Automotive Materials, Measurement and Workshop Practice
This domain tests materials selection, heat treatment and the measuring tools used in the workshop. Expect ferrous and non-ferrous metals and their applications, plastics and composites, heat-treatment names and purposes, least count and readings of vernier, micrometer and dial gauge, and limits and fits.
Materials
- Cast iron: engine blocks, brake drums; good damping and cheap, but brittle.
- Mild steel: brackets and frames. Alloy steel such as Ni-Cr-Mo: gears, crankshafts, connecting rods.
- Aluminium alloy: pistons, cylinder heads, wheels; light with good heat conduction, but needs a liner or coated bore.
- Copper, brass and bronze: wiring, radiators, bushes and bearings.
- Plastics and composites: bumpers, dashboards, GRP panels. Rubber for tyres, seals, hoses and mounts.
- Heat treatment: annealing softens and relieves stress, normalising refines grain, hardening by quenching makes it hard and brittle, tempering restores toughness, case hardening gives a hard skin with a tough core.
Measurement
- Vernier caliper: least count = 1 main scale division minus 1 vernier scale division; common metric value 0.02 mm.
- Micrometer: 0.01 mm; read the barrel and thimble and use the ratchet for consistent pressure.
- Dial gauge: 0.01 mm, for runout, end play and bore ovality with a bore gauge.
- Feeler gauge: valve clearance and spark plug gap. Plastigauge: bearing clearance.
- Fits: clearance (shaft smaller than hole), interference (shaft larger), transition (either possible).
Worked example
A vernier has 1 mm main divisions and 50 vernier divisions, so least count = 1 minus (49/50) = 0.02 mm. The main scale reads 24 mm and the 23rd vernier line coincides. Reading = 24 + (23 x 0.02) = 24.46 mm. Reasoning: always take the best-coinciding vernier line, multiply by the least count, and apply any zero error with the opposite sign.
Traps that cost marks
Ignoring zero error. Reading the wrong micrometer scale (thimble marks are 0.01 mm, barrel marks 0.5 mm). Using a caliper for precision bearing measurement. Confusing hardening with tempering.