Fastener Part Numbering System Explained

When dealing with screws, nuts, and bolts, understanding the cryptic part numbers can simplify ordering and understanding the

The last 2 digits (XX) of the 4 digit fastener part numberprefix (40XX-12345-12) indicates the standard stud size of the bolt or screw. For example, part #4006-01030-12 indicates a #6 stud diameter (.138″) fastener. The 2 digit part number suffix for bolts and screws (-12, -04, -16, etc) indicates the number of 1/16″ length increments of the fastener. For example, part #4004-01003-04 indicates a screw that is 4/16″ long or 1/4″ long. The middle 5 digits describe the fastener head style or other features. Basic diameters and threads of Standard SAE (Society of American Engineers) numeric system small machine screws are shown below. The diameters of machine screws are indicated with the number of threads per inch.

Here is a list of common sizes in inches:

  • 2-56: 0.0860″ or 43/500″ in diameter; 56 threads per inch
  • 4-40: 0.1120″ or 14/125″ in diameter; 40 threads per inch
  • 5-40: 0.1250″ or 1/8″ in diameter; 40 threads per inch
  • 6-32: 0.1380″ or 69/500″ in diameter; 32 threads per inch
  • 8-32: 0.1640″ or 41/250″ in diameter; 32 threads per inch
  • 10-32: 0.1900″ or 19/100″ in diameter; 32 threads per inch
  • 10-24: 0.1900″ or 19/100″ in diameter; 24 threads per inch
  • 12-24: 0.2160″ or 27/125″ in diameter; 24 threads per inch

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