The diameter of an imaginary cylinder touching the roots of an external thread or the crests of an internal thread. For a tapped hole it is set mostly by the tap drill, so it governs percent thread and tapping torque. The minor diameter of a bolt, together with its pitch diameter, sets the tensile stress area and therefore its strength.
Minor diameter on the tapped holes is big, somebody grabbed a number five drill instead of a number seven for the quarter-twenty.
Also heard as
- root diameter
Related terms
Major diameter
The largest diameter of a screw thread, over the crests of an external thread or the roots of an internal one.
Pitch diameter
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.
Percent thread
How deep an internal thread is compared with full theoretical depth, set mainly by the tap drill size.
Tensile strength
The maximum pulling load a fastener can take before breaking, stated as stress over its tensile stress area.