How to Measure the Pitch & Angle of Bolt Threads

Written by c.l. rease
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How to Measure the Pitch & Angle of Bolt Threads
Matching thread pitch and angle allows fasteners to thread together. (Hemera Technologies/ Images)

Knowing the pitch and angle of bolt threads ensures fasteners will thread together and the completed connection will have the correct amount of strength. The pitch of the threads determines how many threads the bolt contains for each inch of its threaded length. This variation means two bolts having the same diameter can have a different thread pitch. Attempting to use a bolt with one thread pitch with a nut having a different thread pitch will damage the threads of both fasteners. A specific tool can read the pitch and angle of bolt threads, eliminating the chances of mismatching threaded fasteners.

Skill level:

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Things you need

  • Wire brush
  • Thread pitch gauge
  • Seat gauge

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  1. 1

    Run a wire brush across the threads of the bolt to be measured. Ensure the threads are clean to avoid a misreading of the thread pitch and thread angle measurements.

  2. 2

    Select a leg of the thread pitch gauge that closely matches the diameter of the bolt. Set the jagged end of the gauge leg against the cleaned area of the bolt threads. Set another leg of the gauge against the threads if the jagged end of the first gauge leg does not sit tight to the bolt.

  3. 3

    Read the numbers etched or stamped in the leg that perfectly mates to the bolt threads. The number will show both the diameter of the bolt and the thread pitch. For example, a 1/4-inch bolt with a 20-thread pitch would read 1/4-20.

  4. 4

    Set the flat side of a seat gauge against the forward edge of the bolt threads. Read the dimension of the gauge blade that sits parallel to the bolt threads when the blade sits against the end threads. The degree marked on the seat gauge blade will show the angle of the bolt threads.

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