How to determine nut & bolt size

Written by teeter allen morrison
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Many people believe that the wrench that fits a nut or bolt is the actual size of the bolt. This is not true -- bolt heads of various sizes are often found on bolts of a given diameter. To make things more complicated, an even larger selection of nuts require various wrench sizes for the same bolt diameter. Knowing the correct size of a nut or bolt is especially important when those parts become worn and need to be replaced.

Skill level:

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Things you need

  • Vernier gauge
  • Drill bit gauge
  • Steel rule
  • Tape measure

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  1. 1

    Determine the diameter of the bolt shaft to correctly identify the bolt size. The easiest way to do this is to place a vernier caliper over the diameter of the bolt and read the size on the vernier scale. If a verier caliper is not available, go on to the next step.

  2. 2

    Remove the nut from the bolt and insert the threaded end of the bolt into a drill bit gauge. A 3/8-inch bolt will fit snugly into a 3/8-inch drill gauge. If a drill gauge is not available, go on to the next step.

  3. 3

    Place a steel rule or tape measure across the centre of the bolt's threads. Determine the size of the bolt diameter by the measurement on the steel rule.

Tips and warnings

  • Another way to determine the size of a bolt is to learn the wrench sizes that fit the bolt diameter. For example, a bolt that uses a 9/16-inch wrench is a 3/8-inch bolt.

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