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.”
    • https://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.

Written ,

21 comments

    How many threads on 1 1/4 inch diameter coil rod are necessary to lift a 5 X 10 road plate that is 1 inch thick and weighs a couple thousand pounds. We had a failure and determined that the worker had not screwed in the lifting eye flush but backed it off a half turn after turning it flush. After review we found that this was common among the workforce. The lifting ring has 1 inch of rod sticking out beyond the base plate. Thread is 3.5 coils per inch. The bottom two and a half threads near the end of the rod stripped.
    Thanks,
    Bob Kelley

    This is great information. Rules of thumb, like for inspection, is a prescriptive approach and for new construction this is best. But sometimes the prescriptive rule cannot be met (new light pole with thicker base plate on an existing pier for example) and we need to be able to sharpen the pencil and calculate what we really need for those few exceptions.

    Nut loosening means that its function is lost and is a matter of time for serious failure, especially in a vibrating structure. So more threads extension, offer no further safety. Most probably stud or mounting will break due to the excess vibration.

    It is quite simple the size of the nut is sized to the diameter of the stud to withstand the tensile forces expected. The stud must extend beyond the nut otherwise the nut will not be exerting it’s calculated holding power because the end thread at least will tend to collapse the last threads as they have no support behind them.
    The rule of thumb is 1 to 11/2 thread beyond the nut.

    @Hessin- We can easily manufacture a 1-1/2 x 7″ A325 structural bolt. but the F3125/A325 standard only covers bolts up to 1-1/2″, so a 2″ diameter bolt would need to be manufactured to a different standard like A449 or A354 gr.BC.

    In Australia, Australian Standard AS 4100 for Assembly of Steel Structures. identifies that for bolted connections section 14.3.6.1 Requires that one full thread is visible above the nut when selecting a bolt length and one full thread and the thread run out should be visible after tightening.
    The application is for the erection of structural steel, such as would be used in buildings, bridges etc.
    AS requirement is stated as a minimum, best practice for structural bolts is generally regarded as 2.5-3 threads past the nut after being tightened.

    Don’t like the idea of a half engaged nut on a bolt, but flush with the top of the bolt is a condition I’m dealing with….I should be ok.

    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.

Leave a Reply

Your email address will not be published. Required fields are marked *