What precision measuring tool is specifically used for measuring crankpin and main bearing journals for out-of-round wear?

The correct answer is the micrometer caliper, which is specifically designed for precision measurements and is particularly well-suited for measuring small dimensions such as those found in crankpins and main bearing journals. This tool allows for extremely accurate measurements due to its design, which includes a calibrated screw mechanism. When measuring for out-of-round wear, precision is crucial since even minor discrepancies can impact the performance and longevity of the engine components.

Micrometers can measure external dimensions, internal dimensions, and depths with varying types of micrometers available for different applications, making them versatile tools in mechanical measurements. The screw mechanism provides a means to measure very small changes in dimensions, which is essential when assessing the roundness and wear of critical components like crankpins and bearing journals.

While calipers and rulers might provide some degree of measurement, they lack the precision required for such specialized applications. Similarly, a vernier scale, although capable of offering decent precision when used properly, does not match the accuracy of a micrometer caliper for very small measurements where minute variances are significant. This makes the micrometer caliper the ideal choice for measuring the roundness and wear condition of these engine components.