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Sketch of thread bold and screw major diameter

Major Diameter Definition

The major diameter of a thread is the diameter of an imaginary cylinder that touches the largest diameter of a thread. The nominal value of the major diameter equals the nominal thread size. For example, in a UNC 1/4-20 thread, the nominal major diameter is 1/4″ (6.35 mm)

External Thread

  • Designation: d
  • The major diameter is measured on the crest of the thread.
  • The major diameter is a critical dimension to control. Together with the pitch diameter, it forms the addendum of the thread and controls if the screw would fit on a bolt. The permissible range for the major diameter is derived from its class.

Determining the required major 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.

Internal Thread

  • Designation: D
  • The major diameter is measured on the root of the thread.
  • The major diameter is not a critical dimension. It only has a requirement to be equal to or larger than the nominal thread diameter.
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