The inside diameter of small holes is very difficult to measure accurately without hole gages like these from Starrett. The hardened ball measuring surface on each tool has a radius less than the minimum diameter to be measured so accurate two point contact is assured. To measure the hole, select the correct ball tool from the set, measure the hole, then measure the ball with a 1"" outside micrometer. Set includes four ball tools, and holes from .0125"" to .500"" can be measured. Vinyl fold over case included.
Sinclair, Issue:2B, Page:039