Cert III Body Repair cheat sheet

Panel alignment tolerance: gaps between adjacent panels should be uniform within about 1-1.5 mm variation; measure at three points per gap before and after repair.

Steel straightening force rule: heat only when specified by the OEM procedure; heated high-strength steel (HSS) above roughly 600 C loses tensile strength and must be replaced, not repaired.

Bonded and riveted panel removal: apply heat to about 90-120 C to soften structural adhesive, cut the adhesive bead, then drill or mill rivets without elongating holes.

MIG plug weld hole size rule of thumb: hole diameter equals 1.2-1.5 times the thickness of the thinner sheet; weld in short runs to limit heat input and distortion.

Spot weld removal: drill only through the top sheet; use a 8 mm cutter for 0.8 mm sheet and check the remaining lower sheet for thinning before plug welding.

Distortion control: use stitch or backstep welding sequences and allow each weld to cool before the next; hammer-and-dolly work is done on-stretch, not off-stretch, on crowned panels.

Paint film build: total film thickness is measured in micrometres; excessive build (generally above about 300 micrometres) risks solvent trapping, cracking, and poor adhesion.

Filler rule: apply body filler only over bare, abraded steel or approved primer, in layers of about 3-6 mm maximum, and never over rust pits or existing cracked filler.

Rust repair decision rule: surface rust is removed mechanically; pitting deeper than about 1 mm or perforation requires cutting out and welding in a patch panel.

Bare steel preparation: clean to bright metal, apply an epoxy primer within the product's recoat window (commonly within 24 hours or per data sheet) to prevent flash rust.

Panel gap and shut line targets on restored classics: aim for even gaps of about 4-6 mm on doors and guards, verified with a gap gauge before final paint.

Media blasting choice: use plastic or soda media on aluminium and thin panels to avoid warping; abrasive grit is limited to heavy steel components.

Lead-loading (body solder) safety: the substrate must be clean and tinned; working temperature is kept just above the solder's melting range to avoid panel distortion.

Primer sequence: epoxy primer for corrosion protection, then a surfacer or high-build primer sanded to 400-600 grit before sealer, base coat, and clear.

Colour matching on aged finishes: blend within adjacent panels rather than panel-painting where possible; metallic and pearl colours require a blend within the panel.

Trim and fastener restoration: replate or replace fasteners to original specification; thread pitch and grade must match because incorrect fasteners alter clamp load.

Chassis straightening requires OEM frame dimension charts; measure diagonals and datum heights before and after pulling, and never exceed the manufacturer's stated pull limits.

Heat rules on frames: most heavy-truck frames are heat-treated; heating above about 400-600 C or quenching is prohibited unless the OEM repair manual permits it.

Frame crack repair: stop-drill both ends of the crack, groove the full depth, weld with the specified electrode, and reinforce with a fishplate only where the OEM allows.

Rivet replacement: use rivets of the same diameter and grip length as original; holes must be reamed to the next oversize only if the OEM permits, otherwise replace the section.

Fifth wheel and kingpin wear limits: check kingpin diameter against the OEM minimum and fifth wheel jaw wear; out-of-limit parts are replaced, not adjusted.

Wheel and hub fastener torque: use the OEM torque value and a calibrated wrench; torque is applied in stages and rechecked after the specified settling period.

Body mounting: check all isolator and U-bolt fasteners for elongation and correct torque; loose body mounts transfer load into the frame and cause cracking.

Air brake and air line routing after body repair: maintain the OEM clearance and support intervals; lines must not contact moving parts or heat sources.