The minor diameter of a thread is the smallest diameter of a screw or bolt’s thread profile, measured across the roots of external threads or crests of internal threads. It represents the diameter of the imaginary cylinder that just touches the innermost points of the thread profile.
In external threads, such as those on a bolt, the minor diameter is the diameter at the bottom of the thread valleys, while in internal threads, such as those inside a nut, it is the diameter at the top of the thread crests. The minor diameter is crucial for ensuring proper fit and engagement between threaded components, as it affects the clearance and the strength of the threaded connection.