Another way to measure a hole like the exit of an internal taper is to measure how far a precision tooling ball seats into the hole, then do a bit of mathematics to obtain the diameter of the hole.
The accuracy of this method depends heavily on the condition of the hole edge. There can be no chamfer or burs and the ball can not be pressed into the hole.
The associated math looks like this...
R = radius of tooling ball
H = distance ball descends into hole
D = diameter of hole
Z = R - H
A = 2 * ACS (Z/ R)
D = 2 * R * SIN (0.5*A)
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