Tools and Measuring Instruments
Hand tools and their correct use
Questions test whether you select the right tool and read it correctly. Spanners: open-ended for accessibility, ring for high torque and reduced slip, combination for general work, socket and torque wrench for controlled tightening. Never extend a spanner with a pipe to gain leverage, and never use a hammer on a hardened tool. Screwdrivers must match slot size to avoid cam-out. Punches, chisels and files have specific uses; files cut on the forward stroke only.
Measuring instruments
Micrometers read: main scale plus thimble scale multiplied by least count. A metric micrometer with 0.5 mm barrel graduations and a 50-division thimble has a least count of 0.01 mm. Vernier callipers: read main scale before the zero of the vernier, then add the vernier division coinciding with a main-scale line times the least count (commonly 0.02 mm). Dial gauges measure runout and backlash; feeler gauges measure clearance; telescopic gauges transfer bore size to a micrometer; plastigauge checks bearing clearance; a bore gauge reads cylinder wear directly.
Traps: reading the vernier from the wrong zero; forgetting to add the 0.5 mm barrel mark; using a micrometer without zeroing; measuring a hot engine; and confusing diameter with circumference.
Worked example: a micrometer shows 12.5 mm on the barrel and thimble division 28. Reading = 12.50 + (28 x 0.01) = 12.78 mm. Rule of thumb: always take three readings at 90 degrees apart and average them to detect ovality.