ITI Mechanic Auto Body Repair (AITT Theory) key terms

A standard document supplied with every hazardous product that lists its ingredients, hazards, safe handling, storage, first-aid and spill procedures. Read the relevant sheet before opening paint, thinner, acid or adhesive containers.

An extraction system that captures dust, mist and solvent vapour close to where they are produced and carries them away from the worker's breathing zone. Spray booths, sanding stations and welding benches depend on it.

Breathing protection worn on the face, such as a half-mask with the correct chemical cartridges or a supplied-air hood. It must be fit-checked, cleaned after use and matched to the contaminant identified on the safety data sheet.

Hand shears with compound-lever handles for cutting sheet steel or aluminium. Colour-coded versions cut left, right or straight; the blade offset determines which curves the tool can follow without distortion.

A wrench that releases or signals at a preset tightening force, used where a fastener must be neither loose nor over-stressed. It should be stored at its lowest setting and recalibrated periodically.

A sliding scale instrument for measuring outside, inside and depth dimensions to a fraction of a millimetre. The jaws must be clean and closed squarely before use, and the reading is taken where the vernier lines align.

A set of thin, accurately ground metal leaves of known thickness, used to check small gaps such as spark plug gaps, valve clearances or panel shut lines. The correct leaf slides with slight drag.

A long adjustable bar with pointed ends used to compare diagonal distances between reference holes on a vehicle body. Equal diagonals indicate a square structure; differences reveal a pushed or twisted chassis.

A number that expresses sheet thickness, where a higher number means thinner material. Because gauge numbers differ between ferrous and non-ferrous standards, the actual thickness should be measured before choosing welding or repair methods.

The tendency of formed metal to spring partly back towards its original shape after the bending force is removed. It is caused by elastic recovery and must be allowed for when forming panels or flanges.

The increase in strength and brittleness that occurs when steel is bent, hammered or stretched repeatedly. Work-hardened metal resists further shaping and may crack, so it is often annealed or replaced.

Arc welding in which a continuous wire electrode is fed through a gun and shielded by gas. It is the commonest method for joining steel body panels because it is fast and produces little spatter.

A weld made by filling a hole drilled or punched in the top panel so that the weld fuses into the panel beneath. It reproduces a factory spot weld where access for a spot welder is impossible.

The band of base metal beside a weld that has been heated but not melted. Its microstructure and strength change, so it must be cleaned, protected against corrosion and never left as a load path on structural parts.

A bit or hole saw that removes the nugget of a resistance spot weld so that the panels can be separated. It cuts only the top sheet, leaving the lower panel undamaged for re-welding.

A vehicle construction in which the floor, pillars and rails form a single welded shell that carries both the body and the suspension loads. There is no separate frame, so any structural damage must be pulled and measured accurately.

A specified reference point on the vehicle underbody or structure from which all dimensions are measured. Datum points are listed by the manufacturer and used with a measuring system to confirm the body is correctly aligned.

Replacing a damaged portion of a panel or rail by cutting along a manufacturer-approved line and joining a new section. The joint position, weld type and corrosion protection are specified because they affect crash performance.

A matched pair of panel-beating tools: the hammer strikes the outside of the panel while a hand-held dolly supports the underside. Used together, they raise low areas and stretch or shrink metal back to shape.

The art of massaging a shallow dent out from behind the panel with rods and levers, keeping the original paint intact. It works only where the paint is unbroken and the metal is not stretched.

The controlled application of heat, usually with a torch or carbon electrode, followed by rapid cooling to shrink stretched or oil-canned metal. Overheating destroys the steel's properties, so heat is applied in small spots.

A plastic that softens when heated and sets again on cooling, so it can be welded or reshaped repeatedly. Most bumpers, trim and interior panels are thermoplastics, and their identification symbol should be checked before repair.

A repair method in which a hot-air gun and a filler rod of the same plastic melt and fuse a crack or tear. Temperature, airflow and rod material must match the parent plastic or the repair will fail.

A sandable coating applied over bare metal or repaired areas to build film thickness and fill minor imperfections. It provides a smooth, uniform surface before the colour coat and improves adhesion.

A two-layer paint system in which the pigmented base coat supplies colour and the transparent clear coat supplies gloss and weather resistance. The clear must be applied within the base coat's specified flash-off window.

A paint defect in which the surface dries with a rippled, citrus-like texture instead of flowing flat. It results from incorrect gun settings, too-thick film, wrong reducer or inadequate atomisation.

A refinishing technique in which new paint is faded into the original finish of an adjacent panel so that no colour difference or edge is visible. It avoids painting the whole panel and is common on metallic colours.

A C-shaped metal fastener squeezed closed with special pliers to attach upholstery covers to seat frames and list wires. It is removed by cutting or prying and is replaced rather than reused.

The fabric-covered panel that forms the vehicle's interior roof surface. It is bonded to a moulded backing and held by clips or trim, and it must be removed to access roof electricals or repair water leaks.

A high-strength moisture-curing adhesive that bonds a windshield or back glass to the body flange. It contributes to structural strength and must be applied to a properly primed, cleaned surface with the correct bead.

Two layers of glass bonded to a plastic interlayer, used for windshields because it holds together when broken and does not shatter into loose pieces. Chips and cracks can often be repaired without replacement.

A rubber or foam seal that fills the gap between a door, glass or lid and the body. It prevents water, dust, wind noise and vibration from entering, and must be correctly seated to avoid leaks.

An instrument for measuring voltage, resistance and current in a circuit. A high-impedance digital meter is essential on modern vehicles so that airbag and electronic modules are not damaged or triggered.

A measurement of the voltage lost across a connection, cable or component while current flows. Comparing the loss with the manufacturer's limit locates high-resistance faults that a simple continuity test would miss.

The small current a vehicle continues to draw from the battery after all systems have gone to sleep. Excessive draw flattens the battery overnight and is found by measuring current across each fuse.

A published time allowance for a specified repair operation, used to price an estimate consistently. The technician is paid for the scheduled time rather than the actual time taken.

An additional estimate submitted when damage found after work has started was not included in the original authorised figure. It requires the customer's or insurer's approval before the extra work proceeds.

A decision that the cost of repairing a vehicle exceeds its economic value, so it is written off or settled instead. The assessor compares repair cost with market value and salvage return.

A signed or recorded approval from the customer or insurer allowing the repairer to carry out stated work at an agreed price. No additional work should begin without renewed authorisation.

The final check before a repaired vehicle is handed over: panel fit, paint finish, function of electricals, cleanliness and a road test. Any defect found should be corrected before the customer collects.