What tool is commonly used to determine piston pin out-of-round wear?

The correct choice for determining piston pin out-of-round wear is a micrometer caliper. A micrometer caliper is specifically designed for precisely measuring small distances, including the diameter of cylindrical objects like piston pins. It can measure the pin's diameter at various points to identify any discrepancies caused by wear or out-of-round conditions. This allows for accurate assessment and adjustment or replacement as necessary.

In contrast, other tools such as caliper gauges and digital gauges, while useful for measurements, may not provide the same level of precision required for determining out-of-round wear. Dial indicators are commonly used for measuring variations in surface flatness or roundness but are not specifically designed for the precise internal measurements of piston pins. Thus, the micrometer caliper stands out as the most suitable tool for this specific measurement task.