Sri Lanka NVQ L3 Motorcycle Mechanic cheat sheet

Personal protective equipment for motorcycle work: safety glasses or goggles for grinding and battery work, nitrile gloves for fuel, oil and solvent, steel-toe boots, ear protection above 85 dB(A), and a respirator when spraying paint or handling brake dust.

Battery charging releases hydrogen; keep chargers in a ventilated area, no flames or sparks within 1 m, and add acid to water (never water to acid) when mixing electrolyte.

Carbon monoxide is odourless: never run an engine indoors or in a closed bay; route exhaust to an extraction hose and treat headache or dizziness as an evacuation signal.

Fuel flash point of petrol is about -43 degrees C, so vapour ignites at room temperature; store fuel only in approved metal containers, bonded and grounded, and keep a dry powder or foam extinguisher within 10 m.

Fire classes: A solid, B flammable liquid, C gas, D metal, F cooking oil; petrol fires need foam, dry powder or CO2, never water, and electrical fires need CO2 or dry powder.

Manual handling limit rule: keep the load close, bend knees not back, and use two persons or a lift for any motorcycle or engine above roughly 25 kg for a single worker.

Hazard hierarchy for control: eliminate, substitute, engineering control, administrative control, then PPE last; PPE protects only the wearer and only if correctly fitted.

Waste segregation in a workshop: used oil and filters to licensed recyclers, brake and clutch dust sealed in bags (assume asbestos in older linings), solvents in labelled closed containers, lead-acid batteries returned to supplier.

Hand-arm vibration: keep powered tool exposure short, use anti-vibration gloves and sharp bits; report tingling or blanching fingers early, as damage is cumulative.

Micrometer reading: main scale in 0.5 mm steps plus thimble divisions; value = main scale + (thimble division x least count), where least count = 0.01 mm for a 50-division thimble.

Vernier caliper: least count = 1 main-scale division / number of vernier divisions; a 1 mm main scale with 20 vernier divisions gives 0.05 mm.

Torque = force x perpendicular distance (T = F x d); a wrench reading is valid only when force is applied at 90 degrees to the handle at the marked point.

Torque wrench rule: tighten to the specified figure in the specified sequence (usually criss-cross or spiral from centre), and recalibrate a click-type wrench at least annually or after a drop.

Bore measurement: use a cylinder bore gauge or inside micrometer at three heights and two planes (parallel and perpendicular to the crankshaft) to detect taper and out-of-round.

Feeler gauge use: stack leaves to the specified clearance, and the correct clearance is the largest stack that enters with slight drag but the next 0.05 mm size does not.

Dial gauge (DTI) checks: crankshaft runout, wheel and disc lateral runout, and gear shaft end float; always zero the gauge against a known surface first.

Plastigauge method: place a strip across the journal, torque the cap to spec without rotating the crank, then read the flattened width against the scale to obtain bearing oil clearance.

Tool care rules: never extend a wrench with a pipe, never use a metric wrench on inch fasteners, keep chrome sockets off impact drivers, and store precision instruments away from magnetic fields and moisture.

Four-stroke cycle: intake, compression, power, exhaust, two crankshaft revolutions per cycle; a two-stroke completes the cycle in one revolution with transfer and scavenge ports.

Compression ratio = (swept volume + clearance volume) / clearance volume; a 150 cc cylinder with 15 cc clearance volume gives (150+15)/15 = 11:1.

Valve clearance is measured with the engine cold at TDC on the compression stroke; typical figures are 0.05-0.10 mm intake and 0.08-0.15 mm exhaust, but always use the manufacturer figure.

Cam timing check: align the T mark on the flywheel or rotor with the crankcase index, then confirm the cam sprocket mark aligns with the head surface; one tooth off shifts timing roughly 10-20 degrees.

Piston ring end gap is measured with the ring squared in the bore using a piston crown; ring side clearance is checked with a feeler gauge in the groove.

Cylinder wear limits: taper and out-of-round beyond about 0.05 mm, or a piston-to-bore clearance beyond spec, requires reboring or replacement.

Overhaul sequence: clean and degrease, measure before dismantling decisions, mark components, replace gaskets, seals, circlips and stretch bolts, then torque in stages to spec.

Crankshaft end float and big-end radial play are checked with a dial gauge; a connecting rod should swing freely sideways but show no up-down movement.

Run-in after overhaul: first 500-1000 km avoid full throttle and sustained high speed, change oil after the first 500 km, and re-check head bolt torque and valve clearance.

Stoichiometric air-fuel ratio for petrol is about 14.7:1 by mass; rich mixtures (12-13:1) give more power, lean mixtures (15-16:1) give better economy but higher combustion temperature.

Carburettor circuits: float and needle valve set fuel level, pilot (slow) circuit covers idle to about 1/4 throttle, slide/needle covers 1/4 to 3/4, main jet covers 3/4 to full throttle.

Fuel level in the float bowl affects all circuits; a high level causes richness and flooding, a low level causes lean running and hesitation.

Air screw (fuel screw on four-strokes) adjusts the idle mixture; set it for the highest steady idle speed, then reset base idle, typically 1,200-1,500 rpm.

Electronic fuel injection sensors: MAP or MAF measures load, TPS measures throttle position, ECT and IAT correct density, and the oxygen sensor provides closed-loop feedback on mixture.

Ignition timing is set in degrees before top dead centre (BTDC); over-advanced timing causes pinging and detonation, over-retarded timing causes overheating and power loss.

CDI and TCI systems: pickup coil or crank sensor triggers the module, which fires the coil; check for spark at a known good plug, then resistance of the pickup and coil primary/secondary against spec.

Spark plug reading: light tan is normal, black soot is rich or weak ignition, white or blistered is lean or overheating; plug heat range must match the engine, not be chosen for tuning convenience.

Exhaust back pressure matters: a blocked or restricted muffler or a leaking header gasket changes mixture and noise; four-stroke exhausts rely on tuned length and a sealing crush washer at the head.

Oil grade SAE 10W-40 means a winter (W) viscosity of 10 and a hot viscosity of 40; the lower the W number, the better cold flow, and motorcycle oils must meet JASO MA or MA2 for wet clutches.

API service categories: later letters are newer, and a diesel category such as CI-4 is not interchangeable with a petrol category such as SN; use the grade the manual specifies.

Wet-sump systems use a pump pickup in the crankcase; dry-sump systems use a separate oil tank and a return pump, and both need the correct oil level checked with the bike upright.

Oil pressure warning at idle can mean low level, a worn pump, a blocked filter or a stuck relief valve; stop the engine and diagnose before further running.

Oil change interval rule: change oil and filter together, replace the crush washer on the drain plug, torque to spec, and run the engine then re-check the level.

Air cooling relies on fins and airflow; a two-stroke or small four-stroke with blocked fins or a missing shroud overheats, so keep fins clean and never run a fan-cooled engine with the fan disconnected.

Liquid cooling: the thermostat opens around 80-90 degrees C, the radiator cap raises boiling point by about 0.7 bar to roughly 120 degrees C, and pressure must be released cold before opening.

Coolant mix is typically 50/50 water and ethylene glycol; check specific gravity or use a refractometer, and never open a hot pressurised system.

Overheating causes: low coolant, air lock, stuck thermostat, failed water pump seal, blocked radiator, lean mixture, retarded timing or a dragging brake.

Clutch operation: the lever releases pressure on the friction plates; free play at the lever is typically 10-20 mm and must exist so the clutch fully engages and the release bearing is unloaded.

Wet multiplate clutch measurement: check friction plate thickness, steel plate flatness with a straight edge, and spring free length against spec; warped steels cause drag and poor disengagement.

Manual gearbox ratios: overall ratio = primary ratio x gear ratio x final drive ratio; a lower numeric ratio gives higher top speed, a higher numeric ratio gives better acceleration.

Constant mesh gearbox: gears are always in mesh, and dogs or shift forks select the pair; worn dogs cause jumping out of gear under load.

Chain drive: slack measured at the midpoint of the lower run, typically 20-30 mm; too tight damages output shaft bearings, too loose risks derailment.

Chain wear check: measure over a set number of links against the service limit, or pull the chain off the rear sprocket; if it lifts more than about half a tooth, replace chain and sprockets as a set.

Belt drive: tension is set by deflection under a specified force, and belts must not be twisted or kinked; misalignment causes edge wear and noise.

Shaft drive: final drive oil is separate from engine oil on most designs, and the pinion and crown wheel backlash and preload must be set with shims to spec.

Two-stroke and small-engine automatic transmissions: a centrifugal clutch engages above a set rpm, and a variator or CVT belt wear changes both engagement speed and top speed.

Hydraulic brakes transmit force by Pascal's law; pressure is equal throughout the system, so a larger caliper piston area gives greater clamping force for the same lever force.

Brake fluid is hygroscopic; DOT 3 and DOT 4 absorb moisture and need replacement typically every 1-2 years, and DOT 5 silicone fluid must never be mixed with glycol fluid.

Brake bleeding removes air, which is compressible; a spongy lever means air or a bulging hose, and fluid level must be monitored so the reservoir never runs dry.

Disc runout limit is typically 0.1-0.3 mm depending on model; below minimum thickness the disc must be replaced, not machined beyond the marked limit.

Tyre markings: 120/70 R17 means 120 mm section width, 70 percent aspect ratio, radial construction, 17 inch rim; the load index and speed symbol must meet or exceed the bike's requirement.

Tyre pressures are set cold; under-inflation causes overheating and poor handling, and tread depth below 1 mm (or the legal local minimum) requires replacement.

Wheel balance and spoke tension: check radial and lateral runout with a dial gauge, true spokes by alternating tightening, and balance with the correct weights on a static balancer.

Suspension setup: sag is measured with the rider aboard, typically 25-35 mm front and 25-40 mm rear; compression and rebound damping adjust separately and should be changed one click at a time.

Steering head bearings: check for free play with the front wheel off the ground and the fork extended, and for notchy feel through the arc; torque the adjuster nut to spec and re-check.

Ohm's law: V = I x R; power P = V x I = I squared x R = V squared / R. A 12 V, 60 W headlamp draws 5 A and has about 2.4 ohms resistance.

Battery: 12 V nominal, six lead-acid cells at about 2.1 V each; open-circuit voltage near 12.6-12.8 V is fully charged, 12.0 V is about 50 percent, and below 11.8 V is discharged.

Charging system check: at 2,000-3,000 rpm the battery should read about 13.5-14.5 V; low voltage suggests a failed regulator/rectifier, stator or wiring, high voltage suggests a failed regulator.

Stator test: measure resistance between phases (typically 0.2-1.0 ohm) and insulation to ground (should be open); also check AC output voltage at cranking or running speed.

Starting circuit: check battery, then the starter relay, then the starter motor draw; a clicking relay with a good battery suggests a relay or earth fault, not a flat battery.

Earth (ground) faults cause dim lights, slow cranking and erratic instruments; measure voltage drop from battery negative to the component body, which should be under about 0.2 V.

Fuse rule: a blown fuse means a fault, not a bigger fuse; replace with the same rating and find the short before restoring power.

LED and electronic loads are polarity sensitive; CAN-bus and ECU-controlled circuits must not be probed with a high-current test lamp, only a high-impedance meter.

Ignition coil test: primary resistance typically 0.1-1 ohm and secondary 5-15 k ohm; open or shorted windings cause a weak or no spark.

Frame types: tubular cradle, trellis, twin-spar aluminium and pressed steel backbone; cracks and bends must be repaired or the frame replaced, never straightened by heat on a heat-treated alloy frame without manufacturer approval.

Steering geometry: rake is the angle of the steering axis from vertical, trail is the horizontal distance from the steering axis to the contact patch; more trail gives more straight-line stability, less trail gives quicker steering.

Wheelbase, rake and trail changes from a bent frame or mismatched forks cause weaving and instability; measure frame alignment with a straight edge and datum points.

Assembly discipline: use new gaskets, seals, circlips and locking fasteners, apply the specified threadlocker, and follow the torque table with the correct sequence.

Fastener grades: 8.8, 10.9 and 12.9 indicate tensile strength; never substitute a lower grade for a critical fastener such as brake caliper or suspension bolts.

Bodywork repair: plastic panels are repaired with the matching plastic welding rod or two-part adhesive, and painted surfaces need primer compatible with the substrate.

Cable and hose routing matters: cables must not kink or rub, brake hoses must not be stretched at full lock, and wiring must clear hot and moving parts.

Pre-delivery inspection checklist: fluid levels, tyre pressures, brake function, lighting, steering and suspension free play, fastener torque, and a road test before handover.

Fairing and screen fitting: use the correct well nuts and rubber grommets, do not overtighten plastic fasteners, and check that the steering lock and lock stops are not obstructed.

Diagnostic order: verify the complaint, check the obvious and free items first (fuel, battery, fuses, connections), then test in a logical sequence from symptom to cause, not by guesswork.

Symptom-to-cause rule: an engine that cranks but does not fire needs fuel, spark and compression; test each with a gauge, a spark tester and a compression gauge before replacing parts.

Compression test: hold the throttle open, crank a warm engine, and compare cylinders; a reading below spec or a large variance between cylinders indicates ring, valve or head gasket faults.

Cylinder leak-down test locates where compression is lost: air from the exhaust is an exhaust valve, from the intake an intake valve, from the crankcase breather rings, and bubbles in the radiator a head gasket.

Electrical diagnosis sequence: measure battery voltage first, then voltage drop across cables and earths, then component resistance; never condemn a component before checking supply and ground.

Service schedule logic: engine oil and filter at the short interval, valve clearance and air filter at the medium interval, brake fluid, coolant and fork oil at the long interval, all per the manual.

Quality control: record measurements before and after repair, use calibrated instruments, torque to spec with a calibrated wrench, and confirm the repair with a road test and a re-check for leaks and noise.

Road test protocol: check brakes at low speed first, listen for unusual noise, confirm instruments and lights, and check for leaks and loose fasteners after the test.

Customer handover: explain the fault found, the work done, the parts replaced and the next service interval, and keep a written record for warranty and traceability.