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Sketch of thread bold and screw minor diameter

Minor Diameter Definition

The minor diameter is the diameter of an imaginary cylinder that touches the smallest diameter of a thread. It depends on the pitch and the thread’s angle.

Minor Diameter Formula

For a 60° thread (ISO metric and Unified Inch standards), it equals to:

\(D_{Minor} = D_{Nom}\,-\,\frac{5}{8}\,\times\,\sqrt{3}\,\times\,P\)
\(\text {For example, for a UNC 1/4-20 screw, the nominal pitch diameter is:}\)
\(0.25-0.625*0.05 = 0.196^”\,(4.98\,mm)\)

Determining the required Minor Diameter in each case is complicated and depends on the threading standard and the selected class. You can find it out by referring to the thread standards or with our Threading Calculator.

External Thread

  • Designation: d1
  • The minor diameter is measured on the root of the thread.
  • The minor diameter is not a critical dimension. It only has a maximum requirement to ensure that the root is reliefed.

Internal Thread

  • Designation: D1
  • The minor diameter is measured on the crest of the thread.
  • The minor diameter is a critical dimension. Together with the pitch diameter, it controls if the bolt would fit on a screw.

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