3-1/2-8 NPSC NPSC (straight, couplings) Internal Thread Specifications
3-1/2-8 NPSC · 3.500-8 NPSC
ASMESTD: ASME B1.20.1SOURCE: Table 4 · p. 14TYPE: Internal (nut/tapped hole)CLASS: —SERIES: NPSC (straight, couplings)TAPER 1:16
Major diameter
4.0005 – 3.9699
max – min, at the gage plane
Pitch diameter
3.9005 – 3.8771
max – min, at the gage plane
Minor diameter
3.8077 – 3.777
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.
Thread length, taper and gage planes
Lengths, taper and gage planes to scale; diameters shortened. D and K at the planes are calculated with the mean truncations.
Limits of size, class —
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter D1Root of the internal thread | 4.0005 | 3.9699 | 0.0306 | |
| Pitch diameter E1Straight thread: the limits apply along the whole length | 3.88881 | 3.9005 | 3.8771 | 0.0234 |
| Minor diameter K1Crest of the internal thread | 3.8077 | 3.777 | 0.0307 | |
| Allowance esNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant | — | |||
| Crest truncation fcTable 1 (both threads) | 0.007750 | 0.004125 | ||
| Root truncation frTable 1 (both threads) | 0.007750 | 0.004125 | ||
| Crest flat FcEquivalent width of flat | 0.009000 | 0.004750 | ||
| Root flat FrEquivalent width of flat | 0.009000 | 0.004750 |
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
- NPT L1 plug gage or L1 internal indicating gage (para. 4.2)
- Gaging tolerance
- ±1-1/2 turns
- Gaging position
- Gaging notch of the NPT L1 plug flush with the face of the coupling
- Sealing
- Pressure-tight with sealant, made up with an NPT external thread (paras. 1.4, 4.1)
NPSC: engagement with an NPT external thread Table 4
- NPT hand-tight at the NPSC PD min, from the small end
- 0.6336 in
- NPT hand-tight at the NPSC PD max, from the small end
- 1.0080 in
L1 plug and ring gages Tables 8 and 9
- Plug gage major dia at the E0 plane
- 3.92075 in
- Plug gage major dia at the gaging notch (E1)
- 3.97207 in
- Plug gage major dia at the large end (E2)
- 3.99888 in
- Gage pitch dia at the E0 plane
- 3.83750 in
- Gage pitch dia at the gaging notch (E1)
- 3.88881 in
- Gage pitch dia at the large end (E2)
- 3.91563 in
- Ring gage minor dia at the E0 plane
- 3.75425 in
- Ring gage minor dia at the gaging notch (E1)
- 3.80556 in
- Thickness of the L1 ring or gage elements
- 0.8210 in
- Working gage crest truncation
- 0.012500 in
- Maximum standoff between plug and ring gages
- 0.059 in
Working gage tolerances Table 7
- On the pitch diameter ±
- 0.0005 in
- Lead, plugs / rings
- 0.0004 / 0.0005 in
- Half angle ±, plugs / rings
- 7 / 10 min
- Taper, plugs / rings
- 0.0010 / 0.0014 in
- Major dia of plugs
- 0.0016 in
- Minor dia of rings
- 0.0016 in
- Total cumulative tolerance on the PD, plugs / rings
- 0.00158 / 0.00211 in
Notes
- Table 4 Note 1: the actual PD of the straight tapped hole will be slightly smaller than the value given when gaged with a taper plug gage (para. 4.2.1). PD sizes correspond to the gaging tolerance of 1-1/2 turns large or small of NPT E1 (para. 4.1.2). Caution (p. 14): when tapered gages are used in straight threads, nonuniformity of gage wear is a particular problem; check the gages directly in addition to checking against a master. NPSC is internal only; the mating external thread is NPT (para. 4.1).
Mates
3-1/2-8 NPSC 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 NPSCevery column of the workbook row
| Designation | 3-1/2-8 NPSC |
|---|---|
| Decimal designation | 3.500-8 NPSC |
| Threads per inch | 8 |
| Thread form | Straight |
| Series | NPSC |
| Series name | Internal Straight Threads in Pipe Couplings |
| 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 4 |
| Printed page | 14 |
| Where the diameter limits apply | Straight thread: the limits apply along the whole length |
| Major dia max (in.) | 4.0005 |
| Major dia min (in.) | 3.9699 |
| Major dia tolerance (in.) | 0.0306 |
| Major dia status | Calculated: PD limits of Table 4 with Table 1 truncation limits (para. 4.1.2) |
| Basic pitch dia at gage plane E1 (in.) | 3.88881 |
| Pitch dia max (in.) | 3.9005 |
| Pitch dia min (in.) | 3.8771 |
| PD tolerance (in.) | 0.0234 |
| Minor dia max (in.) | 3.8077 |
| Minor dia min (in.) | 3.777 |
| Minor dia tolerance (in.) | 0.0307 |
| Minor dia status | Min printed (Table 4 col. 4); max calculated from PD max and Table 1 truncation |
| Allowance source | None: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant |
| 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.) | 0.10000 |
| Thread height h min (in.) | 0.09275 |
| Crest truncation min (in.) | 0.004125 |
| Crest truncation max (in.) | 0.007750 |
| Root truncation min (in.) | 0.004125 |
| Root truncation max (in.) | 0.007750 |
| Crest flat min (in.) | 0.004750 |
| Crest flat max (in.) | 0.009000 |
| Root flat min (in.) | 0.004750 |
| Root flat max (in.) | 0.009000 |
| Sweep cut: start diameter (in.) | 3.777 |
| Sweep cut: tip diameter (in.) | 3.97710 |
| Sweep cut: depth (in.) | 0.100050 |
| Sweep cut: profile width at start diameter (in.) | 0.120250 |
| Sweep cut: tip flat (in.) | 0.004750 |
| Sweep cut: start diameter is | Minor dia min (hole); straight |
| Taper half-angle β (deg) | 0 |
| Taper on diameter | none (straight) |
| Gaging method | NPT L1 plug gage or L1 internal indicating gage (para. 4.2) |
| Gaging tolerance (turns) | ±1-1/2 turns |
| Gaging limit large (turns) | 1.5 |
| Gaging limit small (turns) | 1.5 |
| Basic gaging position | Gaging notch of the NPT L1 plug flush with the face of the coupling |
| Sealing | Pressure-tight with sealant, made up with an NPT external thread (paras. 1.4, 4.1) |
| PD min by formula E1 − 1.5 × 0.0625/n | 3.8771 |
| PD max by formula E1 + 1.5 × 0.0625/n | 3.9005 |
| Minor dia min by formula PDmin − (H − 0.066P) | 3.777 |
| NPT external hand-tight at NPSC PD min, from small end (in.) | 0.6336 |
| NPT external hand-tight at NPSC PD max, from small end (in.) | 1.0080 |
| Gage: plug major dia at E0 plane (in.) | 3.92075 |
| Gage: plug major dia at E1 gaging notch (in.) | 3.97207 |
| Gage: plug major dia at E2 large end (in.) | 3.99888 |
| Gage: pitch dia at E0 plane (in.) | 3.83750 |
| Gage: pitch dia at E1 gaging notch (in.) | 3.88881 |
| Gage: pitch dia at E2 large end (in.) | 3.91563 |
| Gage: ring minor dia at E0 plane (in.) | 3.75425 |
| Gage: ring minor dia at E1 gaging notch (in.) | 3.80556 |
| Gage: thickness of L1 ring or gage elements (in.) | 0.8210 |
| Gage: working gage crest truncation (in.) | 0.012500 |
| Gage: root must clear 0.0381P width (in.) | 0.004763 |
| Gage: plug major at E0 by formula E0 + (H − 2·trunc) | 3.92075 |
| Working gage tol. on PD ± (in.) | 0.0005 |
| Working gage tol. lead, plugs (in.) | 0.0004 |
| Working gage tol. lead, rings (in.) | 0.0005 |
| Working gage tol. half angle, plugs ± (min) | 7 |
| Working gage tol. half angle, rings ± (min) | 10 |
| Working gage tol. taper, plugs + (in.) | 0.0010 |
| Working gage tol. taper, rings − (in.) | 0.0014 |
| Working gage tol. major dia, plugs − (in.) | 0.0016 |
| Working gage tol. minor dia, rings + (in.) | 0.0016 |
| Working gage total cumulative tol. on PD, plugs (in.) | 0.00158 |
| Working gage total cumulative tol. on PD, rings (in.) | 0.00211 |
| Max standoff between plug and ring gages (in.) | 0.059 |
| Check against formulas | OK |
| Notes | Table 4 Note 1: the actual PD of the straight tapped hole will be slightly smaller than the value given when gaged with a taper plug gage (para. 4.2.1). PD sizes correspond to the gaging tolerance of 1-1/2 turns large or small of NPT E1 (para. 4.1.2). Caution (p. 14): when tapered gages are used in straight threads, nonuniformity of gage wear is a particular problem; check the gages directly in addition to checking against a master. NPSC is internal only; the mating external thread is NPT (para. 4.1). |
| Lookup key | 3-1/2-8 NPSC · 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.