NSSCO Motor Mechanics 6189 common mistakes
NSSCO Motor Mechanics 6189 · Understand the traps and how to avoid them.
When asked to identify a hazard in a described workshop scenario, candidates list only the obvious machine hazard (e.g. the rotating chuck) and omit the associated hazards such as flying swarf, entanglement of loose clothing, noise, or inadequate guarding, so the answer is marked incomplete.
Candidates treat 'hazard' as a single item and stop at the first thing they notice, rather than systematically working through hazard categories (mechanical, electrical, chemical, ergonomic, environmental). They also confuse a hazard with a risk or with a control measure.
What to do instead
Practise a fixed checklist for every scenario: energy source, moving parts, material/substance, environment (noise, dust, fumes, lighting, floor), and people (clothing, jewellery, training, lone working). For each hazard named, state who could be harmed and how. Write answers as 'hazard - possible harm - control', which forces full coverage.
Candidates describe the safe procedure for using a hand tool or piece of equipment but omit the pre-use inspection and the post-use storage/maintenance steps, giving only the 'during use' stage.
Attention focuses on the dramatic part of the task (using the tool) and the preparation and close-down stages are mentally skipped. Candidates also assume inspection is someone else's job.
What to do instead
Structure every procedure answer in three stages: before (select correct tool for the job, inspect for damage, check PPE, secure the workpiece), during (correct grip, stance, force direction, awareness of others), after (clean, lubricate where required, return to correct location, report defects). Learn one tool per revision session using this template, e.g. a ball-peen hammer, a hacksaw, a torque wrench, a bench grinder.
Candidates misread a vernier caliper, micrometer or dial gauge and give a reading with the wrong resolution or the wrong number of decimal places, e.g. reading a metric micrometer as 6.5 mm when it is 6.50 mm, or adding the vernier scale in the wrong direction.
The scales are learned as a formula rather than as a physical procedure, so under exam pressure the candidate adds main-scale and vernier/thimble values incorrectly or forgets to account for the sleeve datum line. Zero error is also ignored.
What to do instead
Rehearse the physical steps aloud: read the main/sleeve scale at the last visible graduation before the thimble/vernier edge, then read the fractional scale and add. Always state the resolution of the instrument (0.02 mm for a typical vernier caliper, 0.01 mm for a metric micrometer) and check for zero error before and after. Practise reading from written descriptions of scale positions, since no instrument will be in front of you in the exam.
When explaining the four-stroke cycle, candidates name the strokes correctly but place the valve positions or the ignition point in the wrong stroke, for example saying the inlet valve is open throughout the compression stroke or that the spark occurs at the start of the power stroke with both valves open.
The strokes are memorised as a list of words without linking each to piston direction, valve state, and pressure change. Valve overlap and ignition timing are learned separately and not integrated into the cycle.
What to do instead
Draw the cycle as a four-row table: stroke name, piston direction, inlet valve, exhaust valve, what happens to the charge, and pressure/temperature trend. Then annotate valve lead, lag and overlap on a valve-timing diagram. Test yourself by describing what happens if the camshaft is one tooth out, which only makes sense if the valve-to-stroke mapping is secure.
Candidates confuse two-stroke and four-stroke operation, stating that a two-stroke engine has valves and a camshaft, or that it completes a power stroke every four revolutions of the crankshaft.
The two engine types are revised together and the distinguishing features (ports versus valves, crankcase scavenging, lubrication via the fuel-oil mixture, one power stroke per revolution) blur into one generic description.
What to do instead
Build a comparison table with rows for: number of crankshaft revolutions per power stroke, gas exchange method (ports/reed valve versus poppet valves and camshaft), lubrication method, typical applications, and reasons for each advantage. Write a two-sentence explanation of why a two-stroke needs oil mixed with the fuel or a separate oil injection system.
When describing an overhaul or measurement task, candidates give the measurement or component name without the specified tolerance, sequence, or tightening method, e.g. saying 'check the bore for wear' without stating where and how to measure it, or 'tighten the cylinder head' without a sequence or torque figure.
Candidates recall the general idea of the job but not the controlled procedure, because they have read about it rather than written it out. Torque values and sequences feel like detail to be looked up rather than examinable knowledge.
What to do instead
For each overhaul task, write a numbered procedure including: cleaning before measurement, the measuring instrument and its resolution, measurement positions (e.g. bore measured at top, middle and bottom in two directions at 90 degrees), comparison with the manufacturer's limit, and the reassembly sequence with torque and angle figures where given. Learn the reason for a spiral or centre-outwards tightening sequence, since explanation marks often depend on it.
Candidates describe the lubrication or cooling system as a list of parts without explaining flow direction, pressure regulation or the consequence of a component failing, so questions asking 'what happens if the thermostat sticks open' or 'why does the oil pressure warning light come on' are answered vaguely.
The system is learned as a labelled diagram rather than as a circuit with a pressure and temperature balance. Candidates do not link each component to the fault symptom it produces.
What to do instead
Trace the path in words from sump to bearings and back for lubrication, and from water pump through block, head, thermostat and radiator for cooling. For each component, write one sentence on its function and one on the symptom of its failure (thermostat stuck open gives slow warm-up and poor heater output; stuck closed gives rapid overheating). Practise fault-symptom matching questions in both directions.
Candidates mix up battery, charging and starting circuit behaviour, for example saying a low battery causes the starter to spin slowly because the alternator is faulty, or failing to distinguish a no-crank condition from a slow-crank condition when diagnosing a described fault.
The three circuits are studied separately and the candidate does not build a mental model of how they interact. Voltage drop testing and the difference between open-circuit voltage and cranking voltage are often only half understood.
What to do instead
Learn the diagnostic decision tree: does the engine crank at normal speed (starting circuit and battery likely serviceable, look at fuel/ignition), crank slowly (battery state of charge, terminals, earth straps, starter), or not crank at all (immobiliser, starter solenoid, ignition switch, neutral/clutch switch). Then learn what a voltmeter reading at the battery during cranking should be and what a low reading indicates. Practise describing a voltage drop test step by step.
Candidates describe clutch or gearbox operation without linking component action to power flow, e.g. saying the clutch 'disconnects the engine' without naming the pressure plate, friction disc and release bearing sequence, or confusing the function of the synchromesh with that of the selector fork.
Transmission topics are memorised as component names attached to a system, not as a sequence of torque transmission. The clutch is often reduced to a single sentence in revision notes.
What to do instead
For the clutch, write the engagement and disengagement sequence as a numbered chain from pedal to release bearing to diaphragm spring to pressure plate to disc to flywheel. For the gearbox, trace power from input shaft through the selected gear pair to the output shaft, and separately explain what the synchromesh does and why it is needed. For the differential, explain why the outer wheel must turn faster in a corner.
Candidates answer steering, suspension and tyre questions using everyday language rather than technical terms, e.g. describing 'wheels pointing in different directions' instead of toe-in/toe-out, or confusing camber with caster, and giving tyre wear patterns without linking them to the misalignment that causes them.
The alignment angles are abstract and easily swapped because they are learned as definitions rather than as observable effects. Tyre wear diagnosis is often skipped in favour of the definitions.
What to do instead
Learn each angle with a physical picture and a wear or handling symptom: camber as the wheel leaning in or out viewed from the front (uneven shoulder wear), caster as the steering axis tilt viewed from the side (poor self-centring, heavy or light steering), toe as the wheels viewed from above (feathering across the tread). Practise matching a described wear pattern to a cause and a correction.
Candidates describe braking system faults without distinguishing hydraulic, mechanical and electronic causes, e.g. attributing a spongy pedal to worn pads, or a pulling vehicle to air in the system when a seized caliper is more likely, and they omit the safety-critical checks required before road testing.
Braking is treated as one system rather than three interacting ones, and fault symptoms are matched by guesswork. Candidates also overlook the legal and safety requirements because they seem like common sense rather than examinable content.
What to do instead
Build a symptom-to-cause table for the most common complaints: spongy pedal (air in hydraulics, fluid leak, flexible hose expansion), long pedal travel (rear brake adjustment, fluid loss), pulling to one side (seized caliper or piston, contaminated linings, collapsed hose), vibration (warped disc, wheel bearing). Learn the checks required before a road test and the legal minimum tread depth and brake condition requirements for your jurisdiction.
Candidates treat servicing as a list of items to change rather than a scheduled inspection with specified intervals, torque figures and record-keeping, so answers omit the reason for each service item and the consequences of skipping it.
Routine servicing feels too simple to be examined in depth, so candidates revise it lightly and then cannot explain why a component is replaced on time rather than on condition.
What to do instead
For each service item (engine oil and filter, air filter, brake fluid, coolant, spark plugs, timing belt), write what it does, the typical interval, and the failure mode if it is neglected. Learn the difference between time-based and condition-based replacement and be ready to explain why brake fluid is changed on time even when the vehicle brakes well.
Candidates lose marks on calculation questions by using the wrong formula, mixing units, or giving an answer with no unit or an implausible number of decimal places, e.g. computing engine capacity in cubic centimetres but writing the answer in litres without conversion.
Formulas are memorised without understanding the quantities involved, and unit conversion (mm to cm, cc to litres, bar to kPa) is done carelessly under time pressure. Candidates also skip the sanity check on the final answer.
What to do instead
Write the formula, substitute values with units shown, carry the units through the calculation, then state the answer with its unit and a reasonableness check. Practise the core set: swept volume of one cylinder (pi/4 x bore squared x stroke), total engine capacity, compression ratio (swept plus clearance volume divided by clearance volume), gear ratio and overall ratio, road speed from wheel diameter and rpm, and pulley/belt ratio. Do each one at least twice with different units.
Candidates answer technical drawing questions by describing what a symbol looks like rather than what it means, or they confuse first-angle and third-angle projection, placing views on the wrong side of the front view.
Drawing conventions are learned visually and the candidate cannot translate them into words, which is exactly what the exam requires. The two projection systems look similar until the symbol is understood.
What to do instead
Learn the projection symbol itself: the truncated cone shows the small end towards the observer in first angle and away in third angle. Practise describing a simple component in words: name each view (front, plan, end), state which side it appears on in each system, and identify line types (visible outline, hidden detail, centre line, dimension line, hatching for a section). Work through one small drawing per revision session without looking at the answer.