1/4-18 PTF-SAE-SHORT PTF-SAE SHORT Internal Thread Specifications
1/4-18 PTF-SAE SHORT
ASMESTD: ASME B1.20.3SOURCE: Table 5 · p. 9TYPE: Internal (nut/tapped hole)CLASS: —SERIES: PTF-SAE SHORTTAPER 1:16
Major diameter
0.5277 – 0.5189
max – min, at the gage plane
Pitch diameter
0.48815 – 0.48294
max – min, at the gage plane
Minor diameter
0.4486 – 0.4400
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 | 0.5277 | 0.5189 | 0.0088 | |
| Pitch diameter E1Large end, face of the internal thread (E1 short) | 0.48815 | 0.48815 | 0.48294 | 0.00521 |
| Minor diameter K1Crest of the internal thread | 0.4486 | 0.4400 | 0.0086 | |
| 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 | 0.48815 | 0.48815 | 0.48294 | 0.5259 | 0.5277 | 0.5189 | 0.4469 | 0.4486 | 0.4400 |
| Plane 0 | 0.47739 | 0.47739 | 0.47218 | 0.5151 | 0.5169 | 0.5081 | 0.4362 | 0.4379 | 0.4293 |
| Plane 3 | 0.46700 | 0.46700 | 0.46179 | 0.5047 | 0.5065 | 0.4977 | 0.4258 | 0.4275 | 0.4189 |
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 ±
- 0 / 1.5 turns
- Gage reference plane
- Large end, face of the internal thread (E1 short)
- PD tolerance basis
- Plus 0, minus 1-1/2 turns 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 (primary). Not designed for PTF-SAE SHORT external; at extreme tolerance limits may not assemble with it (Table 1 note 4)
Basic pitch diameters and lengths
- Pitch dia at the small end of the external thread E0
- 0.47739 in
- Pitch dia at the large end of the SAE short internal E1s
- 0.48815 in
- Pitch dia at the small end of the internal thread E3
- 0.467 in
- Hand engagement L1
- 0.1722 · 3.10 in, turns
- Wrench make-up L3
- 0.1667 · 0.01042 in, PD interference
- Full internal thread Lt
- 0.3389 in
- Hole depth for the SAE short tap
- 0.6722 in
- Engagement with a PTF-SAE SHORT external thread Le
- 0.26 in
Notes
- Conforms in all respects to NPTF except that one thread is removed (para. 1.4.1.2): at the small end of the external thread, at the large end of the internal thread. Gage tolerance read as turns of gage engagement: "minus" means fewer turns, i.e. a larger external thread and a smaller internal thread. This reading matches Table 1 notes 3 and 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 5 E1 short (alt): printed 0.48815, formula gives 0.48816 (-0.00001; E1 short = E1 - 0.0625p (one thread removed at large end))
Mates
1/4-18 PTF-SAE SHORT 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 PTF-SAE SHORTevery column of the workbook row
| Designation | 1/4-18 PTF-SAE SHORT |
|---|---|
| Alternate designation | 0.5400 OD-18 PTF-SAE SHORT |
| Threads per inch | 18 |
| Thread form | Dryseal 60°, taper 1:16 |
| Series (Table 1) | PTF-SAE SHORT |
| Series name | Dryseal SAE Short Taper 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 5 |
| Printed page | 9 |
| Thread type (para. 1.2) | Type 2 |
| Gage (reference) plane for the limits | Large end, face of the internal thread (E1 short) |
| Basic pitch dia at gage plane (in.) | 0.48815 |
| 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); L2/L3 gage within ±1/2 turn of the actual L1 position |
| Major dia max, D at gage plane (in.) | 0.5277 |
| Major dia min, D at gage plane (in.) | 0.5189 |
| Major dia tolerance, D (in.) | 0.0088 |
| Pitch dia max, E at gage plane (in.) | 0.48815 |
| Pitch dia min, E at gage plane (in.) | 0.48294 |
| PD tolerance, E (in.) | 0.00521 |
| Minor dia max, K at gage plane (in.) | 0.4486 |
| Minor dia min, K at gage plane (in.) | 0.4400 |
| Minor dia tolerance, K (in.) | 0.0086 |
| 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) | 2.0747 |
| 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.) | 0.4400 |
| Sweep cut: tip diameter at gage plane (in.) | 0.5189 |
| 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 | Rounding only (last digit): Table 5 E1 short (alt): printed 0.48815, formula gives 0.48816 (-0.00001; E1 short = E1 - 0.0625p (one thread removed at large end)) |
| Notes | Conforms in all respects to NPTF except that one thread is removed (para. 1.4.1.2): at the small end of the external thread, at the large end of the internal thread. Gage tolerance read as turns of gage engagement: "minus" means fewer turns, i.e. a larger external thread and a smaller internal thread. This reading matches Table 1 notes 3 and 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.) | 0.47739 |
| E1 short: PD at large end of SAE short internal (in.) | 0.48815 |
| E3: PD at small end of internal, L1 + L3 (in.) | 0.467 |
| Plane 0: distance from small end (ext) or face (int) (in.) | 0.1722 |
| E0 basic: pitch dia at plane 0 (in.) | 0.47739 |
| E0 max (in.) | 0.47739 |
| E0 min (in.) | 0.47218 |
| D0 basic, calculated (in.) | 0.5151 |
| D0 max: major dia at plane 0 (in.) | 0.5169 |
| D0 min (in.) | 0.5081 |
| K0 basic, calculated (in.) | 0.4362 |
| K0 max: minor dia at plane 0 (in.) | 0.4379 |
| K0 min (in.) | 0.4293 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E1 basic: pitch dia at plane 1 (in.) | 0.48815 |
| E1 max (in.) | 0.48815 |
| E1 min (in.) | 0.48294 |
| D1 basic, calculated (in.) | 0.5259 |
| D1 max: major dia at plane 1 (in.) | 0.5277 |
| D1 min (in.) | 0.5189 |
| K1 basic, calculated (in.) | 0.4469 |
| K1 max: minor dia at plane 1 (in.) | 0.4486 |
| K1 min (in.) | 0.4400 |
| Plane 3: distance from small end (ext) or face (int) (in.) | 0.3389 |
| E3 basic: pitch dia at plane 3 (in.) | 0.46700 |
| E3 max (in.) | 0.46700 |
| E3 min (in.) | 0.46179 |
| D3 basic, calculated (in.) | 0.5047 |
| D3 max: major dia at plane 3 (in.) | 0.5065 |
| D3 min (in.) | 0.4977 |
| K3 basic, calculated (in.) | 0.4258 |
| K3 max: minor dia at plane 3 (in.) | 0.4275 |
| K3 min (in.) | 0.4189 |
| 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.) | 0.48815 |
| Hand engagement L1 (in.) | 0.2278 |
| L1 (threads) | 4.1 |
| Hand engagement L1 short (in.) | 0.1722 |
| L1 short (threads) | 3.1 |
| Hand-tight engagement of this thread (in.) | 0.1722 |
| Hand-tight engagement (turns) | 3.10 |
| 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.) | 0.3389 |
| Full internal thread (threads) | 6.1 |
| Hole depth for SAE short tap (in.) | 0.6722 |
| 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.) | 0.31 |
| Table 2: engagement with PTF-SAE SHORT external (in.) | 0.26 |
| Recommended mating thread (Table 1, C.3) | NPTF external (primary). Not designed for PTF-SAE SHORT external; at extreme tolerance limits may not assemble with it (Table 1 note 4) |
| Lookup key | 1/4-18 PTF-SAE SHORT | 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.