3-1/2-8 NPSL NPSL (straight, locknut) Internal Thread Specifications
3-1/2-8 NPSL · 3.500-8 NPSL
ASMESTD: ASME B1.20.1SOURCE: Table 7 · p. 18TYPE: Internal (nut/tapped hole)CLASS: —SERIES: NPSL (straight, locknut)STRAIGHT
Major diameter
— – 4.0112
max – min, at the gage plane
Pitch diameter
3.9396 – 3.9279
max – min, at the gage plane
Minor diameter
— – 3.845
max – min, at the gage plane
Pitch
0.12500
8 threads per inch
Thread profile
Internal 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 —
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter D1Root of the internal thread | — | 4.0112ref. | — | |
| Pitch diameter E1Straight thread: the limits apply along the whole length | 3.88881 | 3.9396 | 3.9279 | 0.0117 |
| Minor diameter K1Crest of the internal thread | — | 3.845 | — | |
| Allowance esTable 7 Note 1: allowance equal to the increase in PD per turn, 0.0625/n | 0.00781 | |||
| Crest truncation fcTable 1 (both threads) | 0.012500 | 0.012500 | ||
| Root truncation frTable 1 (both threads) | 0.012500 | 0.012500 | ||
| Crest flat FcEquivalent width of flat | 0.014434 | 0.014434 | ||
| Root flat FrEquivalent width of flat | 0.014434 | 0.014434 |
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 (tolerance equals 1.5 turns, Table 7 Note 1)
- Gaging position
- not applicable (straight thread)
- Sealing
- Mechanical joint, no sealing requirement (section 6)
Notes
- Table 7 Note 1: major and minor diameters vary with PD (single-point form). External max major = max PD + 0.666025P (truncated depth); commercial pipe may not reach it. An allowance equal to the PD increase per turn is provided, with a tolerance of 1.5 turns for both threads. Gages: GO / NOT GO to the PD limits of Table 7 per ASME B1.2 (para. 6.4.4).
Mates
3-1/2-8 NPSL takes a thread of the same standard at the same size and pitch. The standard mating external 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 NPSLevery column of the workbook row
| Designation | 3-1/2-8 NPSL |
|---|---|
| Decimal designation | 3.500-8 NPSL |
| Threads per inch | 8 |
| Thread form | Straight |
| Series | NPSL |
| Series name | Straight Pipe Threads for Loose-fitting Mechanical Joints with Locknuts |
| External / internal | Internal |
| Class | — |
| 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 7 |
| Printed page | 18 |
| Where the diameter limits apply | Straight thread: the limits apply along the whole length |
| Major dia min (in.) | 4.0112 |
| Major dia status | Not given in the standard; reference min = PD min + 0.666025P (Note 1) |
| Basic pitch dia at gage plane E1 (in.) | 3.88881 |
| Pitch dia max (in.) | 3.9396 |
| Pitch dia min (in.) | 3.9279 |
| PD tolerance (in.) | 0.0117 |
| Minor dia min (in.) | 3.845 |
| Minor dia status | Min only (Table 7 col. 7, Note 1); max not given in the standard |
| Allowance (in.) | 0.00781 |
| Allowance source | Table 7 Note 1: allowance equal to the increase in PD per turn, 0.0625/n |
| 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.) | 0.083253 |
| Thread height h min (in.) | 0.083253 |
| Crest truncation min (in.) | 0.012500 |
| Crest truncation max (in.) | 0.012500 |
| Root truncation min (in.) | 0.012500 |
| Root truncation max (in.) | 0.012500 |
| Crest flat min (in.) | 0.014434 |
| Crest flat max (in.) | 0.014434 |
| Root flat min (in.) | 0.014434 |
| Root flat max (in.) | 0.014434 |
| Sweep cut: start diameter (in.) | 3.845 |
| Sweep cut: tip diameter (in.) | 4.011506 |
| Sweep cut: depth (in.) | 0.083253 |
| Sweep cut: profile width at start diameter (in.) | 0.110566 |
| Sweep cut: tip flat (in.) | 0.014434 |
| Sweep cut: start diameter is | Minor dia min (hole) |
| 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 (tolerance equals 1.5 turns, Table 7 Note 1) |
| Basic gaging position | not applicable (straight thread) |
| Sealing | Mechanical joint, no sealing requirement (section 6) |
| Minor dia min by formula (int) | 3.845 |
| Check against formulas | OK |
| Notes | Table 7 Note 1: major and minor diameters vary with PD (single-point form). External max major = max PD + 0.666025P (truncated depth); commercial pipe may not reach it. An allowance equal to the PD increase per turn is provided, with a tolerance of 1.5 turns for both threads. Gages: GO / NOT GO to the PD limits of Table 7 per ASME B1.2 (para. 6.4.4). |
| Lookup key | 3-1/2-8 NPSL · Internal |
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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.