Understanding Fastener Sizing terms

 
The Key Dimension of a fastener pertains to diameter of screws, bolts, & pins and inner diameter of nuts and washers. Other parameters of bolt are total length, length of thread, head configuration and thread count.  For screws there is overall length, type and head type.

1. Bolts or Fasteners that Require Mating to another fasteners such as a nuts and washers

These fasteners have specific diameters that are denominated in actual inches or fractions which is followed by the number of threads per inch.

When matching a washer with a fastener, the inner diameter of the washer should match the diameter of the fastener. As an example, a 1/2-16 bolt should be matched with a 1/2 washer. In this case the inner diameter of the washer will, in fact, be slightly larger than 1/2. This will allow some room for the washer to fit over the fastener. You must know the inner diamter of the washer when ordering so you can match correctly with the bolt or screw.

Naming of sizes:

Fraction hyphen number: ie 1/2-18 or 1/4-20 or 1-12 OR 2-1/4-4-1/2

  • Fraction is the diameter
  • Number is the threads per inch on the shaft of the bolt

2. Non Mating Fasteners

For fasters that don’t require a mate, – items that drill into or through a surface – are usually identified by industry Numeric Sizes which are preceded by #. Further there is not a number of threads per inch value.  The Standard values run from #0 through #15 with #0 the smallest.

The numeric values indicate standard diameters (American Society of Mechanical Engineers (ASME), American Society of Testing & Materials (ASTM)). The bodies standardized common diameters used in industry. Threads are identified as coarse or fine based on based on ASME Coarse Thread Series (UNC/UNRC) and ASME Fine Thread Series (UNF/UNRF)

Conversion in Inches

  • #0- 0.0600 or 3/50″
  • #1- 0.0730 or 73/1000″
  • #2- 0.0860 or 43/500″
  • #3- 0.0990 or 99/1000″
  • #4- 0.1120 or 14/125″
  • #5- 0.1250 or 1/8″
  • #6- 0.1380 or 69/500″
  • #7- 0.151 or 77/512″
  • #8- 0.1640 or 41/250″
  • #9- 0.1770 or 11/64″
  • #10- 0.1900 or 19/100″
  • #12- 0.2160 or 27/125″
  • #14- 0.2500 or 1/4″
  • #15- 0.3120 or 5/16″
  • #16- 0.3750 or 3/8″ 

3. Machine Screw Industry Standards

Smaller diameter machine screws are identified based on the same above standard with the addition of threads per inch after the hyphen.  Some Examples

  • 2-56: 43/500″ dia- 56 threads per inch
  • 4-40: 14/125″ dia-  40 threads per inch
  • 5-40: 1/8″ in dia- 40 threads per inch
  • 6-32: 69/500″ dia- 32 threads per inch

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