strokes, completed in two crankshaft turns, with valves opening and closing to control the gas flow.

A cycle completed in one crankshaft turn, where the rising piston draws mixture into the crankcase and the falling piston pushes it through transfer ports while burnt gas leaves.

The crankshaft angles, in degrees before or after top dead centre, at which the piston opens or closes the inlet, transfer and exhaust ports of a two-stroke engine.

The volume above the piston at bottom dead centre divided by the volume at top dead centre, showing how strongly the charge is squeezed before ignition.

The small gap, set with the engine cold, between the cam lobe or rocker arm and the valve stem, allowing for heat expansion so the valve still closes fully when hot.

The narrowed throat inside a carburettor where the airflow speeds up and its pressure falls, so fuel is drawn out of the nozzle and mixed with the passing air.

The fuel reservoir in a carburettor, kept at a steady level by a float and needle valve so that the head of fuel at the jets stays constant.

The calibrated brass jet, screwed into the needle jet, that meters fuel at medium and large throttle openings and so sets the mixture strength at high load.

The proportion of air to fuel by mass entering the engine; for petrol about 14.7 to 1 is chemically correct, while richer or leaner mixtures change power and emissions.

A honeycomb unit in the exhaust, coated with precious metal, that changes carbon monoxide, hydrocarbons and nitrogen oxides into carbon dioxide, water and nitrogen.

Capacitor discharge ignition: a system in which a capacitor is charged from a coil and released through the ignition coil by a trigger signal, producing a strong, fast-rising spark.

A number showing how quickly a plug carries heat from its firing tip to the cylinder head; hotter plugs suit light loads, colder plugs suit sustained high-speed running.

A combined unit that changes the alternating output of the alternator into direct current and holds the voltage near 14 volts so the battery charges without bulbs blowing.

The density of a liquid compared with water; measured in battery electrolyte with a hydrometer, it indicates roughly how fully a lead-acid battery is charged.

A clutch pack of alternate friction and steel plates running in engine oil and pressed by springs; operating the lever separates the plates and cuts drive to the gearbox.

The small movement available at the clutch lever or pedal before the plates begin to separate, which makes sure they stay fully pressed together when released.

A gearbox where every gear pair stays in mesh all the time; sliding dogs on the shafts lock the chosen pair to its shaft so drive is transmitted in that ratio.

Continuously variable transmission: a reinforced belt running between two pulleys whose effective diameters change, giving an infinite range of ratios without fixed gears, as used on scooters.

The up-and-down movement of the lower run of a drive chain measured at its mid-point; too little strains bearings, while too much lets the chain jump or whip.

The angle between the steering axis and the vertical; a larger angle improves straight-line stability but makes steering heavier and increases the wheelbase effect.

The horizontal distance between the point where the steering axis meets the ground and the tyre contact patch; it makes the front wheel tend to self-centre.

Front suspension with upper and lower tubes that slide inside each other, compressing a spring and forcing oil through valves to damp out road shocks.

The inflation pressure checked before the tyre has warmed from riding; the maker's figure applies when cold, since running raises pressure and gives a falsely high reading.

The loss of braking force after repeated or prolonged use, caused by overheated friction material or fluid, or by vapour forming in the hydraulic circuit.

The hydraulic liquid that carries pressure from lever to caliper; most types absorb moisture from the air, so their boiling point falls and they need periodic renewal.

Expelling air from a hydraulic brake by pumping fluid through the bleed nipple until no bubbles come out, so the lever feels firm and braking is reliable.

Anti-lock braking system: wheel sensors and a hydraulic modulator that release and reapply pressure many times a second to stop the wheels locking, keeping steering control.

The first distance after an overhaul when new surfaces bed together; speed and load are kept moderate and the oil is changed early to flush away wear particles.

The workshop record of the customer's complaint, vehicle details, approved work, parts fitted and labour time, used to control the job and prepare the bill.

A short ride after service or repair to check starting, acceleration, gear shifting, braking, steering and unusual noises before handing the vehicle back.

Cranking the engine with a gauge fitted in the spark plug hole and comparing the reading with the specification and other cylinders to trace poor sealing.