BTEB Automotive Mechanics cheat sheet

Fire classes: A = solid combustibles (water, foam), B = flammable liquids (foam, CO2, dry powder), C = flammable gases (dry powder, shut off supply first), D = combustible metals (dry powder only; never water), E = electrical (CO2 or dry powder; never water).

A workshop fire extinguisher is checked monthly for seal, gauge pressure and hose condition; hydrostatic test interval is typically 5 years for stored-pressure dry powder units.

Personal protective equipment order of use: eye protection before grinding or blowing with compressed air, gloves for solvents and hot metal, ear protection above 85 dB(A) continuous exposure, respirator for paint mist and brake dust.

Compressed-air safety limit: never use air above about 2 bar (30 psi) for cleaning the body; brake and clutch dust must be removed with a wet method or vacuum extractor, not blown dry.

Lifting rule: jack only under manufacturer-specified lift points, always place axle stands before working under a vehicle, and never rely on a hydraulic jack alone.

Electrical safety: 230 V shock can be fatal above about 30 mA through the body; use RCD/GFCI protection, lock out and tag out before working on mains-powered equipment.

Fuel and solvent storage: flammable liquids in approved closed containers, away from ignition sources, with no more than the permitted quantity in the work area and suitable ventilation.

Vehicle exhaust contains carbon monoxide, which is odourless and binds haemoglobin; never run an engine in a closed workshop without extraction.

Housekeeping rule: spilled oil is the most common cause of workshop slips; clean immediately, use absorbent, and keep walkways and exits clear at all times.

Vernier calliper least count = main scale division ÷ number of vernier divisions; a 1 mm main scale with 50 vernier divisions gives 0.02 mm.

Micrometer least count = pitch of screw ÷ number of thimble divisions; 0.5 mm pitch with 50 divisions gives 0.01 mm.

Micrometer reading = barrel (sleeve) reading + thimble reading; add the 0.5 mm line if the thimble has passed it. Always check zero error before use.

Dial gauge (dial indicator) is used for run-out, end play and backlash; a typical resolution is 0.01 mm with 1 mm total travel.

Cylinder bore is measured with a bore gauge or inside micrometer at three heights and two directions (thrust and axial) to detect taper and ovality.

Torque wrench: torque = force × perpendicular distance; recalibrate after about 5000 cycles or annually; never use it to loosen fasteners.

Feeler gauge measures valve clearance and gap settings; the correct clearance is the largest blade that enters with slight drag.

Telescopic gauge transfers a bore size to an outside micrometer; rock it slightly to find the true diameter.

Common hand-tool rules: use the correct size spanner or socket to avoid rounding, pull rather than push a file, and never extend a spanner with a pipe.

Four-stroke cycle order: intake, compression, power (expansion), exhaust; the crankshaft makes two revolutions per cycle in a four-stroke engine.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; petrol engines typically 8:1 to 12:1, diesel 16:1 to 23:1.

Valve clearance is set with the engine cold for most engines; clearance compensates for thermal expansion of valve train components.

Firing order and valve timing determine when each cylinder's valves open; valve overlap occurs when intake and exhaust are both open near top dead centre.

Engine overhaul sequence: remove, clean, measure, machine or replace, assemble with correct torque and lubricant, then run in and re-torque where specified.

Cylinder wear limits are given by the manufacturer; taper and out-of-round beyond about 0.05 mm usually requires reboring or replacement.

Piston ring end gap is measured in the bore with the ring square; too small a gap causes ring breakage when hot.

Head bolt tightening is usually in a spiral from the centre outwards, often in stages; torque-to-yield bolts must be replaced, not reused.

Clutch transmits torque by friction between flywheel, friction disc and pressure plate; free play must allow the release bearing to clear the pressure plate.

Gear ratio = driven gear teeth ÷ driving gear teeth; overall ratio = product of all stage ratios.

Manual gearbox uses synchromesh cones to equalise shaft speeds before dog engagement, preventing gear clash.

Differential allows the outer wheel to turn faster during a turn while still dividing torque; final drive ratio sets overall road speed per engine rpm.

Road speed (km/h) = (engine rpm × 60 × tyre circumference in km) ÷ (overall gear ratio × 1000).

Automatic transmission uses a torque converter with fluid coupling and a stator; stall speed is the engine speed reached with the output held stationary.

Universal joints and constant-velocity joints transmit drive through changing angles; a worn UJ shows as vibration that changes with load or speed.

Limited-slip differentials use clutch packs or viscous coupling to limit wheel spin; special lubricant or friction modifier is required.

Ohm's law: V = I × R; power P = V × I = I²R = V² ÷ R.

Battery capacity is rated in ampere-hours (Ah) at a 20-hour rate; cold cranking amps (CCA) indicate starting ability at low temperature.

A 12 V lead-acid battery has six cells at about 2.1 V each; a fully charged open-circuit voltage is about 12.6 V, 12.2 V is about half charged.

Alternator output must exceed total vehicle load; a charging voltage of about 13.8 to 14.4 V at the battery indicates normal regulation.

Starter motor draws high current (often 150 to 300 A); excessive voltage drop across cables or earth straps causes slow cranking.

Series circuit: same current through all components, voltages add. Parallel circuit: same voltage across all branches, currents add.

LED and electronic modules are polarity sensitive; test with a digital multimeter rather than a test lamp to avoid damage.

Sensor signals: most modern sensors output 0 to 5 V; a reference voltage of 5 V and a return signal allow the ECU to compute load, temperature or position.

Hydraulic pressure in a brake system is the same throughout the fluid; force at each wheel = pressure × piston area, so larger pistons give more braking force.

Brake fluid is hygroscopic; absorbed moisture lowers boiling point and causes fade, so fluid is usually replaced every 1 to 2 years.

Disc run-out and minimum thickness are checked with a micrometer and dial gauge; below minimum thickness the disc must be replaced, not machined.

Wheel alignment angles: camber (in/out tilt), caster (steering axis tilt), toe (front-to-back difference); toe-in reduces straight-line wander.

Suspension components: springs carry load, shock absorbers control oscillation; a worn shock shows as bouncing, nose dive and uneven tyre wear.

Tyre wear patterns: wear on both edges = under-inflation; centre wear = over-inflation; one-sided wear = camber or toe fault.

Power steering hydraulic systems use a pump and control valve; low fluid or a slipping belt causes heavy steering at low engine speed.

ABS prevents wheel lock by modulating brake pressure; it does not shorten stopping distance on loose gravel and does not replace safe following distance.

Stoichiometric air-fuel ratio is about 14.7:1 by mass for petrol; rich mixture has excess fuel, lean mixture has excess air.

Diesel compression ignition requires high compression (16:1 to 23:1); the injector sprays fuel near the end of the compression stroke.

Fuel pump pressure and volume are tested against manufacturer specification; a clogged filter causes loss of power at high load.

Engine oil viscosity is graded by SAE; a 10W-40 flows like a 10 grade when cold (W = winter) and a 40 grade at operating temperature.

Oil pressure warning at idle with normal pressure at speed often indicates worn bearings or a faulty relief valve, not just low oil level.

Cooling system: thermostat controls flow, radiator dissipates heat, water pump circulates coolant; a stuck-open thermostat causes slow warm-up and poor heater output.

Coolant is a mixture of water and antifreeze (usually ethylene glycol); a 50:50 mix gives protection to about -37 °C and raises boiling point.

Overheating causes: low coolant, failed thermostat, blocked radiator, faulty fan, slipping belt, failed head gasket (bubbles in coolant).

Service intervals are set by distance or time, whichever comes first; severe-duty use shortens oil and filter change intervals.

Daily or pre-start checks: engine oil level, coolant level, brake fluid, tyre pressure and tread, lights, wipers, and warning lamps.

Tyre tread minimum is 1.6 mm in many jurisdictions; measure in the main grooves at several points around the tyre.

Diagnostic method: confirm the complaint, gather symptoms, form a hypothesis, test, repair, then verify the repair under the original conditions.

Battery test sequence: visual inspection, open-circuit voltage, load or conductance test, then charging system test.

Brake inspection: pad thickness, disc condition, fluid level and leaks, hose condition, and parking brake adjustment.

Noise diagnosis: engine speed related noise suggests engine or accessory; road speed related noise suggests tyres, wheel bearings or final drive.

Fluid colours and smells help diagnosis: milky oil suggests coolant contamination; burnt transmission fluid suggests overheating or clutch wear.

Area of a circle = πr²; circumference = 2πr; volume of a cylinder = πr²h.

Pythagoras: in a right triangle, c² = a² + b²; useful for checking squareness and diagonal lengths.

Percentage change = (new value − old value) ÷ old value × 100; efficiency = useful output ÷ input × 100.

Pressure = force ÷ area; 1 bar ≈ 100 kPa ≈ 14.5 psi; torque = force × radius.

First-angle projection: the object is between the observer and the plane, so the view is placed on the far side; third-angle places the view on the near side.

Drawing line conventions: visible outlines are continuous thick lines, hidden edges are dashed, centre lines are chain lines, dimension lines are thin with arrowheads.

Scale drawing: scale 1:10 means 1 unit on paper represents 10 units in reality; always state units.

Temperature conversion: °F = (°C × 9/5) + 32; °C = (°F − 32) × 5/9.

Hierarchy of hazard control: eliminate, substitute, engineering controls, administrative controls, then personal protective equipment as the last line.

Risk = likelihood × severity; a hazard with high severity and high likelihood requires immediate action.

Machine guarding must prevent access to nip points, belts, pulleys and rotating shafts; guards are never removed while the machine is running.

Lock-out/tag-out: isolate the energy source, lock it, tag it, verify zero energy, then work; only the person who applied the lock removes it.

Manual handling rule: keep the load close, bend the knees, keep the back straight, and do not twist while lifting.

Hot work permit is required for welding, cutting and grinding near flammable material; a fire watch must remain after hot work ends.

Noise above 85 dB(A) requires hearing protection and hearing tests; noise above 105 dB(A) requires double protection.

Safety signs: red = prohibition/fire equipment, yellow = warning, blue = mandatory action, green = safe condition or first aid.

Waste segregation: used oil, coolant, solvents, batteries, tyres and metal scrap are separate streams; used oil is never poured into drains or soil.

Used oil can be recycled by re-refining; one litre of oil can contaminate a very large volume of water, so containment and spill kits are mandatory.

Refrigerant recovery: refrigerant must be recovered, not vented to atmosphere; recovery equipment and certification are required.

Brake and clutch dust may contain asbestos in older vehicles; use wet cleaning or a vacuum with a HEPA filter.

Professional practice: record the customer complaint, work performed, parts used and measurements; obtain authorisation before extra work.

Environmental spill response: stop the source, contain with absorbent, protect drains, clean up, and dispose of contaminated material correctly.

Right-to-know: safety data sheets (SDS) for every chemical in the workshop must be available and understood before use.

Professional ethics: do not overstate repairs, do not fit parts below specification, and return replaced parts to the customer on request.