Measuring Instruments and Workshop Practice
Measurement fundamentals
Every measurement has a value, a unit and an uncertainty. Questions test correct instrument choice, reading the scale, and zero/calibration checks.
- Vernier caliper: measures outside, inside and depth. Read main scale plus vernier coincidence; typical resolution 0.02 mm. Check zero with jaws closed. Common error: parallax and jaw wear at the tips.
- Micrometer: outside diameters to 0.01 mm. Read sleeve plus thimble; always use the ratchet or friction stop so you do not over-tighten. Zero-check with the anvils closed; adjust if the error is within the permitted range, otherwise replace.
- Dial gauge / dial indicator and magnetic base: runout, end float, shaft deflection. Mount rigidly; a loose base gives false readings.
- Feeler gauges: valve clearances, spark plug gaps, piston-to-bore clearance checks. Combine blades, never force a thick blade.
- Plastigauge: bearing oil clearance - the crushed strip width is compared to a scale; the journal must be clean and dry.
- Bore gauge / cylinder gauge and telescopic gauges: transfer bore size to a micrometer. Rock the gauge to find the true diameter.
- Torque wrench: click, beam or electronic. Store click types at the lowest setting; calibrate periodically. Apply steady force at the handle, not the head.
Workshop practice
Cleanliness, correct fastener torque sequences (usually spiral from centre outward), thread repair with taps and dies, and the use of soft jaws or a copper mallet to protect finishes. Keep measuring tools separate from cutting tools and wipe them with an oily cloth.
Traps
Mixing up vernier (0.02 mm) and micrometer (0.01 mm) resolution; reading a thimble one division out; using a caliper where a micrometer's precision is needed; measuring a hot engine.
Worked example
A micrometer reads 24.50 mm on the sleeve and 12 divisions on the thimble at 0.01 mm/division: 24.50 + 0.12 = 24.62 mm. Subtract the zero error if the closed anvils read 0.02 mm.