How much thread engagement is recommended, i.e. how many threads should extend beyond the nut?

thread-engagementThis is a difficult question to answer. Depending on your application, the answer could be anywhere from 1/2 engagement, up to having two full threads extend beyond the nut. Look at these conflicting answers found in four separate technical publications.

  1. Federal Highway Administration, “Guidelines for the Installation, Inspection, Maintenance and Repair of Structural Supports for Highway Signs, Luminaires and Traffic Signals.” Section 16.3.1.2 Anchor Rods
  2. Industrial Fasteners Institute, Joe Greenslade, Director of Engineering Technology.
    • “How Many Threads Should Be Beyond The Nut?. The answer is: A minimum of two thread pitches should extend beyond the top surface of the nut.”
  3. RCSC (Research Council for Structural Connections) Specification for Structural Joints Using ASTM A325 or A490 Bolts 2.3.2. Geometry
    • “Heavy-hex structural bolt dimensions shall meet the requirements of ANSI/ASME B18.2.6. The bolt length used shall be such that the end of the bolt extends beyond or is at least flush with the outer face of the nut when properly installed.”
    • http://www.boltcouncil.org/files/2009RCSCSpecification.pdf
  4. AISC Design Guide1 Page 10 Section 2.11.3
  5. AISC Steel Construction Manual, 13th Edition, Section 14-10
    • “Adequate thread engagement for anchor rods is identical to the condition described in the RCSC Specification as adequate for steel to steel structural joints using high strength bolts: having the end of the [anchor rod] flush with or outside the face of the nut”

So we are left a little confused…..which is it? The answer is, it depends. All joints are not created equal. Tightening an A325 structural bolt on a bridge may have different requirements than tightening the nut on an anchor bolt for a traffic signal pole. Some screw and bolt specifications allow for one or more incomplete threads on the point end of the bolt to allow for easier mating, while others do not. Structural bolts like A325s or A490s have a limited thread length, so having too many threads stick out may mean that the nut is engaging into the thread run out and may not be able to develop the full strength needed. Conversely, not enough stick out may leave threads exposed in the shear plane, which may not be desirable. The safest answer is to consult with the project engineer on your job, to assure all variables that are specific to your job are taken into consideration.

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9 comments

    There is a NASA standard MSFC-STD-486B (available online free) that contains requirements for minimum thread protrusion. If you look this standard up, it is given in Table XII. Personally, I don’t want some highway sign dangling over my head with nuts only half engaged. Perhaps there’s a paper trail with calculations that say this condition is acceptable but it still makes me uncomfortable.

    This is totally awesome, this also will elevate misinterpretation on which application its being used. It also make us read and understand the program specification instead of assuming what you have done in the past. Thanks for the information.

    1. I always check the thread engagement at the shank since structural bolts have random thread lengths. Some of the bolts barely have any thread projecting out the top in order to work without “bottoming out”.

    2. If building machinery, use lock washers and flat washers on slots with grade 5 bolts, not A325 bolts. This is our practice.

    ASME B31.3, 335.2.3, states:
    Bolts should extend completely
    through their nuts. Any which fail to do so are
    considered acceptably engaged if the lack of complete
    engagement is not more than one thread.

    I would think that this requirement is dependent on the nature of the objects being fastened together. For example: a industrial machine that will vibrate while in operation could need to have threads extending past the nut to minimize the risk of the nut loosening and failing/coming off.

    Just a thought.

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