What is the significance of a dial indicator?

• A. To measure surface roughness of large features
• B. To measure temperature changes during operation
• C. To measure small distances or variations in surface flatness
• D. To gauge air pressure in a system

Answer: (C)

Dial indicators are precision measuring tools used to detect tiny vertical displacements and variations in a surface or between components. They work by applying a small contact plunger to the surface, and as the surface or part moves, the plunger moves the dial needle in fine, readable increments. This allows you to measure very small distances and assess whether a surface is truly flat or if parts are out of alignment or show runout. In bus maintenance, you’d use a dial indicator to check things like the flatness of a brake rotor mounting face, the parallelism of wheel-hub surfaces, or the concentricity between a shaft and its mating part. It’s not used for measuring surface roughness (that needs a profilometer), temperature changes, or air pressure, which require different tools.