ASME B16.47 · Standard reference
ASME B16.47-2025 flange tolerances (Section 7)
Section 7 of ASME B16.47-2025 sets the permissible manufacturing deviations for large-diameter flanges: raised-face and ring-joint facing dimensions, flange thickness, welding-end outside and inside diameter, backing-ring contact bore, hub length, bolt circle diameter, bolt-hole spacing and bolt-circle concentricity. The values are acceptance limits for new manufacture, not allowances to be added to a design dimension.
B16.47-2025 publishes metric dimensions as primary and U.S. customary values in parentheses; both are reproduced in the table below. Where a tolerance is stated as a single plus value that is intentional — the 1.5 mm raised face carries +0.5 mm and no minus value, and flange thickness carries only a plus tolerance, so a flange may finish thicker than nominal but not thinner. Ring-joint groove dimensions and tolerances are governed by Table 30 (Table 30C) rather than repeated in Section 7.
- Clause
- Section 7, pp. 6-7
- Thickness
- Plus tolerance only (+3.0 to +10.0 mm)
- Bolt circle
- ±1.5 mm (±0.06 in.)
- Bolt-hole spacing
- ±0.8 mm (±0.03 in.)
- Weld-end OD
- +5.0 / -2.0 mm
- Hub length
- +3.0 / -5.0 mm
Section 7 tolerances at a glance
One table collects every limit in Section 7, in the order the standard presents them: facings, thickness, welding ends, hub length and drilling/facing. Where a limit depends on part size, the size bands are the standard's own.
B16.47 · ASME B16.47-2025
| Feature | Size range | Tolerance, mm | Tolerance, in. |
|---|---|---|---|
| Raised-face outside diameter | All sizes | ±2.0 | ±0.08 |
| 1.5 mm (0.06 in.) raised-face height | All sizes | +0.5 | +0.02 |
| 6.4 mm (0.25 in.) raised-face height | All sizes | ±2.0 | ±0.08 |
| Ring-joint groove | All sizes | Per Table 30 | Per Table 30C |
| Flange thickness t | t ≤ 25 mm (≤ 1.0 in.) | +3.0, -0.0 | +0.12, -0.00 |
| Flange thickness t | > 25 to ≤ 50 mm (> 1.0 to ≤ 2.0 in.) | +5.0, -0.0 | +0.20, -0.00 |
| Flange thickness t | > 50 to ≤ 75 mm (> 2.0 to ≤ 3.0 in.) | +8.0, -0.0 | +0.31, -0.00 |
| Flange thickness t | > 75 mm (> 3.0 in.) | +10.0, -0.0 | +0.39, -0.00 |
| Welding-end outside diameter (dim. A, Figure 1) | All sizes | +5.0, -2.0 | +0.20, -0.08 |
| Welding-end inside diameter (dim. B, Figure 1) | All sizes | +3.0, -2.0 | +0.12, -0.08 |
| Welding-end inside diameter (dim. B, Figure 2) | All sizes | +0.0, -2.0 | +0.00, -0.08 |
| Backing-ring contact bore (dim. C, Figure 2) | All sizes | +0.25, -0.0 | +0.01, -0.00 |
| Hub overall length, welding neck | All sizes | +3.0, -5.0 | +0.12, -0.20 |
| Bolt circle diameter | All sizes | ±1.5 | ±0.06 |
| Bolt hole to adjacent bolt hole (centre to centre) | All sizes | ±0.8 | ±0.03 |
| Bolt circle concentricity to machined facing | All sizes | 1.5 max. | 0.06 max. |
| Hub thickness at welding end | All sizes | ≥ 87.5% of nominal pipe wall | — |
- a Ring-joint groove dimensions and tolerances are tabulated in Table 30 (Table 30C); they are not repeated in Section 7.
- b The hub thickness at the welding end overrides the plus/minus limits on dimensions A and B: it must not fall below 87.5% of the nominal pipe wall (or the minimum wall specified by the purchaser).
- c The plus thickness tolerance applies at bolting bearing surfaces whether as-forged, as-cast, spot-faced or back-faced.
Metric values are primary in B16.47-2025; the inch values in the standard are shown in parentheses and are reproduced here separately. Plus and minus signs are as printed: a single plus value means no negative tolerance is permitted.
How the limits are applied
The face and drilling limits are measured on the finished component. Bolt holes are equally spaced in multiples of four (para. 6.2), which is why the adjacent-hole tolerance controls the pattern rather than a single hole position. The bolt-circle concentricity limit is measured against the machined facing diameter, so it is checked after facing rather than before.
Flange thickness is checked at the bolting bearing surface as well as the body. Spot-facing or back-facing may not take the flange below the minimum thickness in the dimension tables, and the plus-only tolerance means a marginally thick forging can be cleaned up by machining while a thin one cannot be recovered. Weld-end dimensions A, B and C refer to the profiles in Figures 1-3; the same figures also limit how much extra thickness may be added to match a heavier pipe.
These are dimensional acceptance limits only. Surface finish, facing-finish imperfections and marking are handled elsewhere in the standard, and the standard does not require flanges to be pressure tested (para. 8.1).
ASME B16.47 tolerances — questions
Can a B16.47 flange be thinner than nominal?
No. Section 7.2 gives only a plus tolerance on flange thickness — +3.0 mm up to 25 mm, rising to +10.0 mm above 75 mm — with no minus allowance. A flange that measures under the tabulated minimum thickness does not conform, even if it is within other dimensional tolerances.
What is the bolt circle tolerance on a B16.47 flange?
The bolt circle diameter has a tolerance of ±1.5 mm (±0.06 in.), the centre-to-centre distance of adjacent bolt holes is ±0.8 mm (±0.03 in.), and the bolt circle must be concentric with the machined facing diameter within 1.5 mm (0.06 in.). Bolt holes are equally spaced in multiples of four.
Which tolerance applies to the weld-end bore?
It depends on the weld-end profile. For the Figure 1 profile the inside diameter tolerance is +3.0/-2.0 mm; for the Figure 2 backing-ring profile it is +0.0/-2.0 mm. In both cases the hub at the welding end must also be at least 87.5% of the nominal pipe wall, which can govern over the dimensional tolerance.
Are the inch tolerances exact conversions of the metric values?
They are the standard's own U.S. customary values, printed alongside the metric values, and some are rounded independently (for example ±2.0 mm is shown as ±0.08 in.). Use the unit system called out on the drawing or purchase order; do not convert one column to check the other, because the rounding will not be exact.