1/8-27 NPSF NPSF (Dryseal fuel, internal straight) Internal Thread Specifications
1/8-27 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
0.3978 – 0.3909
max – min, at the gage plane
Pitch diameter
0.37270 – 0.36920
max – min, at the gage plane
Minor diameter
0.3476 – 0.3406
max – min, at the gage plane
Pitch
0.03704
27 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 | 0.3978 | 0.3909 | 0.0069 | |
| Pitch diameter EAny plane (straight thread); basic = E1 of Table 3 as read by the L1 taper plug gage | 0.3736 | 0.37270 | 0.36920 | 0.00350 |
| Minor diameter KCrest of the internal thread | 0.3476 | 0.3406formula gives 0.3405 | 0.0070 | |
| Crest truncation fcFig. 2, both threads | 0.0035 | 0.0017 | ||
| Root truncation frFig. 2; at least the mating crest’s maximum | 0.0052 | 0.0035 | ||
| Crest flat FcEquivalent width of flat (Fig. 2) | 0.004 | 0.002 | ||
| Root flat FrEquivalent width of flat (Fig. 2) | 0.006 | 0.004 |
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 | — | 0.37270 | 0.36920 | — | 0.3978 | 0.3909 | — | 0.3476 | 0.3406 |
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
- 1° 48′
- 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
- 0.3736 in
- Full internal thread Lt
- 0.31 in
- Engagement with a PTF-SAE SHORT external thread Le
- 0.2 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 minor min: printed 0.3406, formula gives 0.3405 (+0.0001; PD min - H + 2 x min crest truncation (Fig. 2); derived) | Table 2 NPTF_ext_into_NPSF (L1+2p): printed 0.23, formula gives 0.24 (-0.01; L1 + 2p)
Mates
1/8-27 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 1/8-27 NPSFevery column of the workbook row
| Designation | 1/8-27 NPSF |
|---|---|
| Alternate designation | 0.4050 OD-27 NPSF |
| Threads per inch | 27 |
| 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 | 1/8 |
| Pitch P (in.) | 0.03703704 |
| Pipe OD D, basic size (in.) | 0.405 |
| 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.) | 0.3736 |
| 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.) | 0.3978 |
| Major dia min, D at gage plane (in.) | 0.3909 |
| Major dia tolerance, D (in.) | 0.0069 |
| Pitch dia max, E at gage plane (in.) | 0.37270 |
| Pitch dia min, E at gage plane (in.) | 0.36920 |
| PD tolerance, E (in.) | 0.00350 |
| Minor dia max, K at gage plane (in.) | 0.3476 |
| Minor dia min, K at gage plane (in.) | 0.3406 |
| Minor dia tolerance, K (in.) | 0.0070 |
| 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) | 1.8074 |
| Lead angle (deg min) | 1° 48′ |
| Stress area | not given in the standard |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (in.) | 0.032075 |
| H as printed, Fig. 2 (in.) | 0.03208 |
| Thread height h max (in.) | 0.026875 |
| Thread height h min (in.) | 0.023375 |
| Crest truncation min (in.) | 0.0017 |
| Crest truncation max (in.) | 0.0035 |
| Root truncation min (in.) | 0.0035 |
| Root truncation max (in.) | 0.0052 |
| Crest flat min (in.) | 0.002 |
| Crest flat max (in.) | 0.004 |
| Root flat min (in.) | 0.004 |
| Root flat max (in.) | 0.006 |
| Sweep cut: start diameter at gage plane (in.) | 0.3406 |
| Sweep cut: tip diameter at gage plane (in.) | 0.3909 |
| Sweep cut: depth (in.) | 0.02515 |
| Sweep cut: tip flat (in.) | 0.0060 |
| Sweep cut: profile width at start diameter (in.) | 0.03504 |
| Crest flat as modelled (in.) | 0.00200 |
| Sweep cut: taper on diameter (in./in.) | 0.0000 |
| Check against formulas | Rounding only (last digit): Table 6 minor min: printed 0.3406, formula gives 0.3405 (+0.0001; PD min - H + 2 x min crest truncation (Fig. 2); derived) | Table 2 NPTF_ext_into_NPSF (L1+2p): printed 0.23, formula gives 0.24 (-0.01; L1 + 2p) |
| 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.) | 0.3736 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E1 max (in.) | 0.37270 |
| E1 min (in.) | 0.36920 |
| D1 max: major dia at plane 1 (in.) | 0.3978 |
| D1 min (in.) | 0.3909 |
| K1 max: minor dia at plane 1 (in.) | 0.3476 |
| K1 min (in.) | 0.3406 |
| Distance x measured from | any plane (straight) |
| PD change along x (+1 grows, −1 shrinks, 0 straight) | 0 |
| Length of full internal thread (in.) | 0.31 |
| 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.) | 0.23 |
| Table 2: engagement with PTF-SAE SHORT external (in.) | 0.2 |
| Table B1: probable drill oversize cut (in.) | 0.0044 |
| Table B1: minor dia at L1 from large end / straight minor (in.) | 0.3406 |
| Table B1: theoretical drill, no reamer / straight (in.) | 0.3362 |
| Table B1: suggested drill, no reamer / straight | R (0.339) |
| Recommended mating thread (Table 1, C.3) | NPTF external (para. 1.4.1.3, Table 1) |
| Lookup key | 1/8-27 NPSF | Internal |
Related tools
- Thread standardsEvery ISO and ASME thread series and size, with limits and drawings.
- Thread Spec CalculatorDiameters, tolerances and geometry for ISO metric and unified threads.
- Thread Fit CheckPick a bolt and a nut and see the clearance or interference.
- Twist Drill SizesASME B94.11M drills: sizes, lengths, shanks and tolerances.
- Thread Designation GuideHow to read M10×1.5, 1/4-20 UNC-2A, NPT and more.
- Thread Notations & TermsSymbols and terms of a thread profile, explained.
ASME B1.20.3: Dryseal Pipe Threads, Inch. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.