How to Measure Hex Bolts

Written by k.k. lowell
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Hex bolts are the most commonly used bolt type. The hex-shaped head of these bolts is seen in all types of mechanical assemblies, and yet some confusion remains as to the proper way to measure a bolt. The five factors that must be known to determine the proper replacement for a hex bolt are length, shank length, diameter, threads per inch and grade.

Skill level:

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Things you need

  • Measuring scale
  • Thread gauge (optional)

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  1. 1

    Place the zero end of the measuring scale against the bottom of the hex head, and read the scale to determine the total length of the bolt. Bolt length increases in 1/4-inch increments. Remember to use a metric scale when measuring metric hex bolts.

  2. 2

    Measure the unthreaded portion of the bolt. This is the shank length and must be the same on the replacement bolt to insure the proper tensile rating is maintained.

  3. 3

    Measure across the threaded end of the bolt to determine bolt diameter. The smallest bolt is 1/4 inch in diameter. Hex bolts diameters increase in 1/16 inch increments.

  4. 4

    Using a thread gauge, determine the threads per inch. If a thread gauge is not available, count the threads in the first inch of the hex bolt. You must use a thread gauge when measuring metric bolts.

  5. 5

    Note any markings on the bolt head. These are the symbols denoting the hardness grade of the bolt. Replace the bolt with one that has the same markings.

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