1/2-18 F-PTF F-PTF (Dryseal fine) Internal Thread Specifications
1/2-18 F-PTF
ASMESTD: ASME B1.20.3SOURCE: Table C1 · p. 18TYPE: Internal (nut/tapped hole)CLASS: —SERIES: F-PTF (Dryseal fine)TAPER 1:16
Major diameter
0.8350 – 0.8245
max – min, at the gage plane
Pitch diameter
0.79552 – 0.78858
max – min, at the gage plane
Minor diameter
0.7560 – 0.7457
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 | 0.8350 | 0.8245 | 0.0105 | |
| Pitch diameter E1Large end, face of the internal thread (E1) | 0.79205 | 0.79552 | 0.78858 | 0.00694 |
| Minor diameter K1Crest of the internal thread | 0.7560 | 0.7457 | 0.0103 | |
| 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 | 0.79205 | 0.79552 | 0.78858 | 0.8298 | 0.8350 | 0.8245 | 0.7508 | 0.7560 | 0.7457 |
| Plane 0 | 0.77655 | 0.78002 | 0.77308 | 0.8143 | 0.8195 | 0.8090 | 0.7353 | 0.7405 | 0.7302 |
| Plane 3 | 0.76613 | 0.76960 | 0.76266 | 0.8038 | 0.8091 | 0.7986 | 0.7249 | 0.7301 | 0.7197 |
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 or minus the taper of one thread on the diameter (para. C.6)
- Lead angle at the basic PD λ
- 1° 17′
- Crest/root interference at flank contact, min – max δ
- 0.0000 – 0.0035 in
- Recommended mating thread
- F-PTF external of the same size (full length, C.4)
Basic pitch diameters and lengths
- Pitch dia at the small end of the external thread E0
- 0.77655 informula gives 0.77656
- Pitch dia at the hand-tight plane E1
- 0.79205 informula gives 0.79208
- Pitch dia at L2 E2
- 0.80249 informula gives 0.80250
- Pitch dia at the small end of the internal thread E3
- 0.76613 informula gives 0.76614
- Hand engagement L1
- 0.2480 · 4.46 in, turns
- Wrench make-up L3
- 0.1667 · 0.01042 in, PD interference
- Full internal thread Lt
- 0.415 in
Notes
- Fine series: the thread pitch of the next smaller pipe size applied to the next larger size; full length external and internal threads (C.4). 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 C1 E0: printed 0.77655, formula gives 0.77656 (-0.00001; E0 = D - (0.05D + 1.1)p (C.6)) | Table C1 E1: printed 0.79205; exact formula gives 0.79208 (-0.00003). The printed value is E0 + 0.0625 x the printed L1, which is rounded to 3 places, so this is rounding. | Table C1 E2: printed 0.80249, formula gives 0.80250 (-0.00001; E2 = D - 0.675p (C.6)) | Table C1 E3: printed 0.76613, formula gives 0.76614 (-0.00001; E3 = D - (0.05D + 1.2875)p (C.6))
Mates
1/2-18 F-PTF 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 1/2-18 F-PTFevery column of the workbook row
| Designation | 1/2-18 F-PTF |
|---|---|
| Alternate designation | 0.8400 OD-18 F-PTF |
| Threads per inch | 18 |
| Thread form | Dryseal 60°, taper 1:16 |
| Series (Table 1) | F-PTF |
| Series name | Dryseal Fine Taper Pipe Thread |
| External / internal | Internal |
| Class | — |
| Nominal size | 1/2 |
| Pitch P (in.) | 0.05555556 |
| Pipe OD D, basic size (in.) | 0.84 |
| Size category | Special series (Appendix C) |
| Source table | Table C1 |
| Printed page | 18 |
| Thread type (para. 1.2) | App. C.4 |
| Gage (reference) plane for the limits | Large end, face of the internal thread (E1) |
| Basic pitch dia at gage plane (in.) | 0.79205 |
| Gage tolerance plus (turns) | 1 |
| Gage tolerance minus (turns) | 1 |
| PD tolerance basis | Plus or minus the taper of one thread on the diameter (para. C.6) |
| Major dia max, D at gage plane (in.) | 0.8350 |
| Major dia min, D at gage plane (in.) | 0.8245 |
| Major dia tolerance, D (in.) | 0.0105 |
| Pitch dia max, E at gage plane (in.) | 0.79552 |
| Pitch dia min, E at gage plane (in.) | 0.78858 |
| PD tolerance, E (in.) | 0.00694 |
| Minor dia max, K at gage plane (in.) | 0.7560 |
| Minor dia min, K at gage plane (in.) | 0.7457 |
| Minor dia tolerance, K (in.) | 0.0103 |
| 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) | 1.2790 |
| Lead angle (deg min) | 1° 17′ |
| 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.) | 0.7457 |
| Sweep cut: tip diameter at gage plane (in.) | 0.8245 |
| Sweep cut: depth (in.) | 0.03940 |
| Sweep cut: tip flat (in.) | 0.0070 |
| Sweep cut: profile width at start diameter (in.) | 0.05250 |
| Crest flat as modelled (in.) | 0.00306 |
| Sweep cut: taper on diameter (in./in.) | 0.0625 |
| Check against formulas | Rounding only (last digit): Table C1 E0: printed 0.77655, formula gives 0.77656 (-0.00001; E0 = D - (0.05D + 1.1)p (C.6)) | Table C1 E1: printed 0.79205; exact formula gives 0.79208 (-0.00003). The printed value is E0 + 0.0625 x the printed L1, which is rounded to 3 places, so this is rounding. | Table C1 E2: printed 0.80249, formula gives 0.80250 (-0.00001; E2 = D - 0.675p (C.6)) | Table C1 E3: printed 0.76613, formula gives 0.76614 (-0.00001; E3 = D - (0.05D + 1.2875)p (C.6)) |
| Notes | Fine series: the thread pitch of the next smaller pipe size applied to the next larger size; full length external and internal threads (C.4). 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.) | 0.77655 |
| E1: PD at hand-tight plane (in.) | 0.79205 |
| E2: PD at L2, Table 3 (in.) | 0.80249 |
| E3: PD at small end of internal, L1 + L3 (in.) | 0.76613 |
| Plane 0: distance from small end (ext) or face (int) (in.) | 0.2480 |
| E0 basic: pitch dia at plane 0 (in.) | 0.77655 |
| E0 max (in.) | 0.78002 |
| E0 min (in.) | 0.77308 |
| D0 basic, calculated (in.) | 0.8143 |
| D0 max: major dia at plane 0 (in.) | 0.8195 |
| D0 min (in.) | 0.8090 |
| K0 basic, calculated (in.) | 0.7353 |
| K0 max: minor dia at plane 0 (in.) | 0.7405 |
| K0 min (in.) | 0.7302 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E1 basic: pitch dia at plane 1 (in.) | 0.79205 |
| E1 max (in.) | 0.79552 |
| E1 min (in.) | 0.78858 |
| D1 basic, calculated (in.) | 0.8298 |
| D1 max: major dia at plane 1 (in.) | 0.8350 |
| D1 min (in.) | 0.8245 |
| K1 basic, calculated (in.) | 0.7508 |
| K1 max: minor dia at plane 1 (in.) | 0.7560 |
| K1 min (in.) | 0.7457 |
| Plane 3: distance from small end (ext) or face (int) (in.) | 0.4147 |
| E3 basic: pitch dia at plane 3 (in.) | 0.76613 |
| E3 max (in.) | 0.76960 |
| E3 min (in.) | 0.76266 |
| D3 basic, calculated (in.) | 0.8038 |
| D3 max: major dia at plane 3 (in.) | 0.8091 |
| D3 min (in.) | 0.7986 |
| K3 basic, calculated (in.) | 0.7249 |
| K3 max: minor dia at plane 3 (in.) | 0.7301 |
| K3 min (in.) | 0.7197 |
| 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.) | 0.79205 |
| Hand engagement L1 (in.) | 0.248 |
| L1 (threads) | 4.47 |
| Hand-tight engagement of this thread (in.) | 0.2480 |
| Hand-tight engagement (turns) | 4.46 |
| 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.) | 0.415 |
| Full internal thread (threads) | 7.47 |
| Crest/root interference at flank contact, min (radial, in.) | 0.0000 |
| Crest/root interference at flank contact, max (radial, in.) | 0.0035 |
| Recommended mating thread (Table 1, C.3) | F-PTF external of the same size (full length, C.4) |
| Lookup key | 1/2-18 F-PTF | 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.