Can I use black (plain finish) or zinc plated nuts on galvanized bolts?

Heavy Hex NutNo. Hot-dip galvanized nuts need to be used with hot-dip galvanized bolts since galvanized nuts are tapped oversize to accommodate for the extra 2 – 6 mils (.002 – .006) of zinc that is added to the threads of the bolt during the galvanizing process. A common misconception is that bolts that are going to be hot-dip galvanized are threaded undersize since the galvanizing will increase the size of the threads. This is not correct. Bolts are threaded to the same tolerances, regardless of whether they will be galvanized or remain plain finish. Instead, it is the nuts that are tapped oversize to allow for the zinc on the threads of the galvanized bolt. Zinc plated nuts will also not fit a galvanized bolt since they are not tapped oversize. The table below provides overtapping allowances for both Unified National Coarse (UNC) and 8UN thread series. Since galvanized nuts are tapped oversize, they would be too loose to be used effectively on plain finish or zinc plated bolts.

Nut Overtapping Allowances

Nominal Nut Size, in. and Pitch Diametral Allowance, in. Pitch Diameter
Min Max
0.250 - 20 0.016 0.2335 0.2384
0.312 - 18 0.017 0.2934 0.2987
0.375 - 16 0.017 0.3514 0.3571
0.437 - 14 0.018 0.4091 0.4152
0.500 - 13 0.018 0.4680 0.4745
0.0562 - 12 0.020 0.5284 0.5352
0.625 - 11 0.020 0.5860 0.5932
0.750 - 10 0.020 0.7050 0.7127
0.875 - 9 0.022 0.8248 0.8330
1.000 - 8 0.024 0.9428 0.9516
1.125 - 8 0.024 1.0678 1.0768
1.125 - 7 0.024 1.0562 1.0656
1.250 - 8 0.024 1.1928 1.2020
1.250 - 7 0.024 1.1812 1.1908
1.375 - 8 0.027 1.3208 1.3301
1.375 - 6 0.027 1.2937 1.3041
1.500 - 8 0.027 1.4458 1.4553
1.500 - 6 0.027 1.4187 1.4292
1.750 - 5 0.050 1.6701 1.6817
2.000 - 4.5 0.050 1.9057 1.9181
2.250 - 4.5 0.050 2.1557 2.1683
2.500 - 4 0.050 2.3876 2.4011
2.750 - 4 0.050 2.6376 2.6513
3.000 - 4 0.050 2.8876 2.9015
3.250 - 4 0.050 3.1376 3.1517
3.500 - 4 0.050 3.3876 3.4019
3.750 - 4 0.050 3.6376 3.6521
4.000 - 4 0.050 3.8876 3.9023
Inch Fastener Standards. 7th ed. Cleveland: Industrial Fasteners Institute, 2003. B-173. For Nuts: Hot Dipped Galvanized per Specification A153, C1.C
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    HDG on nuts is really not great as the tapping leaves thread raw steel. We use ArmorGalv® thermal diffusion which covers the whole part UNIFOMLY inside and out and gets 3,000 hours of NSS B-117.

    @Martin- It is true that the two processes, hot dip and thermal diffusion, are different and that the hot dip process does leave the nut threads uncoated. However, the zinc coating, being significantly more anodic than the base steel, will protect the uncoated threads for the life of the fastener.

    For 1″-8 ASTM A193 B7 Cap screw with hot dipped galvanized finish, what size tap should I source to make an internal thread in a steel plate? I know the hot dipped galvanize will add an extra layer of thickness and the nut / internal thread will need to be oversize to fit.

    @Khan- If you are threading a HDG bolt into a threaded hole, you will want to tap the internal threads 0.024″ oversize, the same overtap as the nuts.

    Dear Sir, we also do HDG in my country and the nuts already overtapped larger. But after HDG still have many galvanised inside thread of nut then nut can not go in bolt. Athough we dip and spin… Do you have any idea to advise me to solve this problem ? Thank you very much in advance !

    @Ha- Typically the nut is tapped after galvanizing, so the zinc is removed from the threads of the nut. That should create a nice thread fit.

    I have to weld nuts to a steel tube for threading HDG threaded rod. Does anyone know where I can buy the plain finish nuts with oversize tapped threads? My assembly needs to be hot dipped after the nuts are welded on.

    @Steve- You will need to have these nuts tapped, no one is stocking plain, oversized nuts. That said, hot dip galvanized nuts are tapped after galvanizing, so you could use regular nuts, then tap after they are welded and coated.

    Could be safe in operation bolts and nuts hot-dip galvanized with 500g/m2 ?
    Best regards,
    ID Rusu

    @Ion- 500g/m^2 exceed the minimum coating thickness for bolts galvanized per A153 or F2329, so from a corrosion standpoint, that is sufficient. If there are safety issues, you’d need to contact the project engineer.

    @Lahin- Yes, the electro zinc plating is not thick enough to effect the thread fit. A plain nut will assemble just fine.

    As per A563M: Hot-dip zinc coated nuts shall be tapped after zinc coating.

    Is this a violation if we do over sizing tap before galvanizing? My understanding is that re-tap will make bigger size.

    Thanks for your advice.

    @Ragadibaoxa- In order to be in compliance with the A563 standard, the nuts must be tapped after galvanizing. What you can do is to tap them before galvanizing, then simply chase them after. Regardless, the zinc must be removed from the threads or else it will likely not assemble properly.

    @Arvind- The ASTM A563 standard covers overtapping for galvanized nuts. Galvanized lock nuts will typically be overtapped per those guidelines.

    We are installing galvanised studs into drilled and tapped baseplates. As the threaded holes will not receive any coating, my assumption would be that the tapping would not need to be oversized. Please advise as our client is requesting oversized tapped holes.

    @Clive- Yes, because your bolts are galvanized, the holes will need to be tapped oversize or else the bolts will not thread in.

    @Sajith – We typically get ours from a local tool and die shop. They may need to be specially ordered, but a local shop should be able to do that for you.

    @Do Quoc- No, A193 is a high temperature specification, so an A194 high temperature nut should be used in conjunction with it. A 10S nut is manufactured per ASTM A563M and is not rated for high temperatures.

    just wanted to note that care must be used before specifying HDG bolts where galvanic corrosion occurs, such as submerged conditions, as the hot dip galvanizing can go away relatively quickly, especially if the HDG coating is acting as an anode to the steel structure, and you can end up with a loose nut due to the over tapping. In this type of galvanic situation stainless steel can also be problematic if part of the bolt in in an anaerobic environment, such as embedded in timber, and the embedded stainless is no longer passivated, resulting in crevice corrosion.

    Sorry, please ignore my previous comment.

    The metric thread oversize allowance can be found in A563M as mentioned by Dane McKinnon.

    However, looking at A563M, it only provides the diametral allowance. ASTM A563 inch equivalent provides extra informaion for the min & max pitch diameter. How do I get the metric pitch diameter?

    @Celina – I am not sure why the authors of A563M did not include the pitch diameter as they did in A563. You may need to contact a metric nut manufacturer, I do not know where to find that information. Apologies.

    ASTM A563 governs the imperial thread type. How do I get the oversize allowances for metric thread type? What standard should I follow?

    Where can I find the specification for A563 10S and the washer for it?
    I can’t find it in ASTM A563

    No comment, just a thank you for your valuable Fastener Tech Tips. We used one on our NEW feature located on FASTENER NEWS NOW called 60 Seconds with…. We did the one on Hot-Dip Galvanized Nuts Tapped Oversize.

    I hope you approve and we can use others from time to time and you get some inquiries and/or orders.

    You write: “A common misconception is that bolts that are going to be hot-dip galvanized are threaded undersize since the galvanizing will increase the size of the threads. This is not correct.”

    In Europe the old DIN standard 267 Part 10, and now the new ISO Standard 10684 calls for optional processes, i.e. oversized nuts or undersized screws before hot-dip galvaizing.

    Are American Standards such as ASTM A563 only addressing the oversized nuts and do not allow undersizing male threads?

    Is the galvanized bolts, screws and nuts is better than Mild Steel Cadmium bolts, screws and nuts in terms of corrosion resistance ?

    Your advice is appreciated.


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