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 overtapping allowances.
ASTM A563/A563M, metric thread series. These are diametral pitch-diameter overtap allowances for the specified galvanized fastener system, not finished tapping-drill diameters.
| Metric thread | Overtap allowance | Threaded nut system |
|---|---|---|
| M12 × 1.75 | 0.365 mm | ASTM A563/A563M HDG |
| M16 × 2.0 | 0.420 mm | ASTM A563/A563M HDG |
| M20 × 2.5 | 0.530 mm | ASTM A563/A563M HDG |
| M22 × 2.5 | 0.530 mm | ASTM A563/A563M HDG |
| M24 × 3.0 | 0.640 mm | ASTM A563/A563M HDG |
| M27 × 3.0 | 0.640 mm | ASTM A563/A563M HDG |
| M30 × 3.5 | 0.750 mm | ASTM A563/A563M HDG |
| M36 × 4.0 | 0.860 mm | ASTM A563/A563M HDG |
Important: The values are diametral overtap allowances from the ASTM metric table for hot-dip zinc-coated assemblies; the specified thread pitch and acceptance gauge still determine the final finished-nut limits. Do not add these figures to a tap-drill diameter. For ISO 10684 hot-dip galvanized fasteners, use its specified 6AZ/6AX or designated thread system, which is not necessarily interchangeable with ASTM A563/A563M. Verify the applicable current edition and assembly grade before cutting threads. Reference: ASTM International · ISO 10684.
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, United Exim should receive the complete bolt/nut/washer specification rather than only “HDG nut”.