Galvanized nuts & oversize tapping.
Hot-dip galvanizing adds zinc to the external bolt thread. The mating nut is commonly tapped oversize after galvanizing so the coated bolt can assemble correctly.
Coating changes
thread fit.
For HDG threaded assemblies, the bolt normally retains its standard thread while the nut/internal thread receives the required oversize allowance. Nut grade, proof load, lubrication and the governing structural or pressure-bolting specification still have to be respected.
| Nominal inch thread | Diametral allowance | Reference use |
|---|---|---|
| 1/4-20 | 0.016 in | ASTM A563 HDG overtap reference |
| 5/16-18 | 0.017 in | ASTM A563 HDG overtap reference |
| 3/8-16 | 0.017 in | ASTM A563 HDG overtap reference |
| 7/16-14 | 0.018 in | ASTM A563 HDG overtap reference |
| 1/2-13 | 0.018 in | ASTM A563 HDG overtap reference |
| 9/16-12 | 0.020 in | ASTM A563 HDG overtap reference |
| 5/8-11 | 0.020 in | ASTM A563 HDG overtap reference |
| 3/4-10 | 0.020–0.022 in | Verify current governing edition |
| 7/8-9 | 0.022 in | ASTM A563 HDG overtap reference |
| 1-8 | 0.024 in | ASTM A563 HDG overtap reference |
| 1-1/8-7 / 8 | 0.024 in | Pitch-dependent thread limits still apply |
| 1-1/4-7 / 8 | 0.024 in | Pitch-dependent thread limits still apply |
| 1-3/8-6 / 8 | 0.027 in | ASTM A563 HDG overtap reference |
| 1-1/2-6 / 8 | 0.027 in | ASTM A563 HDG overtap reference |
| 1-3/4-5 | 0.050 in | Large-diameter overtap reference |
| 2 to 4 in | 0.050 in | Common large-diameter ASTM A563 allowance range |
Metric: use the metric overtapping requirements of ASTM A563M / the governing project specification; do not convert these inch allowances directly into a metric acceptance limit.
HDG bolt + nut is
a matched system.
For high-strength structural assemblies, overtapping reduces thread engagement, so the permitted nut grade and assembly requirements matter. For critical supply, BIS should receive the complete bolt/nut/washer specification rather than only “HDG nut”.