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

2 1/2-8NPSH EXT

ASMESTD: ASME B1.20.7SOURCE: Table 2 · p. 5TYPE: External (bolt/screw)CLASS: StdSERIES: NPSH (pipe-size hose)
Major diameter
2.8434 – 2.8212
max – min
Pitch diameter
2.7622 – 2.7511
max – min
Minor diameter
2.6810 – 2.6519
max – min
Pitch
0.12500
8 threads per inch

Thread profile

2 1/2-8NPSH EXT: external M profile with its tolerance zone, drawn to scaled = 2.855d2 = 2.774d1 = 2.693P = 0.12560°30°crest 0.015625root 0.015625Major dia zone (crest)2.843 – 2.821 Td 0.022Pitch dia zone (flanks)2.762 – 2.751 Td2 0.011Root zone (minor dia)2.681 – 2.652Fundamental deviationes = −0.012 (allowance)H/80.0135323H/80.040595H/40.027063h = 5H/80.081190H0.108253thread axis, 1.428 in below the major radius (diameters shortened)MALE · external thread · 2 1/2-8NPSH EXT

2 1/2-8NPSH EXT: 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 tolerance2.85542.84342.82120.0222
Pitch diameter d22.77422.76222.75110.0111
Minor diameter d1Max printed; min is a reference value2.69302.68102.6519ref.0.0291ref.
Allowance es0.0120

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 −0.012ei −0.023male pitch dia Stdes −0.012des −0.012ei −0.034male major dia (crest)es −0.012

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 INT
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
C 2 17/32; OD 2 27/32; L 1; l 1/4
Inside diameter of the nipple C
2.53125 in
Approximate OD of the external thread OD
2.84375 in
Length of the nipple L
1.00000 in
Pilot length l
0.25000 in

Ring gages (Table 6)

GO pitch max / min
2.7622 / 2.7617 in
GO minor max / min
2.6930 / 2.6923 in
NOT GO pitch max / min
2.7516 / 2.7511 in
NOT GO minor max / min
2.7247 / 2.7240 in
Z GO major max / min
2.84340 / 2.84320 in
Z NOT GO major max / min
2.82140 / 2.82120 in

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

GO truncated major max / min
2.8263 / 2.8256 in
GO full major max / min
2.8440 / 2.8434 in
GO pitch max / min
2.7622 / 2.7617 in
NOT GO truncated major max / min
2.8052 / 2.8045 in
NOT GO pitch max / min
2.7516 / 2.7511 in

Notes

  • Printed value kept. Misprint: Table 8 col 6 (p. 11), 2 1/2-8NPSH, printed GO full-form major max 2.8440 / min 2.8434; expected max 2.8441 (cols 9, 11 and 12 print 2.8441). Root radius and tensile stress area are not given in the standard.
  • Table 8 col 6 (p. 11), 2 1/2-8NPSH: printed GO full-form major max 2.8440 / min 2.8434; expected max 2.8441 (cols 9, 11 and 12 print 2.8441)

Mates

2 1/2-8NPSH EXT screws into a thread of the same standard at the same size and pitch. The standard mating internal 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 internal
All values for 2 1/2-8NPSH EXTevery column of the workbook row
All values of the workbook row 2 1/2-8NPSH EXT
Designation2 1/2-8NPSH EXT
Decimal / alternate designation2.5000-8NPSH EXT
Threads per inch8
Thread formNPSH
SeriesNPSH
Series nameHose to pipe thread (NPSH)
External / internalExternal
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 2
Printed page5
Major dia max (in.)2.8434
Major dia min (in.)2.8212
Major dia tolerance (in.)0.0222
Allowance es (in.)0.0120
Basic pitch dia D2 (in.)2.7742
Pitch dia max (in.)2.7622
Pitch dia min (in.)2.7511
PD tolerance (in.)0.0111
Basic minor dia D1 (in.)2.6930
Minor dia max (in.)2.6810
Minor dia min (in.)2.6519
Minor dia tolerance (in.)0.0291
Major dia statusToleranced (para. 3.1: tolerance = 2 x PD tolerance)
Minor dia statusMaximum printed; minimum is a reference value (para. 3.1: P/24 flat at min PD)
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.)2.843400
Sweep cut: tip diameter (in.)2.681000
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 INT
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 printedC 2 17/32; OD 2 27/32; L 1; l 1/4
Table 4: C inside dia of nipple (in.)2.53125
Table 4: approx OD of external thread (in.)2.84375
Table 4: L length of nipple (in.)1.00000
Table 4: l pilot length (in.)0.25000
Table 6 ring gage: GO pitch max (in.)2.7622
Table 6 ring gage: GO pitch min (in.)2.7617
Table 6 ring gage: GO minor max (in.)2.6930
Table 6 ring gage: GO minor min (in.)2.6923
Table 6 ring gage: NOT GO pitch max (in.)2.7516
Table 6 ring gage: NOT GO pitch min (in.)2.7511
Table 6 ring gage: NOT GO minor max (in.)2.7247
Table 6 ring gage: NOT GO minor min (in.)2.7240
Table 6 ring gage: Z GO major max (in.)2.84340
Table 6 ring gage: Z GO major min (in.)2.84320
Table 6 ring gage: Z NOT GO major max (in.)2.82140
Table 6 ring gage: Z NOT GO major min (in.)2.82120
Table 8 setting plug: GO truncated major max (in.)2.8263
Table 8 setting plug: GO truncated major min (in.)2.8256
Table 8 setting plug: GO full major max (in.)2.8440
Table 8 setting plug: GO full major min (in.)2.8434
Table 8 setting plug: GO pitch max (in.)2.7622
Table 8 setting plug: GO pitch min (in.)2.7617
Table 8 setting plug: NOT GO truncated major max (in.)2.8052
Table 8 setting plug: NOT GO truncated major min (in.)2.8045
Table 8 setting plug: NOT GO full major max (in.)2.8441
Table 8 setting plug: NOT GO full major min (in.)2.8434
Table 8 setting plug: NOT GO pitch max (in.)2.7516
Table 8 setting plug: NOT GO pitch min (in.)2.7511
Table 8 setting plug: Full-form GO major max (in.)2.8441
Table 8 setting plug: Full-form GO major min (in.)2.8434
Table 8 setting plug: Full-form NOT GO major max (in.)2.8441
Table 8 setting plug: Full-form NOT GO major min (in.)2.8434
Check against formulasTable 8 col 6 (p. 11), 2 1/2-8NPSH: printed GO full-form major max 2.8440 / min 2.8434; expected max 2.8441 (cols 9, 11 and 12 print 2.8441)
NotesPrinted value kept. Misprint: Table 8 col 6 (p. 11), 2 1/2-8NPSH, printed GO full-form major max 2.8440 / min 2.8434; expected max 2.8441 (cols 9, 11 and 12 print 2.8441). Root radius and tensile 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.