NABTEB Motor Vehicle Mechanics cheat sheet

The fire triangle consists of fuel, oxygen and heat; removing any one element stops combustion.

Class B fires involve flammable liquids such as petrol; foam, CO2 or dry powder extinguish them and water jets can spread the fuel.

Lead-acid battery charging releases hydrogen gas, so charging areas require ventilation and no flames or sparks.

A vehicle supported only by a jack is unstable; jack stands placed under a structural point carry the load during work.

Carbon monoxide from exhaust is colourless, odourless and toxic; engine exhaust must be ducted outdoors in enclosed workshops.

Grinding, chipping and compressed-air cleaning generate flying particles; eye or face protection is required at the machine.

Before electric arc welding on a vehicle, the battery negative terminal is disconnected to protect electronic modules from voltage spikes.

Oil spills are absorbed with inert absorbent and disposed of according to local environmental rules; they are not washed into drains.

A metric micrometer with 0.5 mm barrel divisions and a 50-division thimble has a least count of 0.01 mm.

A typical metric vernier caliper has a least count of 0.02 mm, read from the main scale plus the aligned vernier division.

Torque is force multiplied by perpendicular distance, measured in newton metres (N·m).

A torque wrench is set to the specified tightening torque and is stored at its lowest setting to protect the spring.

Feeler gauges measure small gaps such as valve clearances, spark plug gaps and piston ring end gaps.

A dial indicator, mounted on a magnetic base, measures shaft runout, end play and gear backlash.

A compression tester is used with the engine warm, throttle wide open and all spark plugs removed.

A puller removes bearings, gears and pulleys by applying axial force; hammering a shaft can bend it or damage bearing races.

In a four-stroke engine the sequence is intake, compression, power and exhaust, with two crankshaft revolutions per cycle.

Engine displacement = (π/4) × bore² × stroke × number of cylinders.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume.

Compression rings seal combustion gases; the oil control ring scrapes excess oil from the cylinder wall.

Valve clearance is the small gap in the valve train that allows for thermal expansion of valve and pushrod components.

Main bearings support the crankshaft in the block; big-end bearings connect the connecting rod to the crankshaft journal.

The cylinder head gasket seals combustion pressure, coolant passages and oil passages between block and head.

Timing marks align the crankshaft and camshaft; on an interference engine, a broken timing belt can cause valve-to-piston contact.

The firing order specifies the sequence in which cylinders receive spark or injection; it is not the same as cylinder numbering order in all engines.

An engine overhaul is indicated when compression, oil pressure or bearing clearance is outside service limits, or when cylinder taper exceeds specification.

Cylinder bore is measured at top, middle and bottom in two perpendicular directions to find taper and out-of-round.

Plastigauge is placed across a journal and crushed by the bearing cap; the crushed width is compared with a scale to determine clearance.

Cylinder head bolts are tightened in stages from the centre outward in the specified sequence to distribute clamping force evenly.

Piston ring end gap is checked by squaring the ring in the cylinder bore with a piston; the gap must lie within specification.

Crankshaft end play is measured with a dial indicator while the crankshaft is pried axially.

Torque-to-yield bolts stretch during tightening and are replaced rather than reused when specified.

After an overhaul, the engine is run in with reduced load and the oil and filter are changed to remove initial wear particles.

The stoichiometric air-fuel ratio for petrol is about 14.7:1 by mass; for diesel it is about 14.5:1.

Spark ignition timing is measured in degrees before top dead centre; too much advance causes detonation and too little reduces power.

The ignition coil transforms low battery voltage into the high voltage needed to jump the spark plug gap.

Electronic ignition uses crankshaft and camshaft position sensors to determine spark timing and injection sequence.

Injector opening duration controls the quantity of fuel delivered; longer duration gives a richer mixture.

A zirconia oxygen sensor produces a voltage that switches high when the exhaust is rich and low when it is lean.

Diesel engines use compression ignition; fuel is injected near the end of the compression stroke where air temperature is high.

The mass air flow or manifold absolute pressure sensor measures intake air, and the ECU calculates the required fuel mass.

A catalytic converter oxidises CO and HC and reduces NOx, but it requires correct closed-loop mixture control to operate efficiently.

The clutch friction disc, pressure plate and release bearing transmit or interrupt engine torque to the gearbox.

Clutch pedal free play prevents the release bearing from resting on the pressure plate fingers and wearing continuously.

Manual gearbox ratio = driven gear teeth ÷ driving gear teeth; a numerically higher ratio gives more torque and less speed.

The propeller shaft uses universal joints to allow angular movement as the suspension moves, and it must be balanced to avoid vibration.

The differential crown wheel and pinion provide the final drive ratio and turn drive through 90 degrees to the axle shafts.

A limited-slip differential transfers torque to the wheel with more grip when one wheel loses traction.

A torque converter in an automatic transmission couples the engine to the gearbox through fluid flow and multiplies torque at low speed.

Constant-velocity joints allow the front wheels to be driven while steering and moving through suspension travel.

Caster is the forward or backward tilt of the steering axis; positive caster improves straight-line stability and self-centring.

Camber is the vertical tilt of the wheel; negative camber leans the top inward and can improve cornering grip but wear the inner tyre.

Toe is the difference between the front and rear track measurements of the wheels; incorrect toe causes rapid tyre scrub.

Kingpin inclination or steering axis inclination is the inward tilt of the steering axis viewed from the front.

Ackermann geometry makes the inner wheel turn through a larger angle than the outer wheel during a turn.

Shock absorbers damp spring oscillations; worn shock absorbers cause continued bouncing after a bump.

Wheel imbalance produces vibration that increases with road speed and is corrected by fitting balance weights.

A MacPherson strut combines a coil spring, shock absorber and upper steering pivot in one assembly.

Tyre inflation pressure affects contact patch shape, rolling resistance and tread wear pattern.

Hydraulic brake systems obey Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions.

DOT 3 and DOT 4 brake fluids are glycol-based and hygroscopic; DOT 5 is silicone-based and must not be mixed with them.

A spongy brake pedal usually indicates air in the hydraulic system, which is removed by bleeding the brakes.

Brake fade is the reduction in braking force caused by overheating of pads, linings, discs or drums.

Disc brakes dissipate heat well and are self-adjusting; drum brakes use a self-energising action that can increase braking force.

ABS prevents wheel lock by modulating hydraulic pressure at each wheel in response to wheel speed sensor signals.

The parking brake is mechanically actuated, often by cable, so it can hold a vehicle if hydraulic pressure is lost.

Brake pad or lining replacement is required when friction material thickness falls below the manufacturer's minimum.

Ohm's law: V = I × R; electrical power: P = V × I.

A 12-volt lead-acid battery has six cells, each producing about 2.1 volts when fully charged.

The alternator generates alternating current, rectifies it with diodes and regulates output to about 13.8–14.4 volts.

The starter solenoid switches the high current from the battery to the starter motor and engages the drive pinion.

A fuse protects a circuit by melting when current exceeds its rating, breaking the circuit before wiring overheats.

A relay uses a small control current to operate a switch that carries a larger load current.

The vehicle chassis or body is commonly used as the earth return path for electrical circuits.

A voltage drop test measures lost voltage across a cable, connector or earth while current flows; high drop indicates resistance.

A CAN bus allows electronic control units to share data over a twisted pair of wires, reducing wiring complexity.

Service intervals are specified by distance travelled or time, whichever occurs first.

Engine oil must meet the manufacturer's viscosity grade and API or ACEA performance specification.

Coolant level is checked when the engine is cold; a 50:50 antifreeze-water mix raises boiling point and lowers freezing point.

The legal minimum car tyre tread depth is 1.6 mm across the central three-quarters of the tread around the circumference.

Battery terminals are cleaned and tightened; loose or corroded terminals cause high resistance and poor charging.

Brake fluid absorbs moisture over time and is typically renewed every two years or as specified.

A clogged air filter restricts intake air and can increase fuel consumption and reduce power.

Chassis lubrication points, such as suspension and steering nipples, are greased at specified service intervals.

Maintenance records document service dates, mileage, parts fitted and faults found for later diagnosis and resale.

Diagnosis follows a logical path: verify the symptom, identify the affected system, test the component, then confirm the repair.

Low compression that increases after adding oil indicates worn piston rings; no increase indicates valve or head gasket leakage.

A spark test checks whether ignition voltage reaches the spark plug; no spark directs testing to coil, ignition module or crank sensor.

A vacuum gauge reading that is low and fluctuating can indicate a vacuum leak, worn rings or faulty valve sealing.

A scan tool reads diagnostic trouble codes and live data from the ECU; the code identifies the circuit, not necessarily the failed part.

For a no-start complaint, test battery voltage and cranking speed, then fuel pressure and ignition spark before checking compression.

Overheating can be caused by low coolant, a stuck thermostat, a faulty cooling fan, a blocked radiator or a failed water pump.

A vehicle that pulls to one side under braking may have a seized caliper, contaminated lining, unequal tyre pressure or misalignment.

Abnormal noises are localised by operating the vehicle or component and noting whether the noise changes with speed, load or steering.