2-1/2-8 NPT NPT (taper pipe thread) External Thread Specifications

2-1/2-8 NPT · 2.500-8 NPT

ASMESTD: ASME B1.20.1SOURCE: Table 2 · p. 9TYPE: External (bolt/screw)CLASS: —SERIES: NPT (taper pipe thread)TAPER 1:16
Major diameter
2.86997 – 2.84710
max – min, at the gage plane
Pitch diameter
2.76997 – 2.75435
max – min, at the gage plane
Minor diameter
2.67722 – 2.65435
max – min, at the gage plane
Pitch
0.12500
8 threads per inch

Thread profile

gage plane: hand-tight, L1 from small endthread axisD1 = 2.8700–2.8471major dia (crest)E1 = 2.76997–2.75435pitch dia, basic 2.76216K1 = 2.6772–2.6543minor dia (root)P = 0.1250060°⟂ axis30°crest flat = 0.0047–0.0090crest truncation 0.0041–0.0077root flat = 0.0047–0.0090root truncation 0.0041–0.0077H0.108253h0.100000 max0.092750 mintaper 1:16 on dia (1:32 on radius) · β = 1° 47′ 24″, included 2β = 3° 34′ 47″ · bisector ⟂ axisexternal thread (male)

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

plane 0 · E02.71953plane 1 · E12.76216plane 2 · E22.79063pipe OD D 2.8750MALE · 2-1/2-8 NPTtrue scale (lengths and diameters); axis below, not drawnL1 0.6820 (5.46 thd)full thread 1.1375 (9.10 thd)V 0.4338

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 —

DimensionBasicMaxMinTolerance
Major diameter D1Crest of the external thread at the gage plane2.869972.847100.02287
Pitch diameter E1Gage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter2.762162.769972.754350.01562
Minor diameter K1Root of the external thread at the gage plane2.677222.654350.02287
Allowance esNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant—
Crest truncation fcTable 1 (both threads)0.0077500.004125
Root truncation frTable 1 (both threads)0.0077500.004125
Crest flat FcEquivalent width of flat0.0090000.004750
Root flat FrEquivalent width of flat0.0090000.004750

Dimensions in inches. Values are those of the standard (ASME B1.20.1); 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.

Taper, lengths and gaging

Taper, gage and sealing paras. 3, 5; Tables 1, 3

Taper on the diameter T
1 in 16 (0.75 in./ft)
Half apex angle of the pitch cone β
1.789911 °
Taper tolerance on the diameter, max / min
0.1250 / -0.0625 in/ft
Lead tolerance in the effective length ±
0.003 in
60° angle tolerance ±
1.5 °
Lead angle at the basic PD λ
0° 50′
Gaging method
L1 ring gage (external) / L1 plug gage (internal), or L1 indicating gage (para. 3.2)
Gaging tolerance
±1 turn
Gaging position
Small end of pipe flush with small end of ring
Sealing
Pressure-tight: wrench-tight with sealant (para. 1.4)

Basic pitch diameters and lengths Table 3

Pitch dia at the small end of the external thread E0
2.71953 in
Hand-tight engagement L1
0.6820 · 5.46 in, threads
Pitch dia at the hand-tight plane E1
2.76216 in
Effective external thread L2
1.1375 · 9.10 in, threads
Pitch dia at L2 E2
2.79063 in
Wrench make-up length L3
0.2500 · 2 in, threads
Pitch dia at the wrench make-up plane E3
2.70391 in
Complete external threads L5
0.8875 in
Pitch dia at L5 E5
2.77500 in
Vanish thread V
0.4338 · 3.47 in, threads
Overall external thread length L4
1.5712 informula gives 1.5713
Basic minor dia at the small end K0
2.6195 in
Change of diameter per turn
0.00781 in
Hand-tight engagement range, ± 1 turn
0.4320 – 0.9320 in
Wrench make-up interference on the PD
0.01563 in
Hand-tight plus wrench make-up length L1+L3
0.9320 in

L1 plug and ring gages Tables 8 and 9

Plug gage major dia at the E0 plane
2.80278 in
Plug gage major dia at the gaging notch (E1)
2.84541 in
Plug gage major dia at the large end (E2)
2.87388 in
Gage pitch dia at the E0 plane
2.71953 in
Gage pitch dia at the gaging notch (E1)
2.76216 in
Gage pitch dia at the large end (E2)
2.79063 in
Ring gage minor dia at the E0 plane
2.63628 in
Ring gage minor dia at the gaging notch (E1)
2.67890 in
Thickness of the L1 ring or gage elements
0.6820 in
Working gage crest truncation
0.012500 in
Maximum standoff between plug and ring gages
0.059 in

Working gage tolerances Table 7

On the pitch diameter ±
0.0005 in
Lead, plugs / rings
0.0004 / 0.0005 in
Half angle ±, plugs / rings
7 / 10 min
Taper, plugs / rings
0.0010 / 0.0014 in
Major dia of plugs
0.0016 in
Minor dia of rings
0.0016 in
Total cumulative tolerance on the PD, plugs / rings
0.00158 / 0.00211 in

Notes

  • Table 2 Notes: L1 is also the length of the L1 ring gage and the distance from the gaging notch to the small end of the L1 plug; E1 is also the PD at the gaging notch of the L1 plug; L2 is also the threaded length of the L1 plug; L4 is a reference dimension; L5 = L2 − 2P (the next two threads are complete at the root); K0 is given for selecting tap drills (Appendix B). Gaging: ±1 turn from basic (Fig. 4 / Fig. 6); with indicating gages ±0.0625/n on diameter (paras. 3.2.1.2, 3.2.2.2). The gaging tolerance can lengthen or shorten engagement by up to two turns; gage wear reduces it further (para. 3.1.9.1). Diameter limits here are calculated at the gage plane (E1 ± 0.0625/n on PD; major and minor add the Table 1 truncation limits). The standard itself tabulates basic dimensions only. Chamfer: external threads with chamfer smaller than the minor dia are gaged from the last scratch of the chamfer cone (para. 3.2.4.1, Fig. 5); otherwise from the end of the pipe (para. 3.2.4.3).
  • L4 (col. 19): printed 1.5712, formula 1.5713 (1 in last place, rounding)

Mates

2-1/2-8 NPT 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

→ 2-1/2-8 NPTR→ 2-1/2-8 NPSM→ 2-1/2-8 NPSL→ 2-11.5 NPT→ 3-8 NPT→ 2-1/2-8 NPT internal
All values for 2-1/2-8 NPTevery column of the workbook row
All values of the workbook row 2-1/2-8 NPT
Designation2-1/2-8 NPT
Decimal designation2.500-8 NPT
Threads per inch8
Thread formTaper
SeriesNPT
Series nameAmerican National Standard Taper Pipe Thread
External / internalExternal
Class—
Nominal pipe size2-1/2
Pitch P (in.)0.12500000
O.D. of pipe D, basic major dia (in.)2.8750
Size category (Table 3 group)2-1/2 and larger (8 TPI)
Source tableTable 2
Printed page9
Where the diameter limits applyGage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter
Major dia max (in.)2.86997
Major dia min (in.)2.84710
Major dia tolerance (in.)0.02287
Major dia statusCalculated at the gage plane: PD limits with Table 1 truncation limits (para. 2.4)
Basic pitch dia at gage plane E1 (in.)2.76216
Pitch dia max (in.)2.76997
Pitch dia min (in.)2.75435
PD tolerance (in.)0.01562
Minor dia max (in.)2.67722
Minor dia min (in.)2.65435
Minor dia tolerance (in.)0.02287
Minor dia statusCalculated at the gage plane: PD limits with Table 1 truncation limits (para. 2.4)
Allowance sourceNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant
Root radius max (in.)not given in the standard
Root radius min (in.)not given in the standard
Lead tolerance ± in effective length (in.)0.003
60° angle tolerance ± (deg)1.5
Taper tolerance max (in./ft)0.1250
Taper tolerance min (in./ft)-0.0625
Lead / angle / taper sourceTable 3, p. 10; lead ±0.003 in./in. when the effective thread is longer than 1 in. (Note 1)
Lead angle at basic PD (deg)0.825285
Lead angle (deg min)0° 50′
Tensile stress area As (sq in.)not given in the standard
Thread angle (deg)60
Flank angle (deg)30
Sharp V height H (in.)0.1082532
Thread height h max (in.)0.10000
Thread height h min (in.)0.09275
Crest truncation min (in.)0.004125
Crest truncation max (in.)0.007750
Root truncation min (in.)0.004125
Root truncation max (in.)0.007750
Crest flat min (in.)0.004750
Crest flat max (in.)0.009000
Root flat min (in.)0.004750
Root flat max (in.)0.009000
Sweep cut: start diameter (in.)2.81953
Sweep cut: tip diameter (in.)2.61953
Sweep cut: depth (in.)0.100000
Sweep cut: profile width at start diameter (in.)0.120250
Sweep cut: tip flat (in.)0.004750
Sweep cut: start diameter isMajor dia at the small end (E0 plane), basic position; the blank is a 1:16 cone
Taper half-angle β (deg)1.789911
Taper on diameter1 in 16 (0.75 in./ft)
E0 pitch dia at small end (in.)2.71953
L1 hand-tight engagement (in.)0.6820
L1 (threads)5.46
E1 pitch dia at hand-tight plane (in.)2.76216
L2 effective thread, external (in.)1.1375
L2 (threads)9.10
E2 pitch dia at L2 plane (in.)2.79063
L2 − L1 (in.)0.4555
L2 − L1 (threads)3.64
L3 wrench make-up, internal (in.)0.2500
L3 (threads)2
E3 pitch dia at wrench make-up plane (in.)2.70391
V vanish thread (in.)0.4338
V (threads)3.47
L4 overall external thread length, reference (in.)1.5712
L5 complete external threads (in.)0.8875
E5 pitch dia at L5 plane (in.)2.77500
Height of thread h, Table 2 col. 22 (in.)0.10000
Change in dia per turn 0.0625/n (in.)0.00781
K0 basic minor dia at small end (in.)2.6195
E0 by formula D − (0.05D + 1.1)P2.71953
E1 by formula E0 + 0.0625·L12.76216
L2 by formula (0.80D + 6.8)P1.1375
E2 by formula E0 + 0.0625·L22.79063
E3 by formula E0 − 0.0625·L32.70391
L5 by formula L2 − 2P0.8875
E5 by formula E0 + 0.0625·L52.77500
V by formula 3.47P0.4338
L4 by formula L2 + V1.5713
K0 by formula E0 − 0.8P2.6195
Gaging methodL1 ring gage (external) / L1 plug gage (internal), or L1 indicating gage (para. 3.2)
Gaging tolerance (turns)±1 turn
Gaging limit large (turns)1
Gaging limit small (turns)1
Basic gaging positionSmall end of pipe flush with small end of ring
Hand-tight engagement min, ±1 turn both parts (in.)0.4320
Hand-tight engagement max, ±1 turn both parts (in.)0.9320
Wrench make-up interference on PD, basic L3/16 (in.)0.01563
Hand-tight + wrench make-up length L1 + L3 (in.)0.9320
SealingPressure-tight: wrench-tight with sealant (para. 1.4)
Gage: plug major dia at E0 plane (in.)2.80278
Gage: plug major dia at E1 gaging notch (in.)2.84541
Gage: plug major dia at E2 large end (in.)2.87388
Gage: pitch dia at E0 plane (in.)2.71953
Gage: pitch dia at E1 gaging notch (in.)2.76216
Gage: pitch dia at E2 large end (in.)2.79063
Gage: ring minor dia at E0 plane (in.)2.63628
Gage: ring minor dia at E1 gaging notch (in.)2.67890
Gage: thickness of L1 ring or gage elements (in.)0.6820
Gage: working gage crest truncation (in.)0.012500
Gage: root must clear 0.0381P width (in.)0.004763
Gage: plug major at E0 by formula E0 + (H − 2·trunc)2.80278
Working gage tol. on PD ± (in.)0.0005
Working gage tol. lead, plugs (in.)0.0004
Working gage tol. lead, rings (in.)0.0005
Working gage tol. half angle, plugs ± (min)7
Working gage tol. half angle, rings ± (min)10
Working gage tol. taper, plugs + (in.)0.0010
Working gage tol. taper, rings − (in.)0.0014
Working gage tol. major dia, plugs − (in.)0.0016
Working gage tol. minor dia, rings + (in.)0.0016
Working gage total cumulative tol. on PD, plugs (in.)0.00158
Working gage total cumulative tol. on PD, rings (in.)0.00211
Max standoff between plug and ring gages (in.)0.059
Check against formulasL4 (col. 19): printed 1.5712, formula 1.5713 (1 in last place, rounding)
NotesTable 2 Notes: L1 is also the length of the L1 ring gage and the distance from the gaging notch to the small end of the L1 plug; E1 is also the PD at the gaging notch of the L1 plug; L2 is also the threaded length of the L1 plug; L4 is a reference dimension; L5 = L2 − 2P (the next two threads are complete at the root); K0 is given for selecting tap drills (Appendix B). Gaging: ±1 turn from basic (Fig. 4 / Fig. 6); with indicating gages ±0.0625/n on diameter (paras. 3.2.1.2, 3.2.2.2). The gaging tolerance can lengthen or shorten engagement by up to two turns; gage wear reduces it further (para. 3.1.9.1). Diameter limits here are calculated at the gage plane (E1 ± 0.0625/n on PD; major and minor add the Table 1 truncation limits). The standard itself tabulates basic dimensions only. Chamfer: external threads with chamfer smaller than the minor dia are gaged from the last scratch of the chamfer cone (para. 3.2.4.1, Fig. 5); otherwise from the end of the pipe (para. 3.2.4.3).
Lookup key2-1/2-8 NPT · External

Related tools

ASME B1.20.1: Pipe Threads, General Purpose, Inch. Source: workbook data (2026-10-11). Confirm against your licensed copy of the standard before use.