2 1/2-8 NPSH NPSH (pipe-size hose) Internal Thread Specifications

2 1/2-8NPSH INT

ASMESTD: ASME B1.20.7SOURCE: Table 3 · p. 6TYPE: Internal (nut/tapped hole)CLASS: StdSERIES: NPSH (pipe-size hose)
Major diameter
2.8845 – 2.8554
max – min
Pitch diameter
2.7853 – 2.7742
max – min
Minor diameter
2.7152 – 2.6930
max – min
Pitch
0.12500
8 threads per inch

Thread profile

2 1/2-8NPSH INT: internal M profile with its tolerance zone, drawn to scaleD = 2.855D2 = 2.774D1 = 2.693P = 0.12560°30°crest 0.015625root 0.015625Major dia (root)2.855 min (2.885 max ref.)Pitch dia zone (flanks)2.774 – 2.785 TD2 0.011Minor dia zone (crests)2.693 – 2.715 TD1 0.022Fundamental deviationEI = 0, position HH/80.0135323H/80.040595H/40.027063h = 5H/80.081190H0.108253thread axis, 1.428 in below the major radius (diameters shortened)FEMALE · internal thread · 2 1/2-8NPSH INT

2 1/2-8NPSH INT: solid line = maximum material profile, dashed = minimum material, hatched = tolerance zone, grey dashed = basic profile. Drawn to scale; values in inches, diameters shortened.

Limits of size, class Std

DimensionBasicMaxMinTolerance
Major diameter DPara. 3.1: tolerance = 2 × PD tolerance2.85542.8845ref.2.85540.0291ref.
Pitch diameter D22.77422.78532.77420.0111
Minor diameter D12.69302.71522.69300.0222
Allowance es—

Dimensions in inches. Values are those of the standard (ASME B1.20.7); 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.

Tolerance zones relative to the basic size, inches

Tolerance zones relative to the basic sizezero line = basic size+ above · − belowD2ES +0.011EI 0.000female pitch diaD1ES +0.022EI 0.000female minor dia (crest)

Mating, gaging and dimensions

Mating member and fit at the limits

Service
Steam, air, water, and all other hose connections to be made up with standard pipe threads
Mating group (compatible designations)
NPSH-2_1/2
Mating member of the same designation
2 1/2-8NPSH EXT
Other designations in the mating group
none (mates only with its own designation)
Minimum PD clearance (coupling min − nipple max)
0.0120 in
Maximum PD clearance (coupling max − nipple min)
0.0342 in
Minimum major dia clearance
0.0120 in
Minimum minor dia clearance
0.0120 in
Fit at the limits (PD)
clearance (always)

Lead, flank and profile

Lead variation ± (Table 5) ±δP
0.0032 in
Half-angle variation ± (Table 5) ±δα
1° 42′
Lead angle at the basic PD λ
0° 49′
Crest flat (basic P/8)
0.015625 in
Root flat (basic P/8)
0.015625 in
Extreme flat P/24 at minimum PD
0.005208 in
Root radius for the model (worn tool) r
0.013532 in

Table 4 dimensions (nipple and coupling)

As printed
H 15/16; T 11/16; n 5 1/2
Depth of the coupling H
0.93750 in
Thread length of the coupling T
0.68750 in

Setting plugs (Table 8) and plug gages (Table 7)

Plug gage GO major max / min
2.8561 / 2.8554 in
Plug gage GO pitch max / min
2.7747 / 2.7742 in
Plug gage NOT GO pitch max / min
2.7853 / 2.7848 in

Notes

  • Maximum major diameter, major diameter tolerance (internal) are reference values from para. 3.2; root radius and stress area are not given in the standard.

Mates

2 1/2-8NPSH INT takes a thread of the same standard at the same size and pitch. The standard mating external thread is class Std. 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

→ 2-11.5 NPSH→ 3-8 NPSH→ 2 1/2-8 NPSH external
All values for 2 1/2-8NPSH INTevery column of the workbook row
All values of the workbook row 2 1/2-8NPSH INT
Designation2 1/2-8NPSH INT
Decimal / alternate designation2.5000-8NPSH INT
Threads per inch8
Thread formNPSH
SeriesNPSH
Series nameHose to pipe thread (NPSH)
External / internalInternal
ClassStd
Nominal size of hose2 1/2
Pitch P (in.)0.12500
Basic major dia D (in.)2.8554
Size categoryStandard sizes (Tables 1-8)
Source tableTable 3
Printed page6
Major dia max (in.)2.8845
Major dia min (in.)2.8554
Major dia tolerance (in.)0.0291
Basic pitch dia D2 (in.)2.7742
Pitch dia max (in.)2.7853
Pitch dia min (in.)2.7742
PD tolerance (in.)0.0111
Basic minor dia D1 (in.)2.6930
Minor dia max (in.)2.7152
Minor dia min (in.)2.6930
Minor dia tolerance (in.)0.0222
Major dia statusMinimum printed; maximum is a reference value (para. 3.2: PD max + 11h/9)
Minor dia statusToleranced (para. 3.2: tolerance = 2 x PD tolerance)
Root radius max (in.)not given in the standard
Root radius min (in.)not given in the standard
Lead variation ± (in.) (Table 5)0.0032
Half-angle variation ± (Table 5)1° 42′
Lead variation from PD tol (calc) (in.)0.0032
Half-angle variation from PD tol (calc)1° 42′
Lead angle at basic PD (deg)0.8217
Lead angle (deg min)0° 49′
Tensile stress area As (sq in.)not given in the standard
Thread angle (deg)60
Flank angle (deg)30
Sharp V height H (in.)0.108253
Thread height h (in.)0.081190
Crest flat, basic P/8 (in.)0.015625
Root flat, basic P/8 (in.)0.015625
Extreme flat P/24 at minimum PD (in.)0.005208
Truncation of the P/8 flat from the sharp V, f = h/6 (in.)0.013532
Root radius for model (worn tool) (in.)0.013532
Sweep cut: start diameter (in.)2.693000
Sweep cut: tip diameter (in.)2.855400
Sweep cut: depth (in.)0.081200
Sweep cut: profile width at start diameter (in.)0.109387
Crest flat as modelled (in.)0.015613
Tapernone (straight thread)
ServiceSteam, air, water, and all other hose connections to be made up with standard pipe threads
Mating group (compatible designations)NPSH-2_1/2
Mating member of the same designation2 1/2-8NPSH EXT
Other designations in the mating groupnone (mates only with its own designation)
Min PD clearance: coupling min PD - nipple max PD (in.)0.0120
Max PD clearance: coupling max PD - nipple min PD (in.)0.0342
Min major dia clearance: coupling min - nipple max (in.)0.0120
Min minor dia clearance: coupling min - nipple max (in.)0.0120
Fit at the limits (PD)clearance (always)
Table 4 lengths as printedH 15/16; T 11/16; n 5 1/2
Table 4: H depth of coupling (in.)0.93750
Table 4: T thread length of coupling (in.)0.68750
Table 4: approx number of threads in T5.50000
Table 7 plug gage: GO major max (in.)2.8561
Table 7 plug gage: GO major min (in.)2.8554
Table 7 plug gage: GO pitch max (in.)2.7747
Table 7 plug gage: GO pitch min (in.)2.7742
Table 7 plug gage: NOT GO major max (in.)2.8394
Table 7 plug gage: NOT GO major min (in.)2.8387
Table 7 plug gage: NOT GO pitch max (in.)2.7853
Table 7 plug gage: NOT GO pitch min (in.)2.7848
Table 7 plug gage: Z GO minor max (in.)2.69320
Table 7 plug gage: Z GO minor min (in.)2.69300
Table 7 plug gage: Z NOT GO minor max (in.)2.71520
Table 7 plug gage: Z NOT GO minor min (in.)2.71500
Check against formulasOK
NotesMaximum major diameter, major diameter tolerance (internal) are reference values from para. 3.2; root radius and stress area are not given in the standard.

Related tools

ASME B1.20.7: Hose Coupling Screw Threads, Inch. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.