Can I use a hot-dip galvanized nut on a plain finish bolt?

Hot-dip galvanized nuts cannot be used on a plain finish bolt. Hot-dip galvanizing adds between 2 – 6 mils (.002” – .006”) of zinc to the threads of the bolt and due to this increased thread thickness, hot-dip galvanized nuts are tapped oversize to accommodate and create a workable fit. These oversized internal threads on a galvanized nut will not engage properly with a plain finish bolt and the fit of the nut will be too loose to develop the engagement/strength needed.

ASTM 563 specifies the exact amount of oversizing required per each diameter of nut. Portland Bolt has previously answered the opposite question of using plain or zinc plated nuts on a hot-dip galvanized bolt. The answer is the same: only a hot-dip galvanized nut, that is tapped oversize, will work with a hot-dip galvanized bolt. A plain finish or zinc finish nut will not have the oversized threads that will allow for a correct fit, instead the nut will strip and gall a galvanized bolt to the point of not be able to assemble the nut correctly onto the threads, if at all.

In either situation, it is the nut that must be tapped oversize, or not, to accommodate. The bolt is threaded the same if the bolt is plain finish or hot-dip galvanized finish. Never chase or rethread a bolt to remove the added thickness that galvanizing adds. Chasing a bolt will remove the protective zinc coating and may even remove parts of the thread, damaging the corrosion resistance intended for the threads of a bolt, or even the bolt itself. The ASTM F2329 specification for hot-dip galvanizing specifically prohibits thread chasing.

Plain finish bolts must be used with plain finish nuts and hot-dip galvanized bolts must be used with hot-dip galvanized nuts that are tapped oversize. Being a hot-dip galvanizing company with our own specialized galvanizing line in our manufacturing facility, Portland Bolt has the expertise and the manufacturing capabilities to be a qualified and trusted supplier on your next job.

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    @Ron- A hot dip galvanized nut is tapped oversize, so whereas it will thread onto a zinc plated bolt, it will be quite loose and will likely not perform as well as a properly mated nut.

    I remember my lecturer (1986) advising us to use black bolts & nuts (and painting them for protection when installed), rather than using galvanised. A maintenance issue for sure, but better that than the galvanised nut working loose, which he had seen happen.

    @D.Murphy- We are sure there are some applications where painted fasteners are preferable to galvanized, but in general we have not heard of any recurring issues with galvanized fasteners loosening when installed properly.

    Kyle – I have a F1554 GR55 HDG anchor bolt that has been damaged below the threaded portion. I would like to cut it off, thread it, and use a A563 Grade A HDG heavy hex coupling nut with a F1554 GR55 HDG all thread rod. However, I am concerned that the A563 Grade A HDG coupling nut threads will be oversized for the damaged rod that I will thread since the thread will be similar to a plain finish bolt. Have you conquered this application of anchor bolt repair? If so, what is the trick? Thanks for your help!

    @Reggie – Yes, we have encounter this problem before. What we can do is supply a coupler that is tapped oversized on one end, and standard on the other. That way you can fit the plain threads in the standard end, and the galvanized threads in the oversized end. We’d be happy to quote you on these couplers if you would like.

    Dear Mr.Kyle

    I have some problem.please tell me how to remove Black coating of plain finish bolt for galvanize.

    Thank you,

    @Amila- I would need to know what the ‘black’ coating is, but normally either roto-blasting or an acid bath will remove most coatings.

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