Thread Minor Diameter

Thread minor diameter is the smallest diameter of a threaded profile. On external threads (like bolts and screws), it’s measured at the root of the threads, meaning the “valley-to-valley” diameter. On internal threads (like nuts or tapped holes), minor diameter is measured at the crest of the internal thread, which is the smallest diameter inside the hole. Because it reflects the thinnest section of the threaded form, minor diameter is a key dimension for understanding how much material remains in the fastener or how much clearance exists in a tapped hole.

Minor diameter matters for both strength and proper thread engagement. For external threads, a smaller minor diameter can reduce the fastener’s tensile stress area and weaken it, while an oversized root can interfere with assembly or cause poor fit. For internal threads, the minor diameter is closely tied to tap drill sizing and thread percentage; too small and the tap may overload or the threads may be incomplete, too large and thread engagement and holding strength can suffer. Along with major diameter, pitch, and thread class/tolerance, the minor diameter helps ensure fasteners assemble smoothly, clamp reliably, and meet the requirements of the applicable thread standard.