3/8-18 PTF-SPL-EXTRA-SHORT PTF-SPL EXTRA SHORT External Thread Specifications
3/8-18 PTF-SPL EXTRA SHORT
ASMESTD: ASME B1.20.3SOURCE: App. C.2 (from Table 4) · p. 15TYPE: External (bolt/screw)CLASS: —SERIES: PTF-SPL EXTRA SHORTTAPER 1:16
Major diameter
— – —
max – min, at the gage plane
Pitch diameter
— – —
max – min, at the gage plane
Minor diameter
— – —
max – min, at the gage plane
Pitch
0.05556
18 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 —
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter D1Crest of the external thread | — | — | — | |
| Pitch diameter E1Hand-tight plane, L1 short from the small end (E1, Table 3) | 0.62701 | — | — | — |
| Minor diameter K1Root of the external thread | — | — | — | |
| 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 0 | 0.61548 | — | — | 0.6567 | — | — | 0.5778 | — | — |
| Plane 1 | 0.62701 | — | — | 0.6682 | — | — | 0.5893 | — | — |
| Plane 2 | 0.63056 | — | — | 0.6718 | — | — | 0.5928 | — | — |
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 reference plane
- Hand-tight plane, L1 short from the small end (E1, Table 3)
- PD tolerance basis
- Not given: tolerance must be altered, coordinated and specified (paras. C.2, C.3)
- Lead angle at the basic PD λ
- 1° 37′
- Crest/root interference at flank contact, min – max δ
- 0.0000 – 0.0035 in
- Recommended mating thread
- May assemble with PTF-SAE SHORT int, NPSF, PTF-SPL SHORT int, PTF-SPL EXTRA SHORT int (only with tolerances held closer, C.3 *); NPTF or NPSI int (C.3 **)
Basic pitch diameters and lengths
- Short-thread pitch dia at the small end E0s
- 0.61548 informula gives 0.61551
- Pitch dia at the hand-tight plane E1
- 0.62701 in
- Pitch dia at L2 E2
- 0.6375 in
- Hand engagement L1
- 0.1844 · 3.32 in, turns
- Wrench make-up L3
- 0.1667 · 0.01042 in, PD interference
- Full external thread L2
- 0.2411 · 4.34 in, threads
- Threads removed beyond PTF-SAE SHORT
- 2
Notes
- Conforms to PTF-SAE SHORT except two more threads removed: at the large end of external threads, the small end of internal threads (C.2). L2 ring / L3 plug gage cannot be used. Full thread length = L2 short - 2p (calculated from Table 4, C.2); small end and hand-tight plane as PTF-SAE SHORT. Tolerance not given in the standard; limits of size are left blank. The explorer evaluates these at basic size unless a tolerance is entered. 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. E2 calculated: PD at the end of the shortened full thread = E2 (Table 3) - k x 0.0625p.
- DIFFERS: E0 short printed 0.61548; formula D - (0.05D + 1.037)p of App. A.2 gives 0.61551 (-0.00003). The printed value equals E0 + 0.0625p (one thread taper added to Table 3 E0), so the 1.037 constant of A.2 is a rounding of 1.1 - 0.0625 = 1.0375.
Mates
3/8-18 PTF-SPL EXTRA SHORT screws into a thread of the same standard at the same size and pitch. The standard mating internal 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 3/8-18 PTF-SPL EXTRA SHORTevery column of the workbook row
| Designation | 3/8-18 PTF-SPL EXTRA SHORT |
|---|---|
| Alternate designation | 0.6750 OD-18 PTF-SPL EXTRA SHORT |
| Threads per inch | 18 |
| Thread form | Dryseal 60°, taper 1:16 |
| Series (Table 1) | PTF-SPL EXTRA SHORT |
| Series name | Dryseal Special Extra Short Taper Pipe Thread |
| External / internal | External |
| Class | — |
| Nominal size | 3/8 |
| Pitch P (in.) | 0.05555556 |
| Pipe OD D, basic size (in.) | 0.675 |
| Size category | Special series (Appendix C) |
| Source table | App. C.2 (from Table 4) |
| Printed page | 15 |
| Thread type (para. 1.2) | App. C.2 |
| Gage (reference) plane for the limits | Hand-tight plane, L1 short from the small end (E1, Table 3) |
| Basic pitch dia at gage plane (in.) | 0.62701 |
| PD tolerance basis | Not given: tolerance must be altered, coordinated and specified (paras. C.2, C.3) |
| 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) | 1.6155 |
| Lead angle (deg min) | 1° 37′ |
| 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 |
| Check against formulas | DIFFERS: E0 short printed 0.61548; formula D - (0.05D + 1.037)p of App. A.2 gives 0.61551 (-0.00003). The printed value equals E0 + 0.0625p (one thread taper added to Table 3 E0), so the 1.037 constant of A.2 is a rounding of 1.1 - 0.0625 = 1.0375. |
| Notes | Conforms to PTF-SAE SHORT except two more threads removed: at the large end of external threads, the small end of internal threads (C.2). L2 ring / L3 plug gage cannot be used. Full thread length = L2 short - 2p (calculated from Table 4, C.2); small end and hand-tight plane as PTF-SAE SHORT. Tolerance not given in the standard; limits of size are left blank. The explorer evaluates these at basic size unless a tolerance is entered. 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. E2 calculated: PD at the end of the shortened full thread = E2 (Table 3) - k x 0.0625p. |
| 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 short (in.) | 0.61548 |
| E1: PD at hand-tight plane (in.) | 0.62701 |
| E2: PD at L2, Table 3 (in.) | 0.6375 |
| Threads removed beyond PTF-SAE SHORT (App. C) | 2 |
| PD at end of this full external thread (in.) | 0.63056 |
| Plane 0: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E0 basic: pitch dia at plane 0 (in.) | 0.61548 |
| D0 basic, calculated (in.) | 0.6567 |
| K0 basic, calculated (in.) | 0.5778 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.1844 |
| E1 basic: pitch dia at plane 1 (in.) | 0.62701 |
| D1 basic, calculated (in.) | 0.6682 |
| K1 basic, calculated (in.) | 0.5893 |
| Plane 2: distance from small end (ext) or face (int) (in.) | 0.2411 |
| E2 basic: pitch dia at plane 2 (in.) | 0.63056 |
| D2 basic, calculated (in.) | 0.6718 |
| K2 basic, calculated (in.) | 0.5928 |
| 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.) | 0.61548 |
| Hand engagement L1 (in.) | 0.24 |
| L1 (threads) | 4.32 |
| Hand engagement L1 short (in.) | 0.1844 |
| L1 short (threads) | 3.32 |
| Hand-tight engagement of this thread (in.) | 0.1844 |
| Hand-tight engagement (turns) | 3.32 |
| Wrench make-up L3 = 3p (in.) | 0.1667 |
| PD interference from L3 make-up, on dia (in.) | 0.01042 |
| Full thread length of PTF-SAE SHORT, Table 4/5 (in.) | 0.3522 |
| Length of full external thread L2 (in.) | 0.2411 |
| Full external thread (threads) | 4.34 |
| 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) | May assemble with PTF-SAE SHORT int, NPSF, PTF-SPL SHORT int, PTF-SPL EXTRA SHORT int (only with tolerances held closer, C.3 *); NPTF or NPSI int (C.3 **) |
| Lookup key | 3/8-18 PTF-SPL EXTRA SHORT | 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.