1/4-18 NPTF NPTF (Dryseal taper) Internal Thread Specifications
1/4-18 NPTF-1
ASMESTD: ASME B1.20.3SOURCE: Table 3 · p. 7TYPE: Internal (nut/tapped hole)CLASS: 1SERIES: NPTF (Dryseal taper)TAPER 1:16
Major diameter
0.5346 – 0.5241
max – min, at the gage plane
Pitch diameter
0.49510 – 0.48816
max – min, at the gage plane
Minor diameter
0.4556 – 0.4452
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 1
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter D1Root of the internal thread | 0.5346 | 0.5241 | 0.0105 | |
| Pitch diameter E1Large end, face of the internal thread (E1) | 0.49163 | 0.49510 | 0.48816 | 0.00694 |
| Minor diameter K1Crest of the internal thread | 0.4556 | 0.4452 | 0.0104 | |
| Crest truncation fcFig. 2, both threads | 0.0043 | 0.0026 | ||
| Root truncation frFig. 2; at least the mating crest’s maximum | 0.0061 | 0.0043 | ||
| Crest flat FcEquivalent width of flat (Fig. 2) | 0.005 | 0.003 | ||
| Root flat FrEquivalent width of flat (Fig. 2) | 0.007 | 0.005 |
Dimensions in inches. Values are those of the standard (ASME B1.20.3); 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.
Diameters along the taper D major, E pitch, K minor (Table A1 notation), inches
| Plane | E basic | E max | E min | D basic | D max | D min | K basic | K max | K min |
|---|---|---|---|---|---|---|---|---|---|
| Plane 1 | 0.49163 | 0.49510 | 0.48816 | 0.5293 | 0.5346 | 0.5241 | 0.4504 | 0.4556 | 0.4452 |
| Plane 0 | 0.47739 | 0.48086 | 0.47392 | 0.5151 | 0.5204 | 0.5098 | 0.4362 | 0.4413 | 0.4310 |
| Plane 3 | 0.46700 | 0.47047 | 0.46353 | 0.5047 | 0.5100 | 0.4994 | 0.4258 | 0.4310 | 0.4206 |
The standard defines no basic major or minor diameter: D basic and K basic are calculated with the mean Fig. 2 truncations. On the taper every diameter changes by 0.0625 in. per inch of length.
Taper, lengths and gaging
Taper, gage and sealing para. 1.6, Fig. 1, Fig. 2
- Taper on the diameter T
- 0.0625 ± 0.0050 in/in
- Half apex angle of the pitch cone β
- 1° 47′ 24″
- Included taper angle 2β
- 3° 34′ 47″
- Taper on the radius
- 0.03125 in/in
- Taper per thread on the diameter
- 0.003472 in
- Gage tolerance, plus / minus ±
- 1 / 1 turns
- Gage reference plane
- Large end, face of the internal thread (E1)
- PD tolerance basis
- Plus and minus 1 turn of the L1 gage (para. 1.6); L2/L3 gage within ±1/2 turn of the actual L1 position
- Lead angle at the basic PD λ
- 2° 04′
- Crest/root interference at flank contact, min – max δ
- 0.0000 – 0.0035 in
- Recommended mating thread
- NPTF external; PTF-SAE SHORT external (Table 1)
Basic pitch diameters and lengths
- Pitch dia at the small end of the external thread E0
- 0.47739 in
- Pitch dia at the hand-tight plane E1
- 0.49163 in
- Pitch dia at L2 E2
- 0.5025 in
- Pitch dia at the small end of the internal thread E3
- 0.467 in
- Hand engagement L1
- 0.2278 · 4.10 in, turns
- Wrench make-up L3
- 0.1667 · 0.01042 in, PD interference
- Full internal thread Lt
- 0.3945 in
- Engagement with a PTF-SAE SHORT external thread Le
- 0.31 in
Notes
- Longest thread of all Dryseal types; interference (sealing) theoretically at every engaged root and crest (para. 1.4.1.1). Class 1: inspection of root and crest truncation not required. Class 2: inspection required; direct measurement of crest and root truncation is the referee method (paras. 1.1.6, 1.4.2). Limits at the gage plane are calculated: pitch diameter from the basic PD and the gage tolerance in turns x 0.0625p; major and minor diameters from the pitch-diameter limits, H = 0.866025p and the Fig. 2 truncation limits.
Mates
1/4-18 NPTF-1 takes a thread of the same standard at the same size and pitch. The standard mating external thread is class 1. 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 1/4-18 NPTF-1every column of the workbook row
| Designation | 1/4-18 NPTF-1 |
|---|---|
| Alternate designation | 1/4-18 NPTF (class not denoted = Class 1, para. 1.3.1) |
| Threads per inch | 18 |
| Thread form | Dryseal 60°, taper 1:16 |
| Series (Table 1) | NPTF |
| Series name | Dryseal USA (American) Standard Taper Pipe Thread |
| External / internal | Internal |
| Class | 1 |
| Nominal size | 1/4 |
| Pitch P (in.) | 0.05555556 |
| Pipe OD D, basic size (in.) | 0.54 |
| Size category | Standard series (para. 1.2) |
| Source table | Table 3 |
| Printed page | 7 |
| Thread type (para. 1.2) | Type 1 |
| Gage (reference) plane for the limits | Large end, face of the internal thread (E1) |
| Basic pitch dia at gage plane (in.) | 0.49163 |
| Gage tolerance plus (turns) | 1 |
| Gage tolerance minus (turns) | 1 |
| PD tolerance basis | Plus and minus 1 turn of the L1 gage (para. 1.6); L2/L3 gage within ±1/2 turn of the actual L1 position |
| Major dia max, D at gage plane (in.) | 0.5346 |
| Major dia min, D at gage plane (in.) | 0.5241 |
| Major dia tolerance, D (in.) | 0.0105 |
| Pitch dia max, E at gage plane (in.) | 0.49510 |
| Pitch dia min, E at gage plane (in.) | 0.48816 |
| PD tolerance, E (in.) | 0.00694 |
| Minor dia max, K at gage plane (in.) | 0.4556 |
| Minor dia min, K at gage plane (in.) | 0.4452 |
| Minor dia tolerance, K (in.) | 0.0104 |
| Allowance | none (para. 1.3.2) |
| Root radius | not specified; crests and roots may be rounded within the Fig. 2 truncation limits (para. 1.6) |
| Lead variation ± | not given in B1.20.3 (gaging, B1.20.5) |
| Half-angle variation ± | not given in B1.20.3 (gaging, B1.20.5) |
| Lead angle at basic PD (deg) | 2.0600 |
| Lead angle (deg min) | 2° 04′ |
| Stress area | not given in the standard |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (in.) | 0.048113 |
| H as printed, Fig. 2 (in.) | 0.04811 |
| Thread height h max (in.) | 0.041213 |
| Thread height h min (in.) | 0.037713 |
| Crest truncation min (in.) | 0.0026 |
| Crest truncation max (in.) | 0.0043 |
| Root truncation min (in.) | 0.0043 |
| Root truncation max (in.) | 0.0061 |
| Crest flat min (in.) | 0.003 |
| Crest flat max (in.) | 0.005 |
| Root flat min (in.) | 0.005 |
| Root flat max (in.) | 0.007 |
| Sweep cut: start diameter at gage plane (in.) | 0.4452 |
| Sweep cut: tip diameter at gage plane (in.) | 0.5241 |
| Sweep cut: depth (in.) | 0.03945 |
| Sweep cut: tip flat (in.) | 0.0070 |
| Sweep cut: profile width at start diameter (in.) | 0.05255 |
| Crest flat as modelled (in.) | 0.00301 |
| Sweep cut: taper on diameter (in./in.) | 0.0625 |
| Check against formulas | OK |
| Notes | Longest thread of all Dryseal types; interference (sealing) theoretically at every engaged root and crest (para. 1.4.1.1). Class 1: inspection of root and crest truncation not required. Class 2: inspection required; direct measurement of crest and root truncation is the referee method (paras. 1.1.6, 1.4.2). Limits at the gage plane are calculated: pitch diameter from the basic PD and the gage tolerance in turns x 0.0625p; major and minor diameters from the pitch-diameter limits, H = 0.866025p and the Fig. 2 truncation limits. |
| Taper on diameter (in./in.) | 0.0625 |
| Taper tolerance ± (in./in.) | 0.0050 |
| Half apex angle of pitch cone β (deg) | 1.7899 |
| β (deg min sec) | 1° 47′ 24″ |
| Included taper angle 2β (deg) | 3.5798 |
| 2β (deg min sec) | 3° 34′ 47″ |
| Taper on radius (in./in.), 1 in 32 | 0.03125 |
| β min from taper tolerance (deg) | 1.6468 |
| β max from taper tolerance (deg) | 1.9330 |
| Taper per thread on dia, 0.0625p (in.) | 0.003472 |
| E0: PD at small end of external thread (in.) | 0.47739 |
| E1: PD at hand-tight plane (in.) | 0.49163 |
| E2: PD at L2, Table 3 (in.) | 0.5025 |
| E3: PD at small end of internal, L1 + L3 (in.) | 0.467 |
| Plane 0: distance from small end (ext) or face (int) (in.) | 0.2278 |
| E0 basic: pitch dia at plane 0 (in.) | 0.47739 |
| E0 max (in.) | 0.48086 |
| E0 min (in.) | 0.47392 |
| D0 basic, calculated (in.) | 0.5151 |
| D0 max: major dia at plane 0 (in.) | 0.5204 |
| D0 min (in.) | 0.5098 |
| K0 basic, calculated (in.) | 0.4362 |
| K0 max: minor dia at plane 0 (in.) | 0.4413 |
| K0 min (in.) | 0.4310 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E1 basic: pitch dia at plane 1 (in.) | 0.49163 |
| E1 max (in.) | 0.49510 |
| E1 min (in.) | 0.48816 |
| D1 basic, calculated (in.) | 0.5293 |
| D1 max: major dia at plane 1 (in.) | 0.5346 |
| D1 min (in.) | 0.5241 |
| K1 basic, calculated (in.) | 0.4504 |
| K1 max: minor dia at plane 1 (in.) | 0.4556 |
| K1 min (in.) | 0.4452 |
| Plane 3: distance from small end (ext) or face (int) (in.) | 0.3945 |
| E3 basic: pitch dia at plane 3 (in.) | 0.46700 |
| E3 max (in.) | 0.47047 |
| E3 min (in.) | 0.46353 |
| D3 basic, calculated (in.) | 0.5047 |
| D3 max: major dia at plane 3 (in.) | 0.5100 |
| D3 min (in.) | 0.4994 |
| K3 basic, calculated (in.) | 0.4258 |
| K3 max: minor dia at plane 3 (in.) | 0.4310 |
| K3 min (in.) | 0.4206 |
| Distance x measured from | face (large end) of the internal thread |
| PD change along x (+1 grows, −1 shrinks, 0 straight) | -1 |
| E at x = 0, basic (in.) | 0.49163 |
| Hand engagement L1 (in.) | 0.2278 |
| L1 (threads) | 4.1 |
| Hand-tight engagement of this thread (in.) | 0.2278 |
| Hand-tight engagement (turns) | 4.10 |
| Wrench make-up L3 = 3p (in.) | 0.1667 |
| PD interference from L3 make-up, on dia (in.) | 0.01042 |
| Length of full internal thread (in.) | 0.3945 |
| Full internal thread (threads) | 7.1 |
| L2 / L3 gage correlation with L1 ± (turns) | 0.5 |
| Crest/root interference at flank contact, min (radial, in.) | 0.0000 |
| Crest/root interference at flank contact, max (radial, in.) | 0.0035 |
| Table 2: engagement with NPTF external (in.) | 0.37 |
| Table 2: engagement with PTF-SAE SHORT external (in.) | 0.31 |
| Table B1: probable drill oversize cut (in.) | 0.0047 |
| Table B1: minor dia at L1 from large end / straight minor (in.) | 0.4362 |
| Table B1: minor dia at L1 + L3 (in.) | 0.4258 |
| Table B1: theoretical drill, no reamer / straight (in.) | 0.4315 |
| Table B1: suggested drill, no reamer / straight | 7/16 (0.438) |
| Table B1: theoretical drill with reamer (in.) | 0.4211 |
| Table B1: suggested drill with reamer | 27/64 (0.422) |
| Recommended mating thread (Table 1, C.3) | NPTF external; PTF-SAE SHORT external (Table 1) |
| Lookup key | 1/4-18 NPTF-1 | Internal |
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ASME B1.20.3: Dryseal Pipe Threads, Inch. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.