There are many ways to measure a bore, and many factors that influence the accuracy of hole diameter measurements. For example, we know that operator skill in using rocking-type adjustable bore gages can influence the results, as can variations in part geometry. This may make even technically accurate measurements inaccurate from a part-function perspective, so it is important to consider the effects of gage centralization for smaller bores. Selecting the right tool is often based on the bore’s size and tolerance.
Read more on Modern Machine Shop: https://www.mmsonline.com/articles/centralization-errors-grow-as-bores-get-smaller