3-1/2-8 NPTR NPTR (taper, railing) Internal Thread Specifications
3-1/2-8 NPTR · 3.500-8 NPTR
ASMESTD: ASME B1.20.1SOURCE: Table 5 · p. 15TYPE: Internal (nut/tapped hole)CLASS: —SERIES: NPTR (taper, railing)TAPER 1:16
Major diameter
3.99662 – 3.98156
max – min, at the gage plane
Pitch diameter
3.89662 – 3.88881
max – min, at the gage plane
Minor diameter
3.80387 – 3.78881
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 at the gage plane | 3.99662 | 3.98156 | 0.01506 | |
| Pitch diameter E1Gage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter | 3.88881 | 3.89662 | 3.88881 | 0.00781 |
| Minor diameter K1Crest of the internal thread at the gage plane | 3.80387 | 3.78881 | 0.01506 | |
| 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
- 1 in 16 (0.75 in./ft)
- Half apex angle of the pitch cone β
- 1.789911 °
- Taper tolerance on the diameter, max / min
- 0.1250 / -0.0625 in/ft
- Lead tolerance in the effective length ±
- 0.003 in
- 60° angle tolerance ±
- 1.5 °
- Lead angle at the basic PD λ
- 0° 35′
- Gaging method
- NPT L1 thread gages (para. 5.2)
- Gaging tolerance
- 1 turn large, 0 turns small from the modified basic (Table C-1)
- Gaging position
- Gaging notch 5 threads below the face of the fitting (Table 5 col. 18)
- Sealing
- Mechanical joint, not a seal (section 5)
Basic pitch diameters and lengths Table 3
- Pitch dia at the small end of the external thread E0
- 3.83750 in
- Hand-tight engagement L1
- 0.8210 · 6.57 in, threads
- Pitch dia at the hand-tight plane E1
- 3.88881 in
- Effective external thread L2
- 1.2500 · 10.00 in, threads
- Pitch dia at L2 E2
- 3.91563 in
- Wrench make-up length L3
- 0.2500 · 2 in, threads
- Pitch dia at the wrench make-up plane E3
- 3.82188 in
- Complete external threads L5
- 1.0000 in
- Pitch dia at L5 E5
- 3.90000 in
- Vanish thread V
- 0.4338 · 3.47 in, threads
- Overall external thread length L4
- 1.6837 in
- Basic minor dia at the small end K0
- 3.7374 in
- Change of diameter per turn
- 0.00781 in
- Hand-tight engagement range, ± 1 turn
- 0.5710 – 1.0710 in
- Wrench make-up interference on the PD
- 0.01563 in
- Hand-tight plus wrench make-up length L1+L3
- 1.0710 in
NPTR (railing): shortened thread and recess Tables 4, 5
- Shortening of the thread L6
- 0.500 · 4 in, threads
- Pitch dia at the end of the external thread E6
- 3.8688 in
- Effective thread of the NPTR external L2−L6
- 0.750 in
- Maximum total external thread L4−L6
- 1.125 in
- Maximum incomplete threads due to chamfer V
- 0.375 in
- Minimum depth of the recess in the fitting q
- 0.38 in
- Minimum diameter of the recess Q
- 4.04 in
- Minimum length T
- 0.63 in
- Gage notch below the face of the fitting S
- 0.625 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
- NPTR uses the NPT basic dimensions (Table 2) and NPT L1 gages; Table 5 gives the shortening L6, the recess and the gage-notch distance S. Table 5 values were not recalculated in 2013 (Foreword (d)). Para. 5.2 example (1/2-14): the start end of the modified basic thread lies 4 threads inside the face of the fitting; gaging tolerance 4 turns large to 5 turns large. Chamfer: internal threads with chamfer larger than the major dia are gaged from the last scratch of the chamfer cone (para. 3.2.4.2, Fig. 5); otherwise from the end of the fitting (para. 3.2.4.4).
Mates
3-1/2-8 NPTR 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 NPTRevery column of the workbook row
| Designation | 3-1/2-8 NPTR |
|---|---|
| Decimal designation | 3.500-8 NPTR |
| Threads per inch | 8 |
| Thread form | Taper |
| Series | NPTR |
| Series name | Railing Joint Taper Pipe Threads |
| 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 5 |
| Printed page | 15 |
| Where the diameter limits apply | Gage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter |
| Major dia max (in.) | 3.99662 |
| Major dia min (in.) | 3.98156 |
| Major dia tolerance (in.) | 0.01506 |
| Major dia status | Calculated at the gage plane: PD limits with Table 1 truncation limits (para. 2.4) |
| Basic pitch dia at gage plane E1 (in.) | 3.88881 |
| Pitch dia max (in.) | 3.89662 |
| Pitch dia min (in.) | 3.88881 |
| PD tolerance (in.) | 0.00781 |
| Minor dia max (in.) | 3.80387 |
| Minor dia min (in.) | 3.78881 |
| Minor dia tolerance (in.) | 0.01506 |
| Minor dia status | Calculated at the gage plane: PD limits with Table 1 truncation limits (para. 2.4) |
| 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 tolerance ± in effective length (in.) | 0.003 |
| 60° angle tolerance ± (deg) | 1.5 |
| Taper tolerance max (in./ft) | 0.1250 |
| Taper tolerance min (in./ft) | -0.0625 |
| Lead / angle / taper source | Table 3, p. 10 (Table 3 is for NPT; NPTR has the NPT form, para. 5.1.2); lead ±0.003 in./in. when the effective thread is longer than 1 in. (Note 1) |
| 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.) | 0.1000 |
| 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.) | 3.78881 |
| Sweep cut: tip diameter (in.) | 3.98881 |
| Sweep cut: depth (in.) | 0.100000 |
| Sweep cut: profile width at start diameter (in.) | 0.120250 |
| Sweep cut: tip flat (in.) | 0.004750 |
| Sweep cut: start diameter is | Minor dia at the large end of the fitting (E1 plane), basic position; the hole is a 1:16 cone |
| Taper half-angle β (deg) | 1.789911 |
| Taper on diameter | 1 in 16 (0.75 in./ft) |
| E0 pitch dia at small end (in.) | 3.83750 |
| L1 hand-tight engagement (in.) | 0.8210 |
| L1 (threads) | 6.57 |
| E1 pitch dia at hand-tight plane (in.) | 3.88881 |
| L2 effective thread, external (in.) | 1.2500 |
| L2 (threads) | 10.00 |
| E2 pitch dia at L2 plane (in.) | 3.91563 |
| L2 − L1 (in.) | 0.4290 |
| L2 − L1 (threads) | 3.43 |
| L3 wrench make-up, internal (in.) | 0.2500 |
| L3 (threads) | 2 |
| E3 pitch dia at wrench make-up plane (in.) | 3.82188 |
| V vanish thread (in.) | 0.4338 |
| V (threads) | 3.47 |
| L4 overall external thread length, reference (in.) | 1.6837 |
| L5 complete external threads (in.) | 1.0000 |
| E5 pitch dia at L5 plane (in.) | 3.90000 |
| Height of thread h, Table 2 col. 22 (in.) | 0.10000 |
| Change in dia per turn 0.0625/n (in.) | 0.00781 |
| K0 basic minor dia at small end (in.) | 3.7374 |
| E0 by formula D − (0.05D + 1.1)P | 3.83750 |
| E1 by formula E0 + 0.0625·L1 | 3.88881 |
| L2 by formula (0.80D + 6.8)P | 1.2500 |
| E2 by formula E0 + 0.0625·L2 | 3.91563 |
| E3 by formula E0 − 0.0625·L3 | 3.82188 |
| L5 by formula L2 − 2P | 1.0000 |
| E5 by formula E0 + 0.0625·L5 | 3.90000 |
| V by formula 3.47P | 0.4338 |
| L4 by formula L2 + V | 1.6838 |
| K0 by formula E0 − 0.8P | 3.7375 |
| Gaging method | NPT L1 thread gages (para. 5.2) |
| Gaging tolerance (turns) | 1 turn large, 0 turns small from the modified basic (Table C-1) |
| Gaging limit large (turns) | 1 |
| Gaging limit small (turns) | 0 |
| Basic gaging position | Gaging notch 5 threads below the face of the fitting (Table 5 col. 18) |
| Hand-tight engagement min, ±1 turn both parts (in.) | 0.5710 |
| Hand-tight engagement max, ±1 turn both parts (in.) | 1.0710 |
| Wrench make-up interference on PD, basic L3/16 (in.) | 0.01563 |
| Hand-tight + wrench make-up length L1 + L3 (in.) | 1.0710 |
| Sealing | Mechanical joint, not a seal (section 5) |
| E6 pitch dia at end of external thread (in.) | 3.8688 |
| L6 shortening of thread (in.) | 0.500 |
| L6 (threads) | 4 |
| L2 − L6 effective thread (in.) | 0.750 |
| L2 − L6 (threads) | 6.00 |
| L4 − L6 max total external thread (in.) | 1.125 |
| L4 − L6 (threads) | 9.00 |
| V max incomplete threads due to chamfer (in.) | 0.375 |
| V (threads) | 3 |
| q min depth of recess in fitting (in.) | 0.38 |
| Q min dia of recess in fitting (in.) | 4.04 |
| T min length (in.) | 0.63 |
| S distance gage notch below face of fitting (in.) | 0.625 |
| S (threads) | 5 |
| E6 by formula E0 + L6/16 | 3.8688 |
| 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 | NPTR uses the NPT basic dimensions (Table 2) and NPT L1 gages; Table 5 gives the shortening L6, the recess and the gage-notch distance S. Table 5 values were not recalculated in 2013 (Foreword (d)). Para. 5.2 example (1/2-14): the start end of the modified basic thread lies 4 threads inside the face of the fitting; gaging tolerance 4 turns large to 5 turns large. Chamfer: internal threads with chamfer larger than the major dia are gaged from the last scratch of the chamfer cone (para. 3.2.4.2, Fig. 5); otherwise from the end of the fitting (para. 3.2.4.4). |
| Lookup key | 3-1/2-8 NPTR · 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.