Definitions for "MINOR DIAMETER"
This is the diameter of an imaginary cylinder which just touches the roots of an external thread, or the crests of an internal thread.
On a straight thread the minor diameter is that of the minor cylinder. On a taper thread the minor diameter at a given position on the thread axis is that of the minor cone at that position. (See also Minor Cylinder and Minor Cone.)
Diameter of the screw measured at the bottom of the ball track.