To accurately measure the diameter of a shaft, it should be compared to which of the following?

The most appropriate comparison for accurately measuring the diameter of a shaft is the radial internal clearance (RIC). RIC refers to the space between the shaft and the bearing it interacts with, representing the dimensional tolerance and fit. This measurement is critical in ensuring that the shaft fits properly within a bearing system. A precise fit is essential for minimizing friction, wear, and potential failure of the machinery.

In contrast, comparing the shaft diameter with the outer race or inner race may be misleading due to potential variances or wear in these components. The rolling-element circumference is also not directly relevant to measuring shaft diameter, as it's more about the movement and dimensions of the rolling elements within the bearing rather than the shaft's dimensions themselves. Thus, to ensure functional compatibility and effective maintenance, radial internal clearance serves as a key measure when determining shaft diameter in the context of its role within a system.