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NABTEB Motor Vehicle Mechanics study materials
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NABTEB Motor Vehicle Mechanics practice questions

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First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

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Question 1Workshop Safety, Tools and Equipment

Which footwear is most suitable for a mechanic working on a workshop floor?

  1. Canvas shoes
  2. Rubber sandals
  3. Safety boots Correct answer.
  4. Leather shoes

Explanation

Safety boots have reinforced toe caps and oil-resistant soles, protecting the feet from falling tools, heavy components and spilled fluids. Footwear must match the hazards of the workshop floor.

Question 2Workshop Safety, Tools and Equipment

A workshop fire involving petrol or diesel fuel is classified as:

  1. Class F
  2. Class B Correct answer.
  3. Class C
  4. Class A

Explanation

Fires are grouped by the fuel involved. Flammable liquids such as petrol, diesel and paint thinners belong to Class B and are tackled with foam, dry powder or carbon dioxide.

Question 3Workshop Safety, Tools and Equipment

Vehicle exhaust contains a gas that is colourless, odourless and combines with haemoglobin in the blood. This gas is:

  1. Carbon dioxide
  2. Carbon monoxide Correct answer.
  3. Water vapour
  4. Nitrogen

Explanation

Carbon monoxide binds to haemoglobin far more readily than oxygen, so the blood cannot carry oxygen and the victim can lose consciousness without warning. Engines must never be run in an unventilated workshop.

Question 4Workshop Safety, Tools and Equipment

When lifting a heavy component such as a gearbox from the floor, the correct technique is to:

  1. Bend the knees and hold the load close Correct answer.
  2. Hold the load away from the body at arm's length
  3. Keep the legs straight and bend at the waist
  4. Twist the trunk while raising the load

Explanation

Bending the knees uses the strong leg muscles, and keeping the load close keeps the body's centre of gravity over the feet. The back should stay straight and the load should be raised smoothly without twisting.

Question 5Workshop Safety, Tools and Equipment

A ring spanner is preferred to an open-ended spanner for loosening a very tight nut because it:

  1. It can be adjusted to suit the nut
  2. It grips all the flats of the nut Correct answer.
  3. It is lighter to carry
  4. It fits nuts of several sizes

Explanation

A ring spanner completely surrounds the nut and grips all six flats, so it is far less likely to slip or round off a tight or corroded nut. Open-ended spanners grip only two flats.

Question 6Workshop Safety, Tools and Equipment

Engine oil spilled on the workshop floor should be cleaned up at once mainly to:

  1. Keep the workshop looking tidy
  2. Avoid staining the floor paint
  3. Stop the oil from evaporating
  4. Prevent workers from slipping Correct answer.

Explanation

Oil and coolant on a hard floor create a serious slip hazard that can cause falls and injuries. Spills must be absorbed and cleaned immediately, and the area marked until it is dry.

Question 7Workshop Safety, Tools and Equipment

Before anyone works underneath a vehicle raised with a trolley jack, the vehicle must be:

  1. Left on the jack with the handle in place
  2. Supported on a wooden block under the sill
  3. Lowered onto axle stands Correct answer.
  4. Chained to the workshop wall

Explanation

A jack is only a lifting device and can creep or lose pressure. The vehicle must be lowered onto axle stands of adequate rating placed at the manufacturer's jacking points before anyone goes beneath it.

Question 8Workshop Safety, Tools and Equipment

A torque wrench should be stored at its lowest scale setting because this:

  1. Makes it easier to read the scale
  2. Keeps the ratchet mechanism oiled
  3. Prevents the handle from rusting
  4. Relieves tension on the internal spring Correct answer.

Explanation

Leaving a torque wrench wound up keeps the measuring spring compressed and it loses accuracy over time. Backing the setting off after use preserves calibration, which should also be checked periodically.

Question 9Workshop Safety, Tools and Equipment

A feeler gauge is used to measure:

  1. The diameter of a cylinder bore
  2. Small clearances such as valve gaps Correct answer.
  3. The torque of a cylinder head bolt
  4. The depth of a tyre tread

Explanation

A feeler gauge is a set of blades of known thickness used to check narrow gaps such as valve clearances, spark plug gaps and piston ring end gaps. The blade that just slides in gives the clearance.

Question 10Workshop Safety, Tools and Equipment

While a lead-acid battery is being charged, the area must be well ventilated because the battery gives off:

  1. Nitrogen
  2. Carbon dioxide
  3. Hydrogen Correct answer.
  4. Sulphur dioxide

Explanation

Charging liberates hydrogen, which forms an explosive mixture with air. The area must be ventilated and all flames, sparks and smoking kept away from the battery.

Question 11Workshop Safety, Tools and Equipment

If battery acid splashes onto the skin, the first action is to:

  1. Rub the area with a dry cloth
  2. Apply engine oil to the area
  3. Neutralise the acid with a strong alkali
  4. Flood the area with plenty of clean water Correct answer.

Explanation

Sulphuric acid must be diluted and washed away immediately with copious clean water for at least fifteen minutes, then medical help sought. Only after thorough washing should a proper neutralising agent or dressing be considered.

Question 12Workshop Safety, Tools and Equipment

Compressed air from a workshop line should never be used to:

  1. Dry washed components after cleaning
  2. Clean dust off clothing and skin Correct answer.
  3. Blow out a blocked passage
  4. Inflate a vehicle tyre

Explanation

Air jets aimed at clothing or skin can force air and debris into the body and cause fatal injury, so clothing is cleaned with a brush or vacuum. The eyes must also be protected whenever air is used.

Question 13Workshop Safety, Tools and Equipment

When cutting thin-walled metal tubing with a hacksaw, a fine-pitch blade is chosen because it:

  1. It lasts longer on soft metal
  2. It removes metal faster
  3. It keeps many teeth in the cut at once Correct answer.
  4. It needs much less pressure from the operator

Explanation

A fine-pitch blade keeps several teeth in contact with a thin section, so the teeth cannot hook on the edge and strip. At least three teeth should be in the cut at all times.

Question 14Workshop Safety, Tools and Equipment

Before the front of a vehicle is raised with a jack, the rear wheels should be:

  1. Chocked Correct answer.
  2. Removed
  3. Turned
  4. Deflated

Explanation

Wheels still on the ground must be chocked so the vehicle cannot roll or pivot as the jack lifts the other end. The parking brake should also be applied and the transmission left in gear or park.

Question 15Workshop Safety, Tools and Equipment

On oxy-acetylene welding equipment, a flashback arrestor is fitted to:

  1. Filter dirt from the acetylene supply
  2. Control the size of the welding flame
  3. Increase the gas pressure at the torch
  4. Prevent flame travelling back to the cylinders Correct answer.

Explanation

A flashback arrestor contains a non-return valve and a flame trap that stop a flame from running back up the hose into the cylinder or generator. It is fitted at the regulator outlet or the torch end of each hose.

Question 16Workshop Safety, Tools and Equipment

The main purpose of an earth-leakage circuit breaker in a workshop electrical supply is to:

  1. Increase the current available to machines
  2. Reduce the voltage supplied to power tools
  3. Cut the supply when current leaks to earth Correct answer.
  4. Protect electric motors from overheating

Explanation

An earth-leakage device compares current in the live and neutral conductors and trips very quickly when some current finds a path to earth through a person or damaged insulation. This limits the duration of a shock.

Question 17Workshop Safety, Tools and Equipment

A metric micrometer has 50 thimble divisions and one thimble revolution moves the spindle 0.5 mm. If the barrel reads 12.5 mm and the thimble reads 32 divisions, the total reading is:

  1. 12.82 mm Correct answer.
  2. 12.32 mm
  3. 12.66 mm
  4. 16.00 mm

Explanation

Each thimble division represents 0.5 mm divided by 50, which is 0.01 mm. The thimble reading is therefore 32 x 0.01 = 0.32 mm, and adding this to the barrel reading gives 12.5 + 0.32 = 12.82 mm.

Question 18Workshop Safety, Tools and Equipment

The correct tapping drill size for an M10 x 1.5 thread is:

  1. 11.5 mm
  2. 10.0 mm
  3. 7.0 mm
  4. 8.5 mm Correct answer.

Explanation

For a metric thread the tapping drill size is the major diameter minus the pitch, so 10 - 1.5 = 8.5 mm. A hole of this size leaves about 75 per cent thread engagement for the tap to cut.

Question 19Workshop Safety, Tools and Equipment

Before a new grinding wheel is mounted on a bench grinder, the wheel should be:

  1. Ring-tested by light tapping Correct answer.
  2. Coated with light oil
  3. Run at double speed for one minute
  4. Immersed in water for 24 hours

Explanation

A sound vitrified wheel gives a clear ring when tapped lightly with a non-metallic object; a dull thud indicates a crack and the wheel must be discarded. The wheel must also be rated for the machine speed and have the correct blotters and flanges.

Question 20Workshop Safety, Tools and Equipment

Before carrying out work near an airbag module, the recommended precaution is to:

  1. Disconnect the battery and wait before starting Correct answer.
  2. Disconnect the battery positive cable first
  3. Remove the steering wheel with ignition on
  4. Test the circuit with a 12 V test lamp

Explanation

The airbag circuit holds a reserve charge for a period after the battery is disconnected, so the earth cable should be removed first and the specified waiting time observed. The module must never be probed with a test lamp or multimeter that could fire it.

Question 21Engine Construction, Operation and Overhaul

Which component forms the main structural framework of an engine, housing the cylinders and supporting the crankshaft?

  1. Engine block Correct answer.
  2. Camshaft
  3. Flywheel
  4. Cylinder head

Explanation

The engine block is the main casting that contains the cylinders, coolant passages and the bearing saddles that support the crankshaft. The cylinder head closes the top of the block, while the camshaft and flywheel perform different functions.

Question 22Engine Construction, Operation and Overhaul

How many complete crankshaft revolutions are needed to complete one four-stroke cycle in a single cylinder?

  1. Four
  2. Three
  3. Two Correct answer.
  4. One

Explanation

A four-stroke cycle consists of intake, compression, power and exhaust strokes, which require the piston to travel down and up twice. Each piston travel corresponds to half a crankshaft revolution, so the complete cycle takes two revolutions, or 720 degrees.

Question 23Engine Construction, Operation and Overhaul

What is the main function of the compression rings on a piston?

  1. To connect piston to connecting rod
  2. To scrape oil from the bore
  3. To reduce piston weight
  4. To seal the combustion chamber Correct answer.

Explanation

Compression rings press against the cylinder wall to seal combustion gases and help transfer heat from the piston to the bore. The oil control ring, not the compression rings, is mainly responsible for scraping excess oil from the cylinder wall.

Question 24Engine Construction, Operation and Overhaul

Which part converts the reciprocating motion of the piston into rotary motion?

  1. Camshaft
  2. Pushrod
  3. Crankshaft Correct answer.
  4. Rocker arm

Explanation

The connecting rod transmits the piston's linear force to the crankshaft, whose offset journals convert that reciprocating motion into rotation. The camshaft, pushrod and rocker arm are all parts of the valve train.

Question 25Engine Construction, Operation and Overhaul

In a four-stroke spark-ignition engine, the air-fuel charge is compressed during which stroke?

  1. Exhaust (scavenging)
  2. Intake (induction)
  3. Compression Correct answer.
  4. Power (expansion)

Explanation

During the compression stroke the piston moves upward with both valves closed, reducing the cylinder volume and raising the pressure and temperature of the charge. Ignition occurs near the end of this stroke, after which the power stroke begins.

Question 26Engine Construction, Operation and Overhaul

What are the passages cast into the cylinder block and head for coolant circulation called?

  1. Crankcase breathers
  2. Oil galleries
  3. Water jackets Correct answer.
  4. Intake ports

Explanation

Water jackets are the internal coolant passages surrounding the cylinders and combustion chambers. Oil galleries, by contrast, carry lubricating oil, and intake ports carry air or mixture.

Question 27Engine Construction, Operation and Overhaul

The head gasket is fitted between the cylinder block and the cylinder head mainly to:

  1. Guide the pistons into the bores
  2. Reduce engine oil pressure
  3. Support the camshaft bearings
  4. Seal gases and coolant passages Correct answer.

Explanation

The head gasket seals the joint between block and head so that combustion gases, coolant and oil cannot leak across it. It must withstand high pressure and temperature while allowing the head and block to remain properly clamped.

Question 28Engine Construction, Operation and Overhaul

Compression ratio is defined as the ratio of the cylinder volume at bottom dead centre to the:

  1. Volume at TDC Correct answer.
  2. Bore diameter
  3. Stroke length
  4. Engine displacement

Explanation

Compression ratio compares the total cylinder volume when the piston is at bottom dead centre with the clearance volume remaining when it is at top dead centre. It is therefore a volume ratio, not a ratio of stroke or bore dimensions.

Question 29Engine Construction, Operation and Overhaul

Valve clearance is provided in a four-stroke engine to:

  1. Increase the compression ratio
  2. Reduce oil consumption
  3. Allow for thermal expansion Correct answer.
  4. Keep the valve spring compressed

Explanation

Valve-train components expand as the engine warms, so a small clearance prevents the valve from being held off its seat when hot. If clearance is too small, the valve may not seat properly and compression can be lost.

Question 30Engine Construction, Operation and Overhaul

The firing order of a four-cylinder engine is arranged to:

  1. Simplify the valve timing
  2. Eliminate the need for a flywheel
  3. Spread power impulses evenly Correct answer.
  4. Increase the compression ratio

Explanation

A suitable firing order distributes the power strokes evenly around the crankshaft so that the engine runs smoothly and loads on the crankshaft and bearings are more balanced. It does not change compression ratio or remove the need for a flywheel.

Question 31Engine Construction, Operation and Overhaul

A timing belt or chain is used to:

  1. Connect the piston to the crankshaft
  2. Drive the oil pump only
  3. Drive the alternator
  4. Time the camshaft to the crankshaft Correct answer.

Explanation

The timing belt or chain maintains the correct angular relationship between the crankshaft and camshaft so that valves open at the proper point in the cycle. If it slips or is installed one tooth out, valve timing and engine operation are affected.

Question 32Engine Construction, Operation and Overhaul

Cylinder head bolts should be tightened in the specified sequence mainly to:

  1. Speed up assembly
  2. Prevent head distortion Correct answer.
  3. Reduce the need for a gasket
  4. Avoid stretching the bolts

Explanation

Tightening from the centre outward in stages applies clamping force evenly and helps prevent the head from warping or the gasket from being crushed unevenly. The specified torque and sequence together ensure a proper seal.

Question 33Engine Construction, Operation and Overhaul

If the piston ring end gap is too small, the most likely result is:

  1. Excessive oil burning
  2. Piston slap
  3. Ring ends butting Correct answer.
  4. Low compression pressure

Explanation

A ring with too little end gap can expand until its ends butt together as the engine reaches operating temperature. This can break the ring or score the cylinder bore, so the gap must be checked in the bore before assembly.

Question 34Engine Construction, Operation and Overhaul

Piston-to-bore clearance must be measured because:

  1. Prevents seizure and piston slap Correct answer.
  2. It controls the oil pump pressure
  3. It sets the valve opening timing
  4. It determines the engine firing order

Explanation

The correct clearance allows the piston to expand with heat without seizing while keeping it stable enough to avoid slapping against the bore. Too little clearance can cause scoring, and too much can cause noise and blow-by.

Question 35Engine Construction, Operation and Overhaul

Main bearing oil clearance is usually checked with:

  1. A dial indicator on the crankshaft nose
  2. Plastigauge Correct answer.
  3. A torque wrench
  4. A feeler gauge between the journals

Explanation

Plastigauge is a crushable plastic strip placed between the bearing shell and journal; after tightening, its flattened width indicates the clearance. A feeler gauge cannot be inserted reliably into a fully assembled bearing clearance.

Question 36Engine Construction, Operation and Overhaul

The purpose of grinding or cutting a valve seat is to:

  1. Increase the diameter of the port
  2. Increase the maximum valve lift
  3. Seal valve face to seat Correct answer.
  4. Reduce the valve spring tension

Explanation

A valve seat is refinished to restore the correct angle and a gas-tight contact surface for the valve face. This sealing surface is essential for maintaining compression and preventing leakage during the combustion and compression strokes.

Question 37Engine Construction, Operation and Overhaul

In a pushrod engine, the camshaft is located in the block and operates the valves through:

  1. The crankshaft counterweights
  2. Direct acting buckets in the head
  3. The timing belt only
  4. Tappets, pushrods and rockers Correct answer.

Explanation

In a pushrod engine the camshaft acts on tappets or lifters, which move pushrods that operate rocker arms to open the valves. This arrangement places the camshaft in the block rather than in the cylinder head.

Question 38Engine Construction, Operation and Overhaul

The big end of a connecting rod is fitted with bearing shells so that it can:

  1. Slip up and down in the bore
  2. Drive the camshaft
  3. Support the piston pin
  4. Run smoothly on the crankshaft Correct answer.

Explanation

The big-end bearing shells provide a low-friction surface with an oil film between the connecting rod and the crankshaft journal. This allows the rod to rotate around the journal under load without damaging the crankshaft.

Question 39Engine Construction, Operation and Overhaul

After reboring a cylinder, the bore is honed to:

  1. Give correct finish and size Correct answer.
  2. Increase the compression ratio
  3. Harden the cylinder wall
  4. Remove the piston rings

Explanation

Honing produces the final bore size and a controlled crosshatch surface that helps retain oil and seat the rings. It is a finishing operation, not a way to change compression ratio or harden the metal.

Question 40Engine Construction, Operation and Overhaul

Crankshaft end play (axial clearance) is normally controlled by:

  1. The connecting rod bearings
  2. The main bearing shells
  3. The piston pins
  4. The thrust bearing surfaces Correct answer.

Explanation

Thrust bearing surfaces, often part of a main bearing, limit forward and backward movement of the crankshaft along its axis. End play must be within specification so that the crankshaft can expand without binding.

Question 41Engine Construction, Operation and Overhaul

Valve overlap occurs when:

  1. The intake valve closes after BDC
  2. Both valves open near TDC after exhaust Correct answer.
  3. The exhaust valve opens before BDC
  4. Both valves closed at TDC on compression

Explanation

Valve overlap is the period around top dead centre at the end of the exhaust stroke when the intake valve has opened slightly and the exhaust valve has not yet closed. It uses exhaust gas flow to help draw in fresh charge, but excessive overlap can impair idle quality.

Question 42Engine Construction, Operation and Overhaul

Bearing crush in a connecting rod is important because it:

  1. Allows the shell to rotate with the journal
  2. Reduces the big-end bore size
  3. Increases the oil clearance
  4. Holds the shell firmly in its bore Correct answer.

Explanation

Bearing shells are made slightly longer than the housing so that tightening the cap crushes them outward against the bore. This keeps the shell from moving, improves heat transfer and helps maintain the correct oil clearance.

Question 43Engine Construction, Operation and Overhaul

In a diesel engine, ignition of the fuel occurs because:

  1. The injector nozzle heats the fuel electrically
  2. Compression heat ignites the fuel Correct answer.
  3. The fuel is mixed before compression
  4. A spark plug fires near TDC

Explanation

A diesel engine draws in air and compresses it to a high ratio, raising its temperature well above the fuel's ignition point. Fuel is injected near the end of compression and ignites spontaneously, so no spark plug is needed.

Question 44Engine Construction, Operation and Overhaul

A cylinder bore that is worn more at the top than at the bottom is said to have:

  1. A ring ridge only
  2. Out-of-round
  3. Excessive clearance
  4. Taper Correct answer.

Explanation

Taper is a change in diameter along the length of the bore, commonly caused by greater wear near the top where ring pressure and combustion loads are highest. Out-of-round refers to deviation from a true circle at a given height.

Question 45Engine Construction, Operation and Overhaul

Torque-to-yield head bolts are tightened to a specified torque and then turned through an additional angle because:

  1. The gasket must be compressed twice
  2. The thread pitch changes with temperature
  3. The bolt is self-locking
  4. Bolt stretch gives a steady clamp load Correct answer.

Explanation

Torque-to-yield bolts are designed to stretch beyond their elastic range so that they provide a consistent clamping force. The final angle turn controls the stretch more accurately than torque alone, and such bolts are generally not reused.

Question 46Engine Construction, Operation and Overhaul

The main function of the compression rings fitted to a piston is to:

  1. Seal combustion gases from escaping past the piston Correct answer.
  2. Prevent the gudgeon pin from moving sideways out of its boss
  3. Scrape surplus oil from the cylinder bore back to the sump
  4. Keep the piston centred so that it does not touch the bore

Explanation

Compression rings sit in the upper grooves of the piston and are pressed outwards against the bore by gas pressure, forming a seal that keeps combustion pressure above the piston. This seal is what allows the engine to build compression and to transfer force to the piston crown.

Question 47Engine Construction, Operation and Overhaul

The flywheel of an engine is fitted mainly to:

  1. Drive the camshaft at exactly half the speed of the crankshaft
  2. Store energy and smooth out the engine's power impulses Correct answer.
  3. Transmit the engine torque directly to the road wheels through the axle
  4. Increase the compression pressure inside the engine cylinders

Explanation

Because only one stroke in four produces power, the crankshaft speed tends to rise and fall. The mass of the flywheel absorbs energy during the power stroke and gives it back during the other strokes, so the engine runs smoothly and idles steadily.

Question 48Engine Construction, Operation and Overhaul

The valve spring in a four-stroke engine serves to:

  1. Open the valve at the correct moment in the engine cycle
  2. Guide the valve stem so that the valve moves in a straight line
  3. Maintain a fixed clearance between the rocker arm and valve stem
  4. Close the valve firmly on its seat and return it after lifting Correct answer.

Explanation

The cam or rocker only pushes the valve open; something must pull it shut again and hold it shut against cylinder pressure. The compressed valve spring does this, and it also keeps the follower in contact with the cam so that the valve follows the cam profile exactly.

Question 49Engine Construction, Operation and Overhaul

Compared with a dry cylinder liner, a wet cylinder liner is one that:

  1. Is fitted only to engines that are cooled by a stream of air
  2. Needs no sealing rings because it cannot leak at the top or bottom
  3. Is pressed into the block so that it never touches the coolant
  4. Is in direct contact with the coolant in the water jacket Correct answer.

Explanation

A wet liner has its outer surface exposed to the coolant, so heat passes from the liner straight into the water. Because it passes through the water jacket, it must be sealed at the top and bottom with gaskets or O-rings to stop coolant entering the crankcase.

Question 50Engine Construction, Operation and Overhaul

Counterweights are formed on the crankshaft in order to:

  1. Balance the rotating and reciprocating masses of the engine Correct answer.
  2. Throw oil onto the cylinder walls for lubrication
  3. Increase the mass of the crankshaft so that it turns more slowly
  4. Provide a mounting surface for the flywheel and clutch

Explanation

The crankpins, big ends and pistons create out-of-balance forces as they rotate and reciprocate. Counterweights cast opposite each crankpin offset those masses, reducing vibration and bearing loads and allowing the engine to run smoothly at speed.