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Metric Threads

  • Format is M major diameter (mm) X pitch (mm)
  • Example: M12 X 1.75 is a thread having a nominal major diameter of 12 mm and a pitch of 1.75 mm.
  • The letter M is the clue to the metric designation.
  • Similar classification are given for other threads

Unified Threads

  • Dimensions specified in inches
  • Coarse and fine series exist and are termed UNC and UNF respectively.
  • Format is major diameter – threads per inch series
  • Example: 5/8” – 18 UNF is a thread having nominal major diameter of 5/8”, 18 threads per inch, and belongs to the fine thread series.
  • Screw sizes with diameter under ¼” are numbered (i.e., small screw diameters are designated by gauge sizes, and not by fractions of an inch).
  • Unified threads have thread angle of 60.

    Fine Threads vs. Coarse Threads

Reading Data Sheets

  • Static Load Rating (always stay below) – maximum stationary load before total permanent deformation of the fastener.
  • Operating Load Rating (always stay below) - maximum permissible live load before total permanent deformation of the fastener.


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