Which precision tool is used to measure piston ring side clearance?

• A. Thickness gauge or Feeler Gauge
• B. Dial indicator
• C. Micrometer
• D. Vernier caliper

Answer: (A)

Measuring piston ring side clearance requires a tool that can gauge very small gaps with precise thickness. A thickness gauge, or feeler gauge, does exactly that. It provides blades of known thickness that you insert into the gap between the ring’s side and the groove wall. The correct blade is the one that fits with a light drag, giving a direct reading of the clearance in thousandths of an inch (or hundredths of a millimeter). This direct-gap measurement is why the feeler gauge is the best choice. Other tools are less suitable because they measure overall dimensions or movement rather than the tiny internal gap: micrometers and Vernier calipers assess external dimensions, and a dial indicator measures displacement or runout, not the actual clearance inside the groove.