Measuring Instruments and Workshop Practice
What this domain tests
Expect questions on vernier calipers, micrometers, dial gauges, bore gauges, feeler gauges, plastigauge, torque wrenches, surface plates and bench work. You must know least count, zero error, correct reading procedure and the measurement each instrument is suited to.
Core rules
- Vernier caliper least count is usually 0.02 mm: LC = 1 main-scale division − 1 vernier division. Read main scale plus the coinciding vernier division × LC.
- Outside micrometer least count is usually 0.01 mm. Read the sleeve, then the thimble. Clean the anvil faces and use the ratchet or friction thimble.
- Dial gauge and dial bore gauge resolution is typically 0.01 mm; set the bore gauge to a micrometer or setting ring before measuring a cylinder.
- Feeler gauges measure valve clearance and gap; use the blade that gives a light drag, not a forced fit.
- Plastigauge measures bearing oil clearance by crushing a plastic strip and comparing its width with the scale.
- Keep instruments at room temperature, avoid magnetic chucks near calipers and check zero before use.
Worked example
A vernier caliper has a 1 mm main scale and 50 vernier divisions, so LC = 1/50 = 0.02 mm. The main scale reads 12 mm and the 7th vernier line coincides: 12 + (7 × 0.02) = 12.14 mm. For a micrometer, sleeve 5.5 mm and thimble 23 divisions: 5.5 + (23 × 0.01) = 5.73 mm.
Traps that cost marks
Reading the wrong vernier line; parallax error; over-tightening a micrometer; measuring a hot engine part and comparing with a cold specification; forgetting zero error; using a caliper where a micrometer is needed.
Key points
- Vernier LC commonly 0.02 mm.
- Micrometer LC commonly 0.01 mm.
- Zero-check every instrument before measuring.
- Bore gauges are set against a known master.
- Feeler gauge needs light drag, not force.