The side clearance of piston rings are measured with a

• A. Micrometer caliper gauge
• B. Thickness gauge
• C. Dial gauge
• D. Depth gauge

Answer: (B)

Side clearance is the small lateral gap between the side of the piston ring and the groove wall inside the piston. This gap must be measured to ensure proper sealing and oil control, and it’s a true gap between two surfaces rather than the thickness of a single part. A thickness gauge is designed for evaluating such small gaps because you can insert progressively thinner blades between the ring side and groove wall until you find the blade that just fits with slight drag. The blade thickness provides the clearance value directly, which is exactly what you need to compare with specifications. Tools like a micrometer measure the thickness of a single piece, a dial gauge reads displacement or runout, and a depth gauge measures how deep something is—none of these directly quantify the narrow side-gap between two surfaces.