2 1/2-8 PTF-SPL-EXTRA-SHORT PTF-SPL EXTRA SHORT Internal Thread Specifications
2 1/2-8 PTF-SPL EXTRA SHORT
ASMESTD: ASME B1.20.3SOURCE: App. C.2 (from Table 5) · p. 15TYPE: Internal (nut/tapped hole)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
0.12500
8 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 | — | — | — | |
| Pitch diameter E1Large end, face of the internal thread (E1 short) | 2.75435 | — | — | — |
| Minor diameter K1Crest of the internal thread | — | — | — | |
| 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 1 | 2.75435 | — | — | 2.8462 | — | — | 2.6582 | — | — |
| Plane 0 | 2.71953 | — | — | 2.8114 | — | — | 2.6234 | — | — |
| Plane 3 | 2.69610 | — | — | 2.7880 | — | — | 2.5999 | — | — |
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 reference plane
- Large end, face of the internal thread (E1 short)
- PD tolerance basis
- Not given: tolerance must be altered, coordinated and specified (paras. C.2, C.3)
- 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
- May assemble with PTF-SAE SHORT external (C.3 *) and NPTF external (C.3 **), only with tolerances held closer than standard
Basic pitch diameters and lengths
- Pitch dia at the small end of the external thread E0
- 2.71953 in
- Pitch dia at the large end of the SAE short internal E1s
- 2.75435 informula gives 2.75434
- Pitch dia at the small end of the internal thread E3
- 2.6961 in
- Hand engagement L1
- 0.5570 · 4.46 in, turns
- Wrench make-up L3
- 0.3750 · 0.02344 in, PD interference
- Full internal thread Lt
- 0.6820 in
- 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 = (L1 short + L3) - 2p (calculated from Table 5, C.2); large 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.
- Rounding only (last digit): Table 5 E1 short: printed 2.75435, formula gives 2.75434 (+0.00001; E1 short = E0 + 0.0625 L1 short (A.2, with E0 of Table 3)) | Fig. 2 trunc min crest: printed 0.0052, 0.042p = 0.00525 (half-way value printed rounded down).
Mates
2 1/2-8 PTF-SPL EXTRA 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 2 1/2-8 PTF-SPL EXTRA SHORTevery column of the workbook row
| Designation | 2 1/2-8 PTF-SPL EXTRA SHORT |
|---|---|
| Alternate designation | 2.8750 OD-8 PTF-SPL EXTRA SHORT |
| Threads per inch | 8 |
| 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 | Internal |
| Class | — |
| Nominal size | 2 1/2 |
| Pitch P (in.) | 0.12500000 |
| Pipe OD D, basic size (in.) | 2.875 |
| Size category | Special series (Appendix C) |
| Source table | App. C.2 (from Table 5) |
| Printed page | 15 |
| Thread type (para. 1.2) | App. C.2 |
| Gage (reference) plane for the limits | Large end, face of the internal thread (E1 short) |
| Basic pitch dia at gage plane (in.) | 2.75435 |
| 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) | 0.8276 |
| 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 |
| Check against formulas | Rounding only (last digit): Table 5 E1 short: printed 2.75435, formula gives 2.75434 (+0.00001; E1 short = E0 + 0.0625 L1 short (A.2, with E0 of Table 3)) | Fig. 2 trunc min crest: printed 0.0052, 0.042p = 0.00525 (half-way value printed rounded down). |
| 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 = (L1 short + L3) - 2p (calculated from Table 5, C.2); large 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. |
| 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 short: PD at large end of SAE short internal (in.) | 2.75435 |
| E3: PD at small end of internal, L1 + L3 (in.) | 2.6961 |
| Threads removed beyond PTF-SAE SHORT (App. C) | 2 |
| Plane 0: distance from small end (ext) or face (int) (in.) | 0.5570 |
| E0 basic: pitch dia at plane 0 (in.) | 2.71953 |
| D0 basic, calculated (in.) | 2.8114 |
| K0 basic, calculated (in.) | 2.6234 |
| Plane 1: distance from small end (ext) or face (int) (in.) | 0.0000 |
| E1 basic: pitch dia at plane 1 (in.) | 2.75435 |
| D1 basic, calculated (in.) | 2.8462 |
| K1 basic, calculated (in.) | 2.6582 |
| Plane 3: distance from small end (ext) or face (int) (in.) | 0.9320 |
| E3 basic: pitch dia at plane 3 (in.) | 2.69610 |
| D3 basic, calculated (in.) | 2.7880 |
| K3 basic, calculated (in.) | 2.5999 |
| 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.) | 2.75435 |
| Hand engagement L1 (in.) | 0.682 |
| L1 (threads) | 5.46 |
| Hand engagement L1 short (in.) | 0.557 |
| L1 short (threads) | 4.46 |
| Hand-tight engagement of this thread (in.) | 0.5570 |
| Hand-tight engagement (turns) | 4.46 |
| Wrench make-up L3 = 3p (in.) | 0.3750 |
| PD interference from L3 make-up, on dia (in.) | 0.02344 |
| Full thread length of PTF-SAE SHORT, Table 4/5 (in.) | 0.932 |
| Length of full internal thread (in.) | 0.6820 |
| Full internal thread (threads) | 5.46 |
| Crest/root interference at flank contact, min (radial, in.) | 0.0000 |
| Crest/root interference at flank contact, max (radial, in.) | 0.0043 |
| Recommended mating thread (Table 1, C.3) | May assemble with PTF-SAE SHORT external (C.3 *) and NPTF external (C.3 **), only with tolerances held closer than standard |
| Lookup key | 2 1/2-8 PTF-SPL EXTRA SHORT | 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.