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Karnataka II PUC Automobile - Vocational (KSEAB) study materials
Karnataka II PUC Automobile - Vocational (KSEAB) study resources

Karnataka II PUC Automobile - Vocational (KSEAB) practice questions

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Question 1Automobile Fundamentals and Workshop Safety

In an automobile workshop, which personal protective equipment is most important when using a pedestal grinder to sharpen a chisel?

  1. Cotton hand gloves
  2. Safety goggles with side shields Correct answer.
  3. A dust mask only
  4. A leather apron only

Explanation

Grinding throws high-speed metal particles and sparks toward the face, so eye protection with side shields is the primary safeguard. Gloves, aprons and masks do not stop flying fragments from reaching the eyes.

Question 2Automobile Fundamentals and Workshop Safety

A four-stroke petrol engine completes one full working cycle in how many revolutions of the crankshaft?

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

Explanation

The four strokes are suction, compression, power and exhaust, and each stroke occupies half a crankshaft revolution. Therefore one complete cycle needs two revolutions, that is 720 degrees of crank rotation.

Question 3Automobile Fundamentals and Workshop Safety

Which component in a manual transmission allows the driver to engage gears smoothly by matching engine speed to the gearbox input shaft?

  1. Clutch Correct answer.
  2. Differential
  3. Propeller shaft
  4. Torque converter

Explanation

The clutch temporarily disconnects and progressively reconnects the engine from the gearbox, letting the driver change gears and slip it to synchronise speeds. A torque converter does a similar job but belongs to automatic transmissions.

Question 4Automobile Fundamentals and Workshop Safety

The main purpose of a radiator in a liquid-cooled engine cooling system is to:

  1. Increase the coolant boiling point
  2. Reject engine heat to the surrounding air Correct answer.
  3. Filter rust particles from the coolant
  4. Lubricate the water pump bearings

Explanation

Hot coolant from the engine passes through the radiator core, where airflow across the fins removes heat before the coolant returns to the engine. This keeps the engine near its designed operating temperature.

Question 5Automobile Fundamentals and Workshop Safety

Which tool is used to measure the inner diameter of a cylinder bore accurately in an automotive machine shop?

  1. Telescopic gauge with outside micrometer Correct answer.
  2. Feeler gauge
  3. Vernier caliper
  4. Micrometer

Explanation

A telescopic gauge is set to the bore diameter and then measured with an outside micrometer to read the true size. This transfer method gives the accuracy needed for cylinder wear assessment.

Question 6Automobile Fundamentals and Workshop Safety

In a lead-acid starter battery, what is the active material of the positive plate?

  1. Sponge lead
  2. Dilute sulphuric acid
  3. Lead sulphate
  4. Lead dioxide Correct answer.

Explanation

The positive plate is lead dioxide and the negative plate is sponge lead, with dilute sulphuric acid as the electrolyte. During discharge both plates convert to lead sulphate.

Question 7Automobile Fundamentals and Workshop Safety

A tyre is marked 195/65 R15. What does the number 65 represent?

  1. Rim diameter in inches
  2. Aspect ratio of sidewall height to section width Correct answer.
  3. Maximum speed rating in km/h
  4. Load index of the tyre

Explanation

In this size code, 195 is the section width in millimetres, 65 is the aspect ratio and 15 is the rim diameter in inches. Aspect ratio expresses sidewall height as a percentage of section width.

Question 8Automobile Fundamentals and Workshop Safety

During the compression stroke of a diesel engine, the air temperature rises sufficiently because:

  1. Fuel is injected early and burns
  2. The exhaust valve opens slightly
  3. A glow plug heats the air
  4. The compression ratio is high and heat is generated by compressing the air Correct answer.

Explanation

Diesel engines use high compression ratios, typically 16:1 to 22:1, so compressing the air alone raises its temperature above the fuel's ignition point. Fuel injected at the end of compression then ignites without a spark.

Question 9Automobile Fundamentals and Workshop Safety

When jacking up a car in the workshop, where should the jack be placed?

  1. At the manufacturer's designated jacking point on the chassis Correct answer.
  2. On the brake disc
  3. Under the floor pan near the door sill
  4. Under the plastic bumper

Explanation

Manufacturers provide reinforced jacking points designed to carry the vehicle load without damage. Placing the jack elsewhere can crush body panels or cause the vehicle to slip.

Question 10Automobile Fundamentals and Workshop Safety

In a vehicle's steering system, what is the function of the kingpin inclination?

  1. To provide self-centring of the steering after a turn Correct answer.
  2. To reduce braking efficiency
  3. To increase tyre wear on the outer edge
  4. To increase the turning circle radius

Explanation

Kingpin inclination, or steering axis inclination, tilts the steering axis so that lifting the vehicle during a turn creates a self-aligning force. This helps the steering return to the straight-ahead position and reduces driver effort.

Question 11Automobile Fundamentals and Workshop Safety

Which type of suspension uses a transverse leaf spring as both a springing and a locating member?

  1. Double wishbone
  2. Transverse leaf spring suspension Correct answer.
  3. Multi-link suspension
  4. MacPherson strut

Explanation

A transverse leaf spring spans the vehicle width and, when clamped to the chassis, also resists lateral and longitudinal wheel movement. This dual role is characteristic of transverse leaf spring layouts used on some light vehicles.

Question 12Automobile Fundamentals and Workshop Safety

In an engine, the function of the flywheel is to:

  1. Operate the clutch pedal
  2. Store energy and smooth out power pulses between strokes Correct answer.
  3. Filter the engine oil
  4. Increase the compression ratio

Explanation

The flywheel's inertia carries the crankshaft through the non-power strokes and damps cyclic speed fluctuations. It also provides a friction surface for the clutch on manual vehicles.

Question 13Automobile Fundamentals and Workshop Safety

A technician finds the brake pedal feels spongy and travels too far. Which is the most likely cause?

  1. Air trapped in the hydraulic brake system Correct answer.
  2. A loose wheel bearing
  3. A clogged air filter
  4. Worn brake pads with correct fluid level

Explanation

Air is compressible, so bubbles in the hydraulic circuit absorb pedal force and give a spongy feel with excessive travel. The remedy is bleeding the system to remove air.

Question 14Automobile Fundamentals and Workshop Safety

In a carburettor, the choke valve is closed during cold starting to:

  1. Advance the ignition timing
  2. Increase the air supply to the engine
  3. Reduce the fuel level in the float chamber
  4. Enrich the air-fuel mixture by reducing air flow Correct answer.

Explanation

Closing the choke restricts air entering the venturi, which increases the vacuum acting on the fuel jet and delivers a richer mixture. A rich mixture is needed for cold starting because fuel vaporises poorly at low temperatures.

Question 15Automobile Fundamentals and Workshop Safety

Which emission control device uses a ceramic honeycomb coated with platinum, palladium and rhodium to convert harmful gases?

  1. Catalytic converter Correct answer.
  2. Evaporative emission canister
  3. Exhaust gas recirculation valve
  4. Positive crankcase ventilation valve

Explanation

The catalytic converter promotes oxidation of carbon monoxide and hydrocarbons and reduction of oxides of nitrogen on precious-metal catalysts. The three metals named perform these conversion reactions.

Question 16Automobile Fundamentals and Workshop Safety

A 12 V battery is connected to a bulb of resistance 6 ohms. Using Ohm's law, what current flows through the bulb?

  1. 18 A
  2. 2 A Correct answer.
  3. 72 A
  4. 0.5 A

Explanation

Ohm's law states that current equals voltage divided by resistance, so 12 divided by 6 gives 2 amperes. This basic relationship is used constantly when diagnosing automotive electrical faults.

Question 17Automobile Fundamentals and Workshop Safety

In a vehicle with a full-time four-wheel-drive system, the function of the centre differential is to:

  1. Disconnect the rear propeller shaft at high speed
  2. Increase engine torque at low speed
  3. Allow front and rear axles to rotate at different speeds Correct answer.
  4. Lock the front wheels during braking

Explanation

A centre differential permits a speed difference between the front and rear propeller shafts, which is essential when the vehicle turns and the axles cover different path lengths. Without it, transmission wind-up would occur on dry surfaces.

Question 18Automobile Fundamentals and Workshop Safety

An engine has a bore of 80 mm and a stroke of 90 mm with four cylinders. What is the approximate total engine displacement in cubic centimetres?

  1. 1809 cc Correct answer.
  2. 2262 cc
  3. 452 cc
  4. 904 cc

Explanation

Swept volume of one cylinder equals pi/4 times bore squared times stroke, giving about 452 cc. Multiplying by four cylinders gives approximately 1809 cc, which is the engine displacement.

Question 19Automobile Fundamentals and Workshop Safety

During a compression test on a petrol engine, one cylinder reads much lower than the others. Adding a few drops of oil raises that reading significantly. What does this indicate?

  1. Worn piston rings or cylinder bore Correct answer.
  2. Incorrect ignition timing
  3. A leaking head gasket
  4. A burnt exhaust valve

Explanation

Oil temporarily seals the gap between worn rings and the cylinder wall, so the compression rises. A valve or gasket leak would not be sealed by oil and the reading would remain low.

Question 20Automobile Fundamentals and Workshop Safety

A vehicle pulls to one side during braking. Which combination of faults best explains this?

  1. Unequal tyre pressure only
  2. A worn clutch plate
  3. A seized caliper on one side or contaminated lining on one wheel Correct answer.
  4. A faulty fuel pump

Explanation

Braking pull arises when one wheel develops more braking force than the other, for example a sticking caliper or oil-soaked lining on one side. Inspecting both sides of the brake system is the correct diagnostic approach.

Question 21Engine Construction, Operation and Systems

In a four-stroke diesel engine, the camshaft rotates at what speed relative to the crankshaft?

  1. Twice the crankshaft speed
  2. One quarter of the crankshaft speed
  3. Half the crankshaft speed Correct answer.
  4. The same speed as the crankshaft

Explanation

A four-stroke cycle needs two crankshaft revolutions to complete one operating cycle, so the camshaft that opens each valve once per cycle must turn at half the crankshaft speed. The camshaft is driven through timing gears or a timing belt with a 2:1 reduction.

Question 22Engine Construction, Operation and Systems

Which component converts the reciprocating motion of the piston into rotary motion of the crankshaft?

  1. The flywheel
  2. The pushrod
  3. The camshaft
  4. The connecting rod Correct answer.

Explanation

The connecting rod links the piston pin to the crankpin, so as the piston moves up and down the rod swings and forces the crank to rotate. It is the direct mechanical link that transforms linear piston travel into crankshaft rotation.

Question 23Engine Construction, Operation and Systems

The swept volume of a single cylinder is calculated using which relationship?

  1. Bore area multiplied by stroke length Correct answer.
  2. Bore area multiplied by compression ratio
  3. Bore diameter multiplied by stroke length
  4. Clearance volume multiplied by stroke length

Explanation

Swept volume is the cylindrical space the piston displaces in one stroke, found by multiplying the cross-sectional area of the bore by the stroke length. The formula is V = (π/4)d² × L, where d is bore and L is stroke.

Question 24Engine Construction, Operation and Systems

In a diesel engine the fuel is ignited mainly because of which condition inside the cylinder?

  1. A spark from the spark plug
  2. A glow plug left on continuously
  3. Pre-ignition from residual exhaust gas
  4. High compression temperature of the air Correct answer.

Explanation

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

Question 25Engine Construction, Operation and Systems

Which part of the engine seals the combustion chamber at the top of the cylinder bore and carries the valve seats?

  1. The piston crown
  2. The sump
  3. The cylinder head Correct answer.
  4. The cylinder liner

Explanation

The cylinder head bolts onto the top of the block and closes the combustion chamber, and it houses the valve seats, valve guides and often the camshaft. A head gasket seals the joint between head and block.

Question 26Engine Construction, Operation and Systems

What is the main function of the flywheel in an engine?

  1. To drive the camshaft through the timing chain
  2. To store energy and smooth out torque fluctuations between power strokes Correct answer.
  3. To filter the lubricating oil
  4. To increase the compression ratio

Explanation

Because torque is produced only during the power stroke, the flywheel absorbs energy during that stroke and releases it during the other strokes. This keeps the crankshaft turning smoothly and carries the engine past non-power strokes.

Question 27Engine Construction, Operation and Systems

The clearance volume of a cylinder is the volume of the space above the piston when it is at which position?

  1. Mid-stroke
  2. Top dead centre Correct answer.
  3. Bottom dead centre
  4. Halfway through the exhaust stroke

Explanation

Clearance volume is the small space remaining above the piston crown when it reaches the very top of its travel, that is, top dead centre. Together with swept volume it sets the compression ratio of the engine.

Question 28Engine Construction, Operation and Systems

Which cooling system uses a thermostat to regulate coolant flow and relies on a water pump and radiator?

  1. Forced circulation water cooling Correct answer.
  2. Evaporative cooling
  3. Air cooling
  4. Thermosyphon cooling

Explanation

Forced circulation water cooling uses a pump to push coolant through the block and head to a radiator, and a thermostat opens or closes to keep the engine within its operating temperature range. Air cooling uses fins and airflow instead of liquid.

Question 29Engine Construction, Operation and Systems

A four-cylinder four-stroke engine has a bore of 80 mm and a stroke of 90 mm. Using π/4 = 0.785, what is the approximate total engine displacement?

  1. About 226 cc
  2. About 1808 cc Correct answer.
  3. About 7234 cc
  4. About 452 cc

Explanation

Swept volume per cylinder is 0.785 × 8² × 9 = 452 cc, and multiplying by four cylinders gives about 1808 cc. The calculation uses bore area multiplied by stroke, then multiplied by the number of cylinders.

Question 30Engine Construction, Operation and Systems

An engine has a swept volume of 400 cc and a clearance volume of 40 cc per cylinder. What is its compression ratio?

  1. 11:1 Correct answer.
  2. 9:1
  3. 10:1
  4. 40:1

Explanation

Compression ratio equals (swept volume + clearance volume) divided by clearance volume, so (400 + 40) / 40 = 11:1. Forgetting to add the clearance volume to the numerator gives the wrong 10:1 figure.

Question 31Engine Construction, Operation and Systems

Valve overlap in a four-stroke engine refers to which situation?

  1. The exhaust valve opening early during the power stroke
  2. The intake valve closing after bottom dead centre
  3. Both valves open at the same time around top dead centre between exhaust and intake Correct answer.
  4. Both valves closed near top dead centre of compression

Explanation

Valve overlap is the period near the end of the exhaust stroke when the inlet valve has already opened while the exhaust valve is still closing. It uses the inertia of the outgoing exhaust gas to help draw in the fresh charge.

Question 32Engine Construction, Operation and Systems

Why is the exhaust valve of a diesel engine often made slightly smaller than the intake valve in some designs, while the intake valve is kept larger?

  1. To increase the compression ratio
  2. To allow the exhaust gases to leave more slowly
  3. To improve breathing by giving a larger opening to the incoming air Correct answer.
  4. To reduce the weight of the cylinder head

Explanation

The intake valve is commonly made larger so that the incoming air charge can enter with less restriction, improving volumetric efficiency. Exhaust gas leaves under its own pressure, so a slightly smaller exhaust valve is usually acceptable.

Question 33Engine Construction, Operation and Systems

Which type of piston ring is primarily responsible for scraping excess oil off the cylinder wall and returning it to the sump?

  1. The top ring
  2. The compression ring
  3. The gudgeon pin circlip
  4. The oil control ring Correct answer.

Explanation

The oil control ring sits lowest on the piston and has slots and rails that scrape surplus oil from the bore and return it through holes in the piston. The upper rings are compression rings that seal combustion gases.

Question 34Engine Construction, Operation and Systems

A mechanic finds that the engine oil pressure warning lamp stays on after starting and the oil level is normal. Which cause should be checked first?

  1. The radiator cap rating
  2. The battery terminal tightness
  3. The spark plug gap
  4. A faulty oil pressure switch or worn oil pump Correct answer.

Explanation

With correct oil level, persistent low oil pressure points to a defective pressure switch, a worn pump, a blocked pickup or excessive bearing clearance. These are the components of the lubrication circuit that generate and sense pressure.

Question 35Engine Construction, Operation and Systems

In a full-flow lubrication system, what is the purpose of the relief valve fitted near the oil pump?

  1. To bypass the oil filter when it is clogged
  2. To limit maximum oil pressure by diverting excess oil back to the sump Correct answer.
  3. To admit air into the oil gallery
  4. To open the coolant passage when hot

Explanation

The relief valve opens when oil pressure exceeds a set value, usually at high engine speed or with cold thick oil, and returns excess oil to the sump. This protects seals, filters and galleries from excessive pressure.

Question 36Engine Construction, Operation and Systems

What is the role of the thermostat in a water-cooled engine cooling system?

  1. To increase the boiling point of the coolant
  2. To regulate coolant flow to the radiator so the engine reaches and holds its correct temperature Correct answer.
  3. To drive the radiator cooling fan directly
  4. To circulate coolant through the engine

Explanation

The thermostat stays closed while the engine is cold so coolant bypasses the radiator and warms up quickly, then opens at a set temperature to let coolant flow through the radiator. This keeps the engine within its designed operating range.

Question 37Engine Construction, Operation and Systems

A vehicle overheats in slow traffic but runs at normal temperature on the open road. Which fault best fits these symptoms?

  1. An over-rich fuel mixture
  2. A weak or inoperative cooling fan Correct answer.
  3. A stuck-open thermostat
  4. A leaking head gasket

Explanation

At low road speed there is little ram airflow, so the engine depends on the fan to pull air through the radiator. If the fan is weak the engine overheats in traffic but cools normally once forward speed provides airflow.

Question 38Engine Construction, Operation and Systems

Which statement correctly describes the operation of a turbocharger?

  1. It is driven by a belt from the crankshaft
  2. It mixes fuel and air before the intake valve
  3. It uses exhaust gas energy to spin a turbine that drives a compressor in the intake Correct answer.
  4. It cools the exhaust gas before it leaves the engine

Explanation

A turbocharger has a turbine in the exhaust stream coupled by a shaft to a compressor in the intake. Expanding exhaust gas spins the turbine, and the compressor raises intake air density for more power.

Question 39Engine Construction, Operation and Systems

What is the main purpose of providing a fuel injection pump with a governor in a diesel engine?

  1. To vary injected quantity with load and hold a set engine speed Correct answer.
  2. To increase the compression ratio at high speed
  3. To advance the ignition spark
  4. To filter water from the fuel

Explanation

The governor senses engine speed and adjusts the quantity of fuel delivered by the injection pump so the engine holds the selected speed under changing load. It also limits maximum and idling speed to protect the engine.

Question 40Engine Construction, Operation and Systems

In a battery ignition system, what is the function of the condenser connected across the contact breaker points?

  1. To step up battery voltage to the spark plug
  2. To store fuel for cold starting
  3. To rectify alternating current from the alternator
  4. To suppress arcing at the points and speed up the collapse of the primary current Correct answer.

Explanation

The condenser absorbs the current that would otherwise arc across the opening points and helps the primary magnetic field collapse quickly. This rapid collapse induces a high voltage in the secondary winding for the spark.

Question 41Engine Construction, Operation and Systems

A four-cylinder four-stroke engine has a firing order of 1-3-4-2. If cylinder 1 is on its power stroke, what is cylinder 4 doing at that instant?

  1. On the exhaust stroke
  2. Starting the intake stroke Correct answer.
  3. On the compression stroke
  4. On the power stroke

Explanation

In a four-stroke cycle the four strokes are distributed so that with 1 firing, the next cylinder in the order (3) is on compression, cylinder 4 is on intake and cylinder 2 is on exhaust. Thus cylinder 4 is beginning its intake stroke.

Question 42Engine Construction, Operation and Systems

Why does a diesel engine generally require a higher compression ratio than a petrol engine?

  1. To allow the use of a carburettor
  2. To reduce the weight of the engine block
  3. To eliminate the need for a cooling system
  4. To raise the air temperature enough for self-ignition of the injected fuel Correct answer.

Explanation

Compression ignition depends on the air reaching a temperature above the fuel's ignition point at the end of compression. A high compression ratio, typically 16:1 to 22:1, provides that temperature rise, whereas a petrol engine relies on a spark and uses a lower ratio.

Question 43Engine Construction, Operation and Systems

An engine shows blue-grey smoke from the exhaust and high oil consumption, but compression test readings are within specification. Which cause is most consistent with these findings?

  1. Incorrect ignition timing
  2. A blocked fuel filter
  3. Worn valve stem seals allowing oil into the intake port Correct answer.
  4. A leaking head gasket between two cylinders

Explanation

Oil drawn past worn valve stem seals enters the intake charge and burns, producing blue-grey smoke and oil loss while cylinder compression stays normal. Piston ring wear would usually also lower compression readings.

Question 44Engine Construction, Operation and Systems

During a cylinder leakage test, air escapes continuously through the radiator with bubbles appearing. What does this indicate?

  1. A burnt exhaust valve
  2. Worn piston rings
  3. A faulty PCV valve
  4. A leaking head gasket or cracked head allowing air into the coolant jacket Correct answer.

Explanation

Bubbles in the radiator during a leakage test show that compressed air from the cylinder is reaching the coolant passages. This points to a failed head gasket or a crack between the combustion chamber and the water jacket.

Question 45Engine Construction, Operation and Systems

In a multi-cylinder engine, why is the crankshaft fitted with a torsional vibration damper at its front end?

  1. To increase the flywheel mass for smoother idling
  2. To absorb the twisting oscillations set up by periodic power impulses along the shaft Correct answer.
  3. To align the timing marks on the camshaft
  4. To drive the oil pump at constant speed

Explanation

Each power stroke delivers a torque pulse that makes the long crankshaft twist and untwist, and at certain speeds these oscillations can resonate. The damper, usually a pulley with a bonded rubber or viscous element, absorbs this energy to protect the shaft from fatigue failure.

Question 46Engine Construction, Operation and Systems

In a four-stroke petrol engine, the piston moves from top dead centre to bottom dead centre with both valves closed and the spark plug firing near the start of this movement. Which stroke is being described?

  1. Suction (induction) stroke
  2. Power (expansion) stroke Correct answer.
  3. Compression stroke
  4. Exhaust stroke

Explanation

The power stroke begins just after the spark ignites the compressed charge near top dead centre; the burning gases force the piston down to bottom dead centre with both valves held closed. The suction stroke also moves downward but has the intake valve open, and the compression and exhaust strokes move the piston upward.

Question 47Engine Construction, Operation and Systems

A mechanic measures a piston with a nominal size of 80.00 mm and finds it is 79.96 mm. Which term correctly describes the 0.04 mm difference between the nominal size and the actual measured size?

  1. Ovality
  2. Clearance Correct answer.
  3. Allowance
  4. Taper

Explanation

Clearance is the intentional gap between two mating parts, such as piston and cylinder bore, that allows for thermal expansion and lubrication. Taper and ovality describe shape errors of the part itself, while allowance is the minimum intentional difference designed between mating parts before tolerances are applied.

Question 48Engine Construction, Operation and Systems

A four-cylinder, four-stroke engine has a firing order of 1-3-4-2 and the cranks are spaced at equal intervals. At what crank angle interval does each successive cylinder fire?

  1. 180 degrees Correct answer.
  2. 360 degrees
  3. 720 degrees
  4. 90 degrees

Explanation

A four-stroke cycle takes 720 degrees of crankshaft rotation, and with four cylinders firing once each per cycle the interval between successive firings is 720 divided by 4, which equals 180 degrees. This even spacing keeps the power impulses regular and reduces vibration.

Question 49Engine Construction, Operation and Systems

An engine is being assembled and the timing marks on the crankshaft and camshaft sprockets are found to be one tooth out of alignment. Which consequence is most directly expected if the engine is run in this condition?

  1. The oil pump will stop delivering oil to the bearings
  2. Valve timing will shift, so the engine may run rough or valves may contact the piston Correct answer.
  3. The compression ratio will increase permanently
  4. The flywheel will lose its balance and cause vibration

Explanation

The camshaft is driven at half crankshaft speed through the timing chain or belt, so misaligning the marks by one tooth shifts every valve event relative to piston position. This can cause poor running, loss of power, and in severe cases valve-to-piston contact, which is why timing marks must be aligned exactly during assembly.

Question 50Engine Construction, Operation and Systems

A technician observes that an engine with a correctly adjusted valve clearance develops a light tapping noise that increases with engine speed, and a compression test shows normal readings on all cylinders. Which component is the most likely source of the noise?

  1. A partially blocked radiator core tube
  2. Worn valve guides allowing the valve stem to move sideways
  3. Excessive clearance between the valve stem and the rocker arm Correct answer.
  4. A leaking head gasket between two adjacent cylinders

Explanation

A tapping that rises with engine speed while compression stays normal points to clearance in the valve train, where the rocker arm strikes the valve stem tip each cycle. Because the noise tracks camshaft speed and compression is unaffected, the valve train clearance is the strongest candidate.