Can I use black (plain finish) or zinc plated nuts on galvanized bolts?
No. 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 | |||
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.