3/8-18 NPSL NPSL (straight, locknut) Internal Thread Specifications

3/8-18 NPSL · 0.375-18 NPSL

ASMESTD: ASME B1.20.1SOURCE: Table 7 · p. 18TYPE: Internal (nut/tapped hole)CLASS: —SERIES: NPSL (straight, locknut)STRAIGHT
Major diameter
— – 0.6814
max – min, at the gage plane
Pitch diameter
0.6496 – 0.6444
max – min, at the gage plane
Minor diameter
— – 0.607
max – min, at the gage plane
Pitch
0.05556
18 threads per inch

Thread profile

any plane (straight)thread axisD = not givenmajor dia (root)E = 0.64960–0.64440pitch dia, basic 0.62701K = not givenminor dia (crest)P = 0.0555660°⟂ axis30°crest flat = 0.0064–0.0064crest truncation 0.0056–0.0056root flat = 0.0064–0.0064root truncation 0.0056–0.0056H0.048113h0.037001 max0.037001 minstraight thread: pitch line parallel to the axis, bisector ⟂ axisinternal thread (female)

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.

Limits of size, class —

DimensionBasicMaxMinTolerance
Major diameter D1Root of the internal thread—0.6814ref.—
Pitch diameter E1Straight thread: the limits apply along the whole length0.627010.64960.64440.0052
Minor diameter K1Crest of the internal thread—0.607—
Allowance esTable 7 Note 1: allowance equal to the increase in PD per turn, 0.0625/n0.00347
Crest truncation fcTable 1 (both threads)0.0055560.005556
Root truncation frTable 1 (both threads)0.0055560.005556
Crest flat FcEquivalent width of flat0.0064150.006415
Root flat FrEquivalent width of flat0.0064150.006415

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
none (straight)
Half apex angle of the pitch cone β
0 °
Lead angle at the basic PD λ
1° 37′
Gaging method
GO / NOT GO thread gages or indicating gages to ASME B1.2 (para. 6.4); NOT GO may enter up to 3 turns
Gaging tolerance
limits of size (tolerance equals 1.5 turns, Table 7 Note 1)
Gaging position
not applicable (straight thread)
Sealing
Mechanical joint, no sealing requirement (section 6)

Notes

  • Table 7 Note 1: major and minor diameters vary with PD (single-point form). External max major = max PD + 0.666025P (truncated depth); commercial pipe may not reach it. An allowance equal to the PD increase per turn is provided, with a tolerance of 1.5 turns for both threads. Gages: GO / NOT GO to the PD limits of Table 7 per ASME B1.2 (para. 6.4.4).

Mates

3/8-18 NPSL 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

→ 3/8-18 NPT→ 3/8-18 NPSC→ 3/8-18 NPSM→ 1/4-18 NPSL→ 1/2-14 NPSL→ 3/8-18 NPSL external
All values for 3/8-18 NPSLevery column of the workbook row
All values of the workbook row 3/8-18 NPSL
Designation3/8-18 NPSL
Decimal designation0.375-18 NPSL
Threads per inch18
Thread formStraight
SeriesNPSL
Series nameStraight Pipe Threads for Loose-fitting Mechanical Joints with Locknuts
External / internalInternal
Class—
Nominal pipe size3/8
Pitch P (in.)0.05555556
O.D. of pipe D, basic major dia (in.)0.675
Size category (Table 3 group)1/4, 3/8 (18 TPI)
Source tableTable 7
Printed page18
Where the diameter limits applyStraight thread: the limits apply along the whole length
Major dia min (in.)0.6814
Major dia statusNot given in the standard; reference min = PD min + 0.666025P (Note 1)
Basic pitch dia at gage plane E1 (in.)0.62701
Pitch dia max (in.)0.6496
Pitch dia min (in.)0.6444
PD tolerance (in.)0.0052
Minor dia min (in.)0.607
Minor dia statusMin only (Table 7 col. 7, Note 1); max not given in the standard
Allowance (in.)0.00347
Allowance sourceTable 7 Note 1: allowance equal to the increase in PD per turn, 0.0625/n
Root radius max (in.)not given in the standard
Root radius min (in.)not given in the standard
Lead / angle / taper sourceNot given in the standard for straight threads
Lead angle at basic PD (deg)1.615514
Lead angle (deg min)1° 37′
Tensile stress area As (sq in.)not given in the standard
Thread angle (deg)60
Flank angle (deg)30
Sharp V height H (in.)0.0481125
Thread height h max (in.)0.037001
Thread height h min (in.)0.037001
Crest truncation min (in.)0.005556
Crest truncation max (in.)0.005556
Root truncation min (in.)0.005556
Root truncation max (in.)0.005556
Crest flat min (in.)0.006415
Crest flat max (in.)0.006415
Root flat min (in.)0.006415
Root flat max (in.)0.006415
Sweep cut: start diameter (in.)0.607
Sweep cut: tip diameter (in.)0.681002
Sweep cut: depth (in.)0.037001
Sweep cut: profile width at start diameter (in.)0.049141
Sweep cut: tip flat (in.)0.006415
Sweep cut: start diameter isMinor dia min (hole)
Taper half-angle β (deg)0
Taper on diameternone (straight)
Gaging methodGO / NOT GO thread gages or indicating gages to ASME B1.2 (para. 6.4); NOT GO may enter up to 3 turns
Gaging tolerance (turns)limits of size (tolerance equals 1.5 turns, Table 7 Note 1)
Basic gaging positionnot applicable (straight thread)
SealingMechanical joint, no sealing requirement (section 6)
Minor dia min by formula (int)0.607
Check against formulasOK
NotesTable 7 Note 1: major and minor diameters vary with PD (single-point form). External max major = max PD + 0.666025P (truncated depth); commercial pipe may not reach it. An allowance equal to the PD increase per turn is provided, with a tolerance of 1.5 turns for both threads. Gages: GO / NOT GO to the PD limits of Table 7 per ASME B1.2 (para. 6.4.4).
Lookup key3/8-18 NPSL · Internal

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.