3-1/2-8 NPSM NPSM (straight, mechanical) External Thread Specifications
3-1/2-8 NPSM · 3.500-8 NPSM
ASMESTD: ASME B1.20.1SOURCE: Table 6 · p. 17TYPE: External (bolt/screw)CLASS: 2ASERIES: NPSM (straight, mechanical)STRAIGHT
Major diameter
3.968 – 3.953
max – min, at the gage plane
Pitch diameter
3.8865 – 3.8788
max – min, at the gage plane
Minor diameter
3.8053 – —
max – min, at the gage plane
Pitch
0.12500
8 threads per inch
Thread profile
External profile over three pitches at the gage plane, drawn at maximum material (sharpest crest, flattest root); hatched bands are the truncation zones. Diameters shortened.
Limits of size, class 2A
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter D1Crest of the external thread | 3.968 | 3.953 | 0.015 | |
| Pitch diameter E1Straight thread: the limits apply along the whole length | 3.88881 | 3.8865 | 3.8788 | 0.0077 |
| Minor diameter K1Root of the external thread | 3.8053ref. | — | — | |
| Allowance esTable 6 col. 4 (external, Class 2A) | 0.0023 | |||
| Crest truncation fcTable 1 (both threads) | 0.013531 | 0.013531 | ||
| Root truncation frTable 1 (both threads) | 0.013531 | 0.013531 | ||
| Crest flat FcEquivalent width of flat | 0.015625 | 0.015625 | ||
| Root flat FrEquivalent width of flat | 0.015625 | 0.015625 |
Dimensions in inches. Values are those of the standard (ASME B1.20.1); 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.
Taper, lengths and gaging
Taper, gage and sealing paras. 3, 5; Tables 1, 3
- Taper on the diameter T
- none (straight)
- Half apex angle of the pitch cone β
- 0 °
- Lead angle at the basic PD λ
- 0° 35′
- Gaging method
- GO / NOT GO thread gages or indicating gages to ASME B1.2 (para. 6.4); NOT GO may enter up to 3 turns
- Gaging tolerance
- limits of size
- Gaging position
- not applicable (straight thread)
- Sealing
- Mechanical joint, no sealing requirement (section 6)
Notes
- Table 6 General Note (a): NPSM has a form similar to UN with tolerances similar to 2A/2B; the internal thread minimum PD equals E1 of NPT (Note 1). Values were not recalculated in 2013 (Foreword (d)). Gages: GO / NOT GO to the PD limits of Table 6 per ASME B1.2 (para. 6.4.4): GO plug min major = int PD min + 0.64951905P; NOT GO plug max major = int PD max + 0.43301270P; GO ring max minor = ext PD max − 0.43301300P; NOT GO ring min minor = ext PD min − 0.21650635P.
Mates
3-1/2-8 NPSM 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 3-1/2-8 NPSMevery column of the workbook row
| Designation | 3-1/2-8 NPSM |
|---|---|
| Decimal designation | 3.500-8 NPSM |
| Threads per inch | 8 |
| Thread form | Straight |
| Series | NPSM |
| Series name | Straight Pipe Threads for Free-fitting Mechanical Joints for Fixtures |
| External / internal | External |
| Class | 2A |
| Nominal pipe size | 3-1/2 |
| Pitch P (in.) | 0.12500000 |
| O.D. of pipe D, basic major dia (in.) | 4.000 |
| Size category (Table 3 group) | 2-1/2 and larger (8 TPI) |
| Source table | Table 6 |
| Printed page | 17 |
| Where the diameter limits apply | Straight thread: the limits apply along the whole length |
| Major dia max (in.) | 3.968 |
| Major dia min (in.) | 3.953 |
| Major dia tolerance (in.) | 0.015 |
| Major dia status | Toleranced (Table 6) |
| Basic pitch dia at gage plane E1 (in.) | 3.88881 |
| Pitch dia max (in.) | 3.8865 |
| Pitch dia min (in.) | 3.8788 |
| PD tolerance (in.) | 0.0077 |
| Minor dia max (in.) | 3.8053 |
| Minor dia status | Reference only: produced by commercial dies (General Note (b)); max = PD max − (H − 2 × 0.10825P) |
| Allowance (in.) | 0.0023 |
| Allowance source | Table 6 col. 4 (external, Class 2A) |
| Root radius max (in.) | not given in the standard |
| Root radius min (in.) | not given in the standard |
| Lead / angle / taper source | Not given in the standard for straight threads |
| Lead angle at basic PD (deg) | 0.586207 |
| 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.) | 0.1082532 |
| Thread height h max (in.) | 0.081190 |
| Thread height h min (in.) | 0.081190 |
| Crest truncation min (in.) | 0.013531 |
| Crest truncation max (in.) | 0.013531 |
| Root truncation min (in.) | 0.013531 |
| Root truncation max (in.) | 0.013531 |
| Crest flat min (in.) | 0.015625 |
| Crest flat max (in.) | 0.015625 |
| Root flat min (in.) | 0.015625 |
| Root flat max (in.) | 0.015625 |
| Sweep cut: start diameter (in.) | 3.968 |
| Sweep cut: tip diameter (in.) | 3.805620 |
| Sweep cut: depth (in.) | 0.081190 |
| Sweep cut: profile width at start diameter (in.) | 0.109375 |
| Sweep cut: tip flat (in.) | 0.015625 |
| Sweep cut: start diameter is | Major dia max (cylinder) |
| Taper half-angle β (deg) | 0 |
| Taper on diameter | none (straight) |
| Gaging method | GO / NOT GO thread gages or indicating gages to ASME B1.2 (para. 6.4); NOT GO may enter up to 3 turns |
| Gaging tolerance (turns) | limits of size |
| Basic gaging position | not applicable (straight thread) |
| Sealing | Mechanical joint, no sealing requirement (section 6) |
| PD limit by formula (NPSM ext: E1 − allowance; int: E1) | 3.8865 |
| Major dia max by formula (ext) | 3.968 |
| Check against formulas | OK |
| Notes | Table 6 General Note (a): NPSM has a form similar to UN with tolerances similar to 2A/2B; the internal thread minimum PD equals E1 of NPT (Note 1). Values were not recalculated in 2013 (Foreword (d)). Gages: GO / NOT GO to the PD limits of Table 6 per ASME B1.2 (para. 6.4.4): GO plug min major = int PD min + 0.64951905P; NOT GO plug max major = int PD max + 0.43301270P; GO ring max minor = ext PD max − 0.43301300P; NOT GO ring min minor = ext PD min − 0.21650635P. |
| Lookup key | 3-1/2-8 NPSM · External 2A |
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ASME B1.20.1: Pipe Threads, General Purpose, Inch. Source: workbook data (2026-10-11). Confirm against your licensed copy of the standard before use.