1/4-18 NPSF NPSF (Dryseal fuel, internal straight) Internal Thread Specifications
1/4-18 NPSF
ASMESTD: ASME B1.20.3SOURCE: Table 6 · p. 10TYPE: Internal (nut/tapped hole)CLASS: —SERIES: NPSF (Dryseal fuel, internal straight)TAPER 1:16
Major diameter
0.5299 – 0.5211
max – min, at the gage plane
Pitch diameter
0.49040 – 0.48520
max – min, at the gage plane
Minor diameter
0.4509 – 0.4422
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 DRoot of the internal thread | 0.5299 | 0.5211 | 0.0088 | |
| Pitch diameter EAny plane (straight thread); basic = E1 of Table 3 as read by the L1 taper plug gage | 0.49163 | 0.49040formula gives 0.49030 | 0.48520formula gives 0.48510 | 0.00520 |
| Minor diameter KCrest of the internal thread | 0.4509 | 0.4422formula gives 0.4423 | 0.0087 | |
| 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 | — | 0.49040 | 0.48520 | — | 0.5299 | 0.5211 | — | 0.4509 | 0.4422 |
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 radius
- 0.00000 in/in
- Gage tolerance, plus / minus ±
- 0 / 1.5 turns
- Gage reference plane
- Any plane (straight thread); basic = E1 of Table 3 as read by the L1 taper plug gage
- PD tolerance basis
- Plus 0, minus 1-1/2 turns of the L1 gage (para. 1.6); PD limits printed in Table 6
- 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 (para. 1.4.1.3, Table 1)
Basic pitch diameters and lengths
- Pitch dia at the hand-tight plane E1
- 0.49163 in
- Full internal thread Lt
- 0.47 in
- Engagement with a PTF-SAE SHORT external thread Le
- 0.28 in
Notes
- Straight internal only. Root and crest interference will not occur for all threads; for soft or ductile materials or thick sections (para. 1.4.1.3). PD limits are printed (Table 6); the taper plug gage reads about 3/8 turn larger (note a). Major and minor diameters vary with the pitch diameter (note b). There is no external straight Dryseal thread (Table 1 note 5). 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.
- Rounding only (last digit): Table 6 PD max: printed 0.4904, formula gives 0.4903 (+0.0001; E1 - 0.375 turn x 0.0625p (note e)) | Table 6 PD min: printed 0.4852, formula gives 0.4851 (+0.0001; PD max - 1 1/2 turns x 0.0625p (note d)) | Table 6 minor min: printed 0.4422, formula gives 0.4423 (-0.0001; PD min - H + 2 x min crest truncation (Fig. 2); derived)
Mates
1/4-18 NPSF takes a thread of the same standard at the same size and pitch. The standard mating external thread is class —. 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 NPSFevery column of the workbook row
| Designation | 1/4-18 NPSF |
|---|---|
| Alternate designation | 0.5400 OD-18 NPSF |
| Threads per inch | 18 |
| Thread form | Dryseal 60°, straight |
| Series (Table 1) | NPSF |
| Series name | Dryseal USA (American) Standard Fuel Internal Straight Pipe Thread |
| External / internal | Internal |
| Class | — |
| 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 6 |
| Printed page | 10 |
| Thread type (para. 1.2) | Type 3 |
| Gage (reference) plane for the limits | Any plane (straight thread); basic = E1 of Table 3 as read by the L1 taper plug gage |
| Basic pitch dia at gage plane (in.) | 0.49163 |
| Gage tolerance plus (turns) | 0 |
| Gage tolerance minus (turns) | 1.5 |
| PD tolerance basis | Plus 0, minus 1-1/2 turns of the L1 gage (para. 1.6); PD limits printed in Table 6 |
| Major dia max, D at gage plane (in.) | 0.5299 |
| Major dia min, D at gage plane (in.) | 0.5211 |
| Major dia tolerance, D (in.) | 0.0088 |
| Pitch dia max, E at gage plane (in.) | 0.49040 |
| Pitch dia min, E at gage plane (in.) | 0.48520 |
| PD tolerance, E (in.) | 0.00520 |
| Minor dia max, K at gage plane (in.) | 0.4509 |
| Minor dia min, K at gage plane (in.) | 0.4422 |
| Minor dia tolerance, K (in.) | 0.0087 |
| 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.) | 0.4422 |
| Sweep cut: tip diameter at gage plane (in.) | 0.5211 |
| 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.) | 0.0000 |
| Check against formulas | Rounding only (last digit): Table 6 PD max: printed 0.4904, formula gives 0.4903 (+0.0001; E1 - 0.375 turn x 0.0625p (note e)) | Table 6 PD min: printed 0.4852, formula gives 0.4851 (+0.0001; PD max - 1 1/2 turns x 0.0625p (note d)) | Table 6 minor min: printed 0.4422, formula gives 0.4423 (-0.0001; PD min - H + 2 x min crest truncation (Fig. 2); derived) |
| Notes | Straight internal only. Root and crest interference will not occur for all threads; for soft or ductile materials or thick sections (para. 1.4.1.3). PD limits are printed (Table 6); the taper plug gage reads about 3/8 turn larger (note a). Major and minor diameters vary with the pitch diameter (note b). There is no external straight Dryseal thread (Table 1 note 5). 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.) | 0.0000 |
| Half apex angle of pitch cone β (deg) | 0.0000 |
| Taper on radius (in./in.), 1 in 32 | 0.00000 |
| E1: PD at hand-tight plane (in.) | 0.49163 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E1 max (in.) | 0.49040 |
| E1 min (in.) | 0.48520 |
| D1 max: major dia at plane 1 (in.) | 0.5299 |
| D1 min (in.) | 0.5211 |
| K1 max: minor dia at plane 1 (in.) | 0.4509 |
| K1 min (in.) | 0.4422 |
| Distance x measured from | any plane (straight) |
| PD change along x (+1 grows, −1 shrinks, 0 straight) | 0 |
| Length of full internal thread (in.) | 0.47 |
| Full internal thread (threads) | 8.44 |
| 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.) | 0.34 |
| Table 2: engagement with PTF-SAE SHORT external (in.) | 0.28 |
| Table B1: probable drill oversize cut (in.) | 0.0047 |
| Table B1: minor dia at L1 from large end / straight minor (in.) | 0.4422 |
| Table B1: theoretical drill, no reamer / straight (in.) | 0.4375 |
| Table B1: suggested drill, no reamer / straight | 7/16 (0.438) |
| Recommended mating thread (Table 1, C.3) | NPTF external (para. 1.4.1.3, Table 1) |
| Lookup key | 1/4-18 NPSF | 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.