2 1/2-8 NPTF NPTF (Dryseal taper) External Thread Specifications
2 1/2-8 NPTF-1
ASMESTD: ASME B1.20.3SOURCE: Table 3 · p. 7TYPE: External (bolt/screw)CLASS: 1SERIES: NPTF (Dryseal taper)TAPER 1:16
Major diameter
2.8678 – 2.8488
max – min, at the gage plane
Pitch diameter
2.76997 – 2.75435
max – min, at the gage plane
Minor diameter
2.6807 – 2.6599
max – min, at the gage plane
Pitch
0.12500
8 threads per inch
Thread profile
External 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 D1Crest of the external thread | 2.8678 | 2.8488 | 0.0190 | |
| Pitch diameter E1Hand-tight plane, L1 from the small end (E1) | 2.76216 | 2.76997 | 2.75435 | 0.01562 |
| Minor diameter K1Root of the external thread | 2.6807 | 2.6599 | 0.0208 | |
| Crest truncation fcFig. 2, both threads | 0.0069 | 0.0052 | ||
| Root truncation frFig. 2; at least the mating crest’s maximum | 0.0095 | 0.0069 | ||
| Crest flat FcEquivalent width of flat (Fig. 2) | 0.008 | 0.006 | ||
| Root flat FrEquivalent width of flat (Fig. 2) | 0.011 | 0.008 |
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 0 | 2.71953 | 2.72734 | 2.71172 | 2.8157 | 2.8252 | 2.8062 | 2.6277 | 2.6381 | 2.6173 |
| Plane 1 | 2.76216 | 2.76997 | 2.75435 | 2.8583 | 2.8678 | 2.8488 | 2.6703 | 2.6807 | 2.6599 |
| Plane 2 | 2.79062 | 2.79843 | 2.78281 | 2.8868 | 2.8963 | 2.8773 | 2.6988 | 2.7092 | 2.6884 |
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
- 0.007813 in
- Gage tolerance, plus / minus ±
- 1 / 1 turns
- Gage reference plane
- Hand-tight plane, L1 from the small end (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
- 0° 50′
- Crest/root interference at flank contact, min – max δ
- 0.0000 – 0.0043 in
- Recommended mating thread
- NPTF internal; PTF-SAE SHORT internal; NPSF; NPSI (Table 1)
Basic pitch diameters and lengths
- Pitch dia at the small end of the external thread E0
- 2.71953 in
- Pitch dia at the hand-tight plane E1
- 2.76216 in
- Pitch dia at L2 E2
- 2.79062 in
- Pitch dia at the small end of the internal thread E3
- 2.6961 in
- Hand engagement L1
- 0.6820 · 5.46 in, turns
- Wrench make-up L3
- 0.3750 · 0.02344 in, PD interference
- Full external thread L2
- 1.1375 · 9.1 in, threads
- Vanish threads V
- 0.3781 · 3.025 in, threads
- Shoulder length Ls
- 1.5156 in
- External thread for draw
- 0.4555 · 3.64 in, threads
- OD of fitting D2
- 2.893 in
- Engagement in a PTF-SAE SHORT internal thread Le
- 0.87 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.
- Rounding only (last digit): Fig. 2 trunc min crest: printed 0.0052, 0.042p = 0.00525 (half-way value printed rounded down).
Mates
2 1/2-8 NPTF-1 screws into a thread of the same standard at the same size and pitch. The standard mating internal 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 2 1/2-8 NPTF-1every column of the workbook row
| Designation | 2 1/2-8 NPTF-1 |
|---|---|
| Alternate designation | 2 1/2-8 NPTF (class not denoted = Class 1, para. 1.3.1) |
| Threads per inch | 8 |
| Thread form | Dryseal 60°, taper 1:16 |
| Series (Table 1) | NPTF |
| Series name | Dryseal USA (American) Standard Taper Pipe Thread |
| External / internal | External |
| Class | 1 |
| Nominal size | 2 1/2 |
| Pitch P (in.) | 0.12500000 |
| Pipe OD D, basic size (in.) | 2.875 |
| 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 | Hand-tight plane, L1 from the small end (E1) |
| Basic pitch dia at gage plane (in.) | 2.76216 |
| 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.) | 2.8678 |
| Major dia min, D at gage plane (in.) | 2.8488 |
| Major dia tolerance, D (in.) | 0.0190 |
| Pitch dia max, E at gage plane (in.) | 2.76997 |
| Pitch dia min, E at gage plane (in.) | 2.75435 |
| PD tolerance, E (in.) | 0.01562 |
| Minor dia max, K at gage plane (in.) | 2.6807 |
| Minor dia min, K at gage plane (in.) | 2.6599 |
| Minor dia tolerance, K (in.) | 0.0208 |
| 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) | 0.8253 |
| Lead angle (deg min) | 0° 50′ |
| Stress area | not given in the standard |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (in.) | 0.108253 |
| H as printed, Fig. 2 (in.) | 0.10825 |
| Thread height h max (in.) | 0.096153 |
| Thread height h min (in.) | 0.091853 |
| Crest truncation min (in.) | 0.0052 |
| Crest truncation max (in.) | 0.0069 |
| Root truncation min (in.) | 0.0069 |
| Root truncation max (in.) | 0.0095 |
| Crest flat min (in.) | 0.006 |
| Crest flat max (in.) | 0.008 |
| Root flat min (in.) | 0.008 |
| Root flat max (in.) | 0.011 |
| Sweep cut: start diameter at gage plane (in.) | 2.8678 |
| Sweep cut: tip diameter at gage plane (in.) | 2.6807 |
| Sweep cut: depth (in.) | 0.09355 |
| Sweep cut: tip flat (in.) | 0.0110 |
| Sweep cut: profile width at start diameter (in.) | 0.11902 |
| Crest flat as modelled (in.) | 0.00598 |
| Sweep cut: taper on diameter (in./in.) | 0.0625 |
| Check against formulas | Rounding only (last digit): Fig. 2 trunc min crest: printed 0.0052, 0.042p = 0.00525 (half-way value printed rounded down). |
| 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.) | 0.007813 |
| E0: PD at small end of external thread (in.) | 2.71953 |
| E1: PD at hand-tight plane (in.) | 2.76216 |
| E2: PD at L2, Table 3 (in.) | 2.79062 |
| E3: PD at small end of internal, L1 + L3 (in.) | 2.6961 |
| PD at end of this full external thread (in.) | 2.79062 |
| PD at small end of external, max (in.) | 2.72734 |
| PD at small end of external, min (in.) | 2.71172 |
| Plane 0: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E0 basic: pitch dia at plane 0 (in.) | 2.71953 |
| E0 max (in.) | 2.72734 |
| E0 min (in.) | 2.71172 |
| D0 basic, calculated (in.) | 2.8157 |
| D0 max: major dia at plane 0 (in.) | 2.8252 |
| D0 min (in.) | 2.8062 |
| K0 basic, calculated (in.) | 2.6277 |
| K0 max: minor dia at plane 0 (in.) | 2.6381 |
| K0 min (in.) | 2.6173 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.6820 |
| E1 basic: pitch dia at plane 1 (in.) | 2.76216 |
| E1 max (in.) | 2.76997 |
| E1 min (in.) | 2.75435 |
| D1 basic, calculated (in.) | 2.8583 |
| D1 max: major dia at plane 1 (in.) | 2.8678 |
| D1 min (in.) | 2.8488 |
| K1 basic, calculated (in.) | 2.6703 |
| K1 max: minor dia at plane 1 (in.) | 2.6807 |
| K1 min (in.) | 2.6599 |
| Plane 2: distance from small end (ext) or face (int) (in.) | 1.1375 |
| E2 basic: pitch dia at plane 2 (in.) | 2.79062 |
| E2 max (in.) | 2.79843 |
| E2 min (in.) | 2.78281 |
| D2 basic, calculated (in.) | 2.8868 |
| D2 max: major dia at plane 2 (in.) | 2.8963 |
| D2 min (in.) | 2.8773 |
| K2 basic, calculated (in.) | 2.6988 |
| K2 max: minor dia at plane 2 (in.) | 2.7092 |
| K2 min (in.) | 2.6884 |
| Distance x measured from | small end of the external thread |
| PD change along x (+1 grows, −1 shrinks, 0 straight) | 1 |
| E at x = 0, basic (in.) | 2.71953 |
| Hand engagement L1 (in.) | 0.682 |
| L1 (threads) | 5.46 |
| Hand-tight engagement of this thread (in.) | 0.6820 |
| Hand-tight engagement (turns) | 5.46 |
| Wrench make-up L3 = 3p (in.) | 0.3750 |
| PD interference from L3 make-up, on dia (in.) | 0.02344 |
| Length of full external thread L2 (in.) | 1.1375 |
| Full external thread (threads) | 9.1 |
| Vanish thds V + 1p + 1/2p (in.) | 0.3781 |
| V + 1p + 1/2p (threads) | 3.025 |
| Shoulder length L2 + 3p approx. (in.) | 1.5156 |
| External thread for draw (in.) | 0.4555 |
| Draw (threads) | 3.64 |
| OD of fitting D2 (in.) | 2.893 |
| 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.) | 0.0043 |
| Table 2: engagement in NPSI internal (in.) | 1.06 |
| Table 2: engagement in NPTF internal (in.) | 0.99 |
| Table 2: engagement in NPSF internal (in.) | 0.93 |
| Table 2: engagement in PTF-SAE SHORT internal (in.) | 0.87 |
| Recommended mating thread (Table 1, C.3) | NPTF internal; PTF-SAE SHORT internal; NPSF; NPSI (Table 1) |
| Lookup key | 2 1/2-8 NPTF-1 | External |
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.