BTEB Automotive Mechanics flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Class B fire

Fire involving flammable liquids such as petrol, diesel, oil or paint. Extinguish with foam, carbon dioxide or dry powder; never use a water jet, which can spread the burning liquid.

RCD protection

Residual current device that trips when current leaks to earth, typically at 30 mA within milliseconds. It protects people from electric shock on workshop power tools and sockets.

Vernier least count

Smallest measurement a vernier calliper can read, found by dividing one main scale division by the number of vernier divisions. A 1 mm scale with 50 divisions reads to 0.02 mm.

Micrometer least count

Screw pitch divided by the number of thimble divisions. A 0.5 mm pitch screw with a 50-division thimble reads to 0.01 mm, so readings are taken to two decimal places.

Torque wrench use

Instrument that applies a set turning force. It is used only to tighten fasteners, is set to the specified value, and is recalibrated periodically to keep accuracy.

Compression ratio

Swept volume plus clearance volume, divided by clearance volume. Petrol engines run about 8:1 to 12:1, while diesel engines use 16:1 to 23:1 for compression ignition.

Valve overlap

Period near top dead centre when both intake and exhaust valves are open. It helps scavenge exhaust gas and fill the cylinder, and is set by camshaft timing.

Torque-to-yield bolt

Head or main bolt tightened past its elastic limit, usually by angle after a low initial torque. It must be replaced rather than reused because it has permanently stretched.

Gear ratio

Number of teeth on the driven gear divided by the teeth on the driving gear. A ratio above one reduces speed and multiplies torque at the output shaft.

Synchromesh

Cone clutch inside a manual gearbox that frictionally equalises the speed of the gear and shaft before the dog teeth engage, preventing gear clash during a shift.

Differential function

Gear set that lets the driven wheels turn at different speeds in a turn while transmitting torque to both, and also provides the final drive reduction ratio.

Torque converter stall speed

Engine speed reached when the accelerator is fully applied with the output shaft held stationary. It indicates converter and engine matching and is used in transmission diagnosis.

Ohm's law

Current through a conductor equals voltage divided by resistance, written I = V/R. It is the basis for calculating circuit current, voltage drop and component resistance.

Battery open-circuit voltage

Voltage measured at rest with no load. A healthy fully charged 12 V lead-acid battery reads about 12.6 V; 12.2 V indicates roughly half charge.

Series and parallel circuits

In series, the same current flows through all parts and voltages add. In parallel, the same voltage applies to each branch and currents add.

Alternator charging voltage

Regulated output measured at the battery with the engine running, normally about 13.8 to 14.4 V. Higher or lower readings indicate a faulty regulator, wiring or battery.

Hydraulic brake pressure

Pressure applied to brake fluid is transmitted equally throughout the system. Force at each wheel equals pressure multiplied by the wheel cylinder or calliper piston area.

Caster angle

Tilt of the steering axis from vertical when viewed from the side. Positive caster improves straight-line stability and steering self-centring after a turn.

Tyre wear diagnosis

Wear on both edges indicates under-inflation; centre wear indicates over-inflation; one-sided or feathered wear indicates incorrect camber or toe alignment.

Stoichiometric ratio

Chemically ideal air-to-fuel mass ratio for complete combustion, about 14.7:1 for petrol. Richer or leaner mixtures change power, emissions and fuel consumption.

Oil viscosity grade

SAE rating showing resistance to flow. In a 10W-40 oil, the 10W grade describes cold flow and the 40 grade describes thickness at operating temperature.

Thermostat function

Valve that stays closed while the engine is cold to speed warm-up, then opens at a set temperature to send coolant through the radiator for cooling.

Coolant antifreeze mixture

Water plus ethylene glycol, commonly 50:50. It lowers the freezing point to about -37 degrees Celsius, raises the boiling point and inhibits corrosion in the cooling system.

Tyre tread depth limit

Minimum legal tread depth, commonly 1.6 mm in the main grooves. It is checked at several points around the tyre and replaced before the limit is reached.

Systematic diagnosis

Fault-finding method: confirm the complaint, gather symptoms, form a likely cause, test it, repair, then verify the repair under the original operating conditions.

Battery load test

Test that applies a heavy current load briefly and observes voltage fall. A battery that drops below the specified value under load is weak or worn out.

Cylinder volume formula

Volume of a cylinder equals pi times radius squared times height. It is used for engine swept volume, tank capacity and hydraulic cylinder displacement calculations.

First-angle projection

Drawing convention where the object sits between the observer and the projection plane, so the view appears on the far side. Common in European and Asian drawing standards.

Risk assessment formula

Risk is estimated as likelihood of harm multiplied by severity of harm. High likelihood with high severity demands immediate control before work continues.

Lock-out tag-out

Procedure that isolates an energy source, applies a personal lock and tag, and verifies zero energy before maintenance. Only the person who applied the lock removes it.

Used oil disposal

Drained engine oil is stored in a closed labelled container and sent for recycling or re-refining. It must never be poured into drains, soil or general waste.

Safety data sheet

Document supplied with a chemical that lists hazards, safe handling, protective equipment, first aid and spill response. It must be available in the workshop.