3/8-18 NPT NPT (taper pipe thread) Internal Thread Specifications
3/8-18 NPT · 0.375-18 NPT
ASMESTD: ASME B1.20.1SOURCE: Table 2 · p. 9TYPE: Internal (nut/tapped hole)CLASS: —SERIES: NPT (taper pipe thread)TAPER 1:16
Major diameter
0.67493 – 0.66187
max – min, at the gage plane
Pitch diameter
0.63048 – 0.62354
max – min, at the gage plane
Minor diameter
0.59215 – 0.57909
max – min, at the gage plane
Pitch
0.05556
18 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 | 0.67493 | 0.66187 | 0.01306 | |
| 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 | 0.62701 | 0.63048 | 0.62354 | 0.00694 |
| Minor diameter K1Crest of the internal thread at the gage plane | 0.59215 | 0.57909 | 0.01306 | |
| Allowance esNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant | — | |||
| Crest truncation fcTable 1 (both threads) | 0.004889 | 0.001833 | ||
| Root truncation frTable 1 (both threads) | 0.004889 | 0.001833 | ||
| Crest flat FcEquivalent width of flat | 0.005667 | 0.002111 | ||
| Root flat FrEquivalent width of flat | 0.005667 | 0.002111 |
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 ±
- 2.0 °
- Lead angle at the basic PD λ
- 1° 37′
- Gaging method
- L1 ring gage (external) / L1 plug gage (internal), or L1 indicating gage (para. 3.2)
- Gaging tolerance
- ±1 turn
- Gaging position
- Gaging notch of plug flush with the large end of the fitting
- Sealing
- Pressure-tight: wrench-tight with sealant (para. 1.4)
Basic pitch diameters and lengths Table 3
- Pitch dia at the small end of the external thread E0
- 0.61201 in
- Hand-tight engagement L1
- 0.2400 · 4.32 in, threads
- Pitch dia at the hand-tight plane E1
- 0.62701 in
- Effective external thread L2
- 0.4078 · 7.34 in, threads
- Pitch dia at L2 E2
- 0.63750 in
- Wrench make-up length L3
- 0.1667 · 3 in, threads
- Pitch dia at the wrench make-up plane E3
- 0.60160 in
- Complete external threads L5
- 0.2967 in
- Pitch dia at L5 E5
- 0.63056 in
- Vanish thread V
- 0.1928 · 3.47 in, threads
- Overall external thread length L4
- 0.6006 in
- Basic minor dia at the small end K0
- 0.5675 informula gives 0.5676
- Change of diameter per turn
- 0.00347 in
- Hand-tight engagement range, ± 1 turn
- 0.1289 – 0.3511 in
- Wrench make-up interference on the PD
- 0.01042 in
- Hand-tight plus wrench make-up length L1+L3
- 0.4067 in
L1 plug and ring gages Tables 8 and 9
- Plug gage major dia at the E0 plane
- 0.64802 in
- Plug gage major dia at the gaging notch (E1)
- 0.66302 in
- Plug gage major dia at the large end (E2)
- 0.67350 in
- Gage pitch dia at the E0 plane
- 0.61201 in
- Gage pitch dia at the gaging notch (E1)
- 0.62701 in
- Gage pitch dia at the large end (E2)
- 0.63750 in
- Ring gage minor dia at the E0 plane
- 0.57601 in
- Ring gage minor dia at the gaging notch (E1)
- 0.59101 in
- Thickness of the L1 ring or gage elements
- 0.2400 in
- Working gage crest truncation
- 0.006056 in
- Maximum standoff between plug and ring gages
- 0.036 in
Working gage tolerances Table 7
- On the pitch diameter ±
- 0.0002 in
- Lead, plugs / rings
- 0.0002 / 0.0003 in
- Half angle ±, plugs / rings
- 15 / 20 min
- Taper, plugs / rings
- 0.0004 / 0.0007 in
- Major dia of plugs
- 0.0006 in
- Minor dia of rings
- 0.0006 in
- Total cumulative tolerance on the PD, plugs / rings
- 0.00092 / 0.00134 in
Notes
- Table 2 Notes: L1 is also the length of the L1 ring gage and the distance from the gaging notch to the small end of the L1 plug; E1 is also the PD at the gaging notch of the L1 plug; L2 is also the threaded length of the L1 plug; L4 is a reference dimension; L5 = L2 − 2P (the next two threads are complete at the root); K0 is given for selecting tap drills (Appendix B). Gaging: ±1 turn from basic (Fig. 4 / Fig. 6); with indicating gages ±0.0625/n on diameter (paras. 3.2.1.2, 3.2.2.2). The gaging tolerance can lengthen or shorten engagement by up to two turns; gage wear reduces it further (para. 3.1.9.1). Diameter limits here are calculated at the gage plane (E1 ± 0.0625/n on PD; major and minor add the Table 1 truncation limits). The standard itself tabulates basic dimensions only. 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).
- K0 (col. 24): printed 0.5675, formula 0.5676 (1 in last place, rounding)
Mates
3/8-18 NPT 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 3/8-18 NPTevery column of the workbook row
| Designation | 3/8-18 NPT |
|---|---|
| Decimal designation | 0.375-18 NPT |
| Threads per inch | 18 |
| Thread form | Taper |
| Series | NPT |
| Series name | American National Standard Taper Pipe Thread |
| External / internal | Internal |
| Class | — |
| Nominal pipe size | 3/8 |
| Pitch P (in.) | 0.05555556 |
| O.D. of pipe D, basic major dia (in.) | 0.6750 |
| Size category (Table 3 group) | 1/4, 3/8 (18 TPI) |
| Source table | Table 2 |
| Printed page | 9 |
| 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.) | 0.67493 |
| Major dia min (in.) | 0.66187 |
| Major dia tolerance (in.) | 0.01306 |
| 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.) | 0.62701 |
| Pitch dia max (in.) | 0.63048 |
| Pitch dia min (in.) | 0.62354 |
| PD tolerance (in.) | 0.00694 |
| Minor dia max (in.) | 0.59215 |
| Minor dia min (in.) | 0.57909 |
| Minor dia tolerance (in.) | 0.01306 |
| 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) | 2.0 |
| Taper tolerance max (in./ft) | 0.1250 |
| Taper tolerance min (in./ft) | -0.0625 |
| Lead / angle / taper source | Table 3, p. 10 |
| Lead angle at basic PD (deg) | 1.615514 |
| Lead angle (deg min) | 1° 37′ |
| Tensile stress area As (sq in.) | not given in the standard |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (in.) | 0.0481125 |
| Thread height h max (in.) | 0.04444 |
| Thread height h min (in.) | 0.03833 |
| Crest truncation min (in.) | 0.001833 |
| Crest truncation max (in.) | 0.004889 |
| Root truncation min (in.) | 0.001833 |
| Root truncation max (in.) | 0.004889 |
| Crest flat min (in.) | 0.002111 |
| Crest flat max (in.) | 0.005667 |
| Root flat min (in.) | 0.002111 |
| Root flat max (in.) | 0.005667 |
| Sweep cut: start diameter (in.) | 0.58256 |
| Sweep cut: tip diameter (in.) | 0.67146 |
| Sweep cut: depth (in.) | 0.044450 |
| Sweep cut: profile width at start diameter (in.) | 0.053445 |
| Sweep cut: tip flat (in.) | 0.002111 |
| 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.) | 0.61201 |
| L1 hand-tight engagement (in.) | 0.2400 |
| L1 (threads) | 4.32 |
| E1 pitch dia at hand-tight plane (in.) | 0.62701 |
| L2 effective thread, external (in.) | 0.4078 |
| L2 (threads) | 7.34 |
| E2 pitch dia at L2 plane (in.) | 0.63750 |
| L2 − L1 (in.) | 0.1678 |
| L2 − L1 (threads) | 3.02 |
| L3 wrench make-up, internal (in.) | 0.1667 |
| L3 (threads) | 3 |
| E3 pitch dia at wrench make-up plane (in.) | 0.60160 |
| V vanish thread (in.) | 0.1928 |
| V (threads) | 3.47 |
| L4 overall external thread length, reference (in.) | 0.6006 |
| L5 complete external threads (in.) | 0.2967 |
| E5 pitch dia at L5 plane (in.) | 0.63056 |
| Height of thread h, Table 2 col. 22 (in.) | 0.04444 |
| Change in dia per turn 0.0625/n (in.) | 0.00347 |
| K0 basic minor dia at small end (in.) | 0.5675 |
| E0 by formula D − (0.05D + 1.1)P | 0.61201 |
| E1 by formula E0 + 0.0625·L1 | 0.62701 |
| L2 by formula (0.80D + 6.8)P | 0.4078 |
| E2 by formula E0 + 0.0625·L2 | 0.63750 |
| E3 by formula E0 − 0.0625·L3 | 0.60160 |
| L5 by formula L2 − 2P | 0.2967 |
| E5 by formula E0 + 0.0625·L5 | 0.63056 |
| V by formula 3.47P | 0.1928 |
| L4 by formula L2 + V | 0.6006 |
| K0 by formula E0 − 0.8P | 0.5676 |
| Gaging method | L1 ring gage (external) / L1 plug gage (internal), or L1 indicating gage (para. 3.2) |
| Gaging tolerance (turns) | ±1 turn |
| Gaging limit large (turns) | 1 |
| Gaging limit small (turns) | 1 |
| Basic gaging position | Gaging notch of plug flush with the large end of the fitting |
| Hand-tight engagement min, ±1 turn both parts (in.) | 0.1289 |
| Hand-tight engagement max, ±1 turn both parts (in.) | 0.3511 |
| Wrench make-up interference on PD, basic L3/16 (in.) | 0.01042 |
| Hand-tight + wrench make-up length L1 + L3 (in.) | 0.4067 |
| Sealing | Pressure-tight: wrench-tight with sealant (para. 1.4) |
| Gage: plug major dia at E0 plane (in.) | 0.64802 |
| Gage: plug major dia at E1 gaging notch (in.) | 0.66302 |
| Gage: plug major dia at E2 large end (in.) | 0.67350 |
| Gage: pitch dia at E0 plane (in.) | 0.61201 |
| Gage: pitch dia at E1 gaging notch (in.) | 0.62701 |
| Gage: pitch dia at E2 large end (in.) | 0.63750 |
| Gage: ring minor dia at E0 plane (in.) | 0.57601 |
| Gage: ring minor dia at E1 gaging notch (in.) | 0.59101 |
| Gage: thickness of L1 ring or gage elements (in.) | 0.2400 |
| Gage: working gage crest truncation (in.) | 0.006056 |
| Gage: root must clear 0.0381P width (in.) | 0.002117 |
| Gage: plug major at E0 by formula E0 + (H − 2·trunc) | 0.64802 |
| Working gage tol. on PD ± (in.) | 0.0002 |
| Working gage tol. lead, plugs (in.) | 0.0002 |
| Working gage tol. lead, rings (in.) | 0.0003 |
| Working gage tol. half angle, plugs ± (min) | 15 |
| Working gage tol. half angle, rings ± (min) | 20 |
| Working gage tol. taper, plugs + (in.) | 0.0004 |
| Working gage tol. taper, rings − (in.) | 0.0007 |
| Working gage tol. major dia, plugs − (in.) | 0.0006 |
| Working gage tol. minor dia, rings + (in.) | 0.0006 |
| Working gage total cumulative tol. on PD, plugs (in.) | 0.00092 |
| Working gage total cumulative tol. on PD, rings (in.) | 0.00134 |
| Max standoff between plug and ring gages (in.) | 0.036 |
| Check against formulas | K0 (col. 24): printed 0.5675, formula 0.5676 (1 in last place, rounding) |
| Notes | Table 2 Notes: L1 is also the length of the L1 ring gage and the distance from the gaging notch to the small end of the L1 plug; E1 is also the PD at the gaging notch of the L1 plug; L2 is also the threaded length of the L1 plug; L4 is a reference dimension; L5 = L2 − 2P (the next two threads are complete at the root); K0 is given for selecting tap drills (Appendix B). Gaging: ±1 turn from basic (Fig. 4 / Fig. 6); with indicating gages ±0.0625/n on diameter (paras. 3.2.1.2, 3.2.2.2). The gaging tolerance can lengthen or shorten engagement by up to two turns; gage wear reduces it further (para. 3.1.9.1). Diameter limits here are calculated at the gage plane (E1 ± 0.0625/n on PD; major and minor add the Table 1 truncation limits). The standard itself tabulates basic dimensions only. 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 | 3/8-18 NPT · Internal |
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