Mostly, these reamers have an even number of cutting edges two of which are facing each other. As a result, the diameter of the reamer can be determined by an external micrometer. However, this is not very useful.
Straight reamers produce a bore hole within a set range of accuracy.
One cannot infer the exact diameter of the bore from the exact diameter of the reamer.
Faults with the process of reaming or dull reamers may cause deviations of several hundredth of a millimeter.
Therefore, a test bore must always be made: on the basis of the checked bore it can be found out whether the respective reamer can be used or not.
Readjustable reamers must be adjusted by a ring gauge or external micrometer before reaming; then, a test bore has to be made.