3 1/2-8 NPSH NPSH (pipe-size hose) External Thread Specifications
3 1/2-8NPSH EXT
ASMESTD: ASME B1.20.7SOURCE: Table 2 · p. 5TYPE: External (bolt/screw)CLASS: StdSERIES: NPSH (pipe-size hose)
Major diameter
3.9700 – 3.9478
max – min
Pitch diameter
3.8888 – 3.8777
max – min
Minor diameter
3.8076 – 3.7785
max – min
Pitch
0.12500
8 threads per inch
Thread profile
3 1/2-8NPSH EXT: solid line = maximum material profile, dashed = minimum material, hatched = tolerance zone, grey dashed = basic profile. Drawn to scale; values in inches, diameters shortened.
Limits of size, class Std
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter dPara. 3.1: tolerance = 2 × PD tolerance | 3.9820 | 3.9700 | 3.9478 | 0.0222 |
| Pitch diameter d2 | 3.9008 | 3.8888 | 3.8777 | 0.0111 |
| Minor diameter d1Max printed; min is a reference value | 3.8196 | 3.8076 | 3.7785ref. | 0.0291ref. |
| Allowance es | 0.0120 |
Dimensions in inches. Values are those of the standard (ASME B1.20.7); calculated cells follow the standard’s formulas. Where the standard prints a value that differs from its formula, the printed value is shown with a note.
Tolerance zones relative to the basic size, inches
Mating, gaging and dimensions
Mating member and fit at the limits
- Service
- Steam, air, water, and all other hose connections to be made up with standard pipe threads
- Mating group (compatible designations)
- NPSH-3_1/2
- Mating member of the same designation
- 3 1/2-8NPSH INT
- Other designations in the mating group
- none (mates only with its own designation)
- Minimum PD clearance (coupling min − nipple max)
- 0.0120 in
- Maximum PD clearance (coupling max − nipple min)
- 0.0342 in
- Minimum major dia clearance
- 0.0120 in
- Minimum minor dia clearance
- 0.0120 in
- Fit at the limits (PD)
- clearance (always)
Lead, flank and profile
- Lead variation ± (Table 5) ±δP
- 0.0032 in
- Half-angle variation ± (Table 5) ±δα
- 1° 42′
- Lead angle at the basic PD λ
- 0° 35′
- Crest flat (basic P/8)
- 0.015625 in
- Root flat (basic P/8)
- 0.015625 in
- Extreme flat P/24 at minimum PD
- 0.005208 in
- Root radius for the model (worn tool) r
- 0.013532 in
Table 4 dimensions (nipple and coupling)
- As printed
- C 3 17/32; OD 3 31/32; L 1 1/8; l 1/4
- Inside diameter of the nipple C
- 3.53125 in
- Approximate OD of the external thread OD
- 3.96875 in
- Length of the nipple L
- 1.12500 in
- Pilot length l
- 0.25000 in
Ring gages (Table 6)
- GO pitch max / min
- 3.8888 / 3.8883 in
- GO minor max / min
- 3.8196 / 3.8189 in
- NOT GO pitch max / min
- 3.8782 / 3.8777 in
- NOT GO minor max / min
- 3.8513 / 3.8506 in
- Z GO major max / min
- 3.97000 / 3.96980 in
- Z NOT GO major max / min
- 3.94800 / 3.94780 in
Setting plugs (Table 8) and plug gages (Table 7)
- GO truncated major max / min
- 3.9529 / 3.9522 in
- GO full major max / min
- 3.9707 / 3.9700 in
- GO pitch max / min
- 3.8888 / 3.8883 in
- NOT GO truncated major max / min
- 3.9318 / 3.9311 in
- NOT GO pitch max / min
- 3.8782 / 3.8777 in
Notes
- Root radius and tensile stress area are not given in the standard.
Mates
3 1/2-8NPSH EXT screws into a thread of the same standard at the same size and pitch. The standard mating internal thread is class Std. Check the fit with the standard pair →
Working out the mating options…
Different pitch or a different thread family never mate; the fit check runs over both tolerance zones.
Related thread sizes
Same diameter other pitch, and neighbouring sizes
All values for 3 1/2-8NPSH EXTevery column of the workbook row
| Designation | 3 1/2-8NPSH EXT |
|---|---|
| Decimal / alternate designation | 3.5000-8NPSH EXT |
| Threads per inch | 8 |
| Thread form | NPSH |
| Series | NPSH |
| Series name | Hose to pipe thread (NPSH) |
| External / internal | External |
| Class | Std |
| Nominal size of hose | 3 1/2 |
| Pitch P (in.) | 0.12500 |
| Basic major dia D (in.) | 3.9820 |
| Size category | Standard sizes (Tables 1-8) |
| Source table | Table 2 |
| Printed page | 5 |
| Major dia max (in.) | 3.9700 |
| Major dia min (in.) | 3.9478 |
| Major dia tolerance (in.) | 0.0222 |
| Allowance es (in.) | 0.0120 |
| Basic pitch dia D2 (in.) | 3.9008 |
| Pitch dia max (in.) | 3.8888 |
| Pitch dia min (in.) | 3.8777 |
| PD tolerance (in.) | 0.0111 |
| Basic minor dia D1 (in.) | 3.8196 |
| Minor dia max (in.) | 3.8076 |
| Minor dia min (in.) | 3.7785 |
| Minor dia tolerance (in.) | 0.0291 |
| Major dia status | Toleranced (para. 3.1: tolerance = 2 x PD tolerance) |
| Minor dia status | Maximum printed; minimum is a reference value (para. 3.1: P/24 flat at min PD) |
| Root radius max (in.) | not given in the standard |
| Root radius min (in.) | not given in the standard |
| Lead variation ± (in.) (Table 5) | 0.0032 |
| Half-angle variation ± (Table 5) | 1° 42′ |
| Lead variation from PD tol (calc) (in.) | 0.0032 |
| Half-angle variation from PD tol (calc) | 1° 42′ |
| Lead angle at basic PD (deg) | 0.5844 |
| Lead angle (deg min) | 0° 35′ |
| Tensile stress area As (sq in.) | not given in the standard |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (in.) | 0.108253 |
| Thread height h (in.) | 0.081190 |
| Crest flat, basic P/8 (in.) | 0.015625 |
| Root flat, basic P/8 (in.) | 0.015625 |
| Extreme flat P/24 at minimum PD (in.) | 0.005208 |
| Truncation of the P/8 flat from the sharp V, f = h/6 (in.) | 0.013532 |
| Root radius for model (worn tool) (in.) | 0.013532 |
| Sweep cut: start diameter (in.) | 3.970000 |
| Sweep cut: tip diameter (in.) | 3.807600 |
| Sweep cut: depth (in.) | 0.081200 |
| Sweep cut: profile width at start diameter (in.) | 0.109387 |
| Crest flat as modelled (in.) | 0.015613 |
| Taper | none (straight thread) |
| Service | Steam, air, water, and all other hose connections to be made up with standard pipe threads |
| Mating group (compatible designations) | NPSH-3_1/2 |
| Mating member of the same designation | 3 1/2-8NPSH INT |
| Other designations in the mating group | none (mates only with its own designation) |
| Min PD clearance: coupling min PD - nipple max PD (in.) | 0.0120 |
| Max PD clearance: coupling max PD - nipple min PD (in.) | 0.0342 |
| Min major dia clearance: coupling min - nipple max (in.) | 0.0120 |
| Min minor dia clearance: coupling min - nipple max (in.) | 0.0120 |
| Fit at the limits (PD) | clearance (always) |
| Table 4 lengths as printed | C 3 17/32; OD 3 31/32; L 1 1/8; l 1/4 |
| Table 4: C inside dia of nipple (in.) | 3.53125 |
| Table 4: approx OD of external thread (in.) | 3.96875 |
| Table 4: L length of nipple (in.) | 1.12500 |
| Table 4: l pilot length (in.) | 0.25000 |
| Table 6 ring gage: GO pitch max (in.) | 3.8888 |
| Table 6 ring gage: GO pitch min (in.) | 3.8883 |
| Table 6 ring gage: GO minor max (in.) | 3.8196 |
| Table 6 ring gage: GO minor min (in.) | 3.8189 |
| Table 6 ring gage: NOT GO pitch max (in.) | 3.8782 |
| Table 6 ring gage: NOT GO pitch min (in.) | 3.8777 |
| Table 6 ring gage: NOT GO minor max (in.) | 3.8513 |
| Table 6 ring gage: NOT GO minor min (in.) | 3.8506 |
| Table 6 ring gage: Z GO major max (in.) | 3.97000 |
| Table 6 ring gage: Z GO major min (in.) | 3.96980 |
| Table 6 ring gage: Z NOT GO major max (in.) | 3.94800 |
| Table 6 ring gage: Z NOT GO major min (in.) | 3.94780 |
| Table 8 setting plug: GO truncated major max (in.) | 3.9529 |
| Table 8 setting plug: GO truncated major min (in.) | 3.9522 |
| Table 8 setting plug: GO full major max (in.) | 3.9707 |
| Table 8 setting plug: GO full major min (in.) | 3.9700 |
| Table 8 setting plug: GO pitch max (in.) | 3.8888 |
| Table 8 setting plug: GO pitch min (in.) | 3.8883 |
| Table 8 setting plug: NOT GO truncated major max (in.) | 3.9318 |
| Table 8 setting plug: NOT GO truncated major min (in.) | 3.9311 |
| Table 8 setting plug: NOT GO full major max (in.) | 3.9707 |
| Table 8 setting plug: NOT GO full major min (in.) | 3.9700 |
| Table 8 setting plug: NOT GO pitch max (in.) | 3.8782 |
| Table 8 setting plug: NOT GO pitch min (in.) | 3.8777 |
| Table 8 setting plug: Full-form GO major max (in.) | 3.9707 |
| Table 8 setting plug: Full-form GO major min (in.) | 3.9700 |
| Table 8 setting plug: Full-form NOT GO major max (in.) | 3.9707 |
| Table 8 setting plug: Full-form NOT GO major min (in.) | 3.9700 |
| Check against formulas | OK |
| Notes | Root radius and tensile stress area are not given in the standard. |
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ASME B1.20.7: Hose Coupling Screw Threads, Inch. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.