--- type: "thread" name: "3/8-18 NPTF" designation: "3/8-18 NPTF" series: "nptf" series_name: "NPTF Dryseal Pipe Thread" system: "pipe" size_label: "3/8" tpi: 18 pitch_in: 0.055556 pitch_mm: 1.4111 basic_major_dia_in: 0.675 basic_major_dia_mm: 17.145 basic_pitch_dia_in: 0.62701 basic_pitch_dia_mm: 15.9261 basic_minor_dia_in: 0.5826 basic_minor_dia_mm: 14.798 tap_drill: "37/64" tap_drill_dia_in: 0.578125 tap_drill_dia_mm: 14.684375 standard: "ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A (1984)" verified: "August 1, 2026" url: https://shopmath.org/threads/nptf/3-8 --- # 3/8-18 NPTF Thread — Dimensions, Tap Drill & Clearance The tap drill for 3/8-18 NPTF taper pipe thread is 37/64 — 0.5781 in (14.684 mm). Basic pitch diameter is 0.6270 in (15.926 mm) and basic minor diameter is 0.5826 in (14.798 mm). - **Series**: NPTF Dryseal Pipe Thread - **Designation**: 3/8-18 NPTF - **Threads per inch**: 18 - **Tap drill**: 37/64 - **Standard**: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A (1984) ## Basic Thread Dimensions for 3/8-18 NPTF | Property | Inch | Millimetre | | --- | --- | --- | | Basic major diameter | 0.6750 | 17.145 | | Threads per inch | 18 | — | | Pitch | 0.0556 | 1.411 | | Basic pitch diameter | 0.6270 | 15.926 | | Basic minor diameter | 0.5826 | 14.798 | | Sharp V thread height (H) | 0.0481 | 1.222 | _Basic dimensions are the theoretical values before any tolerance class is applied._ ## Taper Pipe Thread Geometry for 3/8-18 NPTF 3/8-18 NPTF is a taper thread: the diameter changes along the thread, so every diameter below is stated at a named plane. The taper is 0.7500 in per foot on diameter. | Property | Inch | Millimetre | | --- | --- | --- | | Pipe outside diameter | 0.6750 | 17.145 | | Taper on diameter, per foot | 0.7500 | 19.050 | | Pitch Ø at end of pipe (E₀) | 0.6120 | 15.545 | | Pitch Ø at hand-tight plane (E₁) | 0.6270 | 15.926 | | Pitch Ø at effective plane (E₂) | 0.6375 | 16.192 | | Hand-tight engagement (L₁) | 0.2400 | 6.096 | | Effective thread length (L₂) | 0.4078 | 10.358 | | Overall external thread length (L₄) | 0.6006 | 15.255 | | Thread height (h) | 0.0444 | 1.129 | | Sharp V height (H) | 0.0481 | 1.222 | | Minor Ø at hand-tight plane | 0.5826 | 14.798 | _ASME B1.20.1 publishes these dimensions in inches only; the millimetre column is an exact conversion (1 in ≡ 25.4 mm). L₁ is the hand-tight engagement length and is tabulated in the standard, not derived._ ## Other Thread Pitches in the Same Size as 3/8-18 NPTF A 3/8 fastener is not always 3/8-18 NPTF. These are the other standard pitches cut on the same nominal size. | Thread | Series | Pitch | Tap drill | | --- | --- | --- | --- | | [3/8-18 NPT](https://shopmath.org/threads/npt/3-8.md) | NPT Taper Pipe Thread | 18 TPI | 37/64 | ## Sources and Method for 3/8-18 NPTF ### Basic dimensions - **Source**: ASME B1.20.1 taper pipe thread formulas; E0 and L2 reproduced to 5 decimals in docs/research/data-sourcing.md §3.3. Public-domain primary: FED-STD-H28/7A (1984).; pipe OD / tpi / L1 / L4: ASME B1.20.1 pipe OD / tpi / L1 / L4 via Wikipedia National pipe thread (CC BY-SA 4.0); OD and tpi cross-checked against FreeCAD FeatureHole.cpp NPT block (LGPL-2.1+). NPTF shares the NPT taper geometry; dryseal crest/root truncation limits are NOT included (single-source only — see generator header). - **Method**: computed+compiled - **Verified against**: Wikipedia National pipe thread standard-sizes table (CC BY-SA 4.0) — 98 checks passed to 1e-5 in ### Taper pipe geometry - **Source**: ASME B1.20.1 taper pipe thread formulas; E0 and L2 reproduced to 5 decimals in docs/research/data-sourcing.md §3.3. Public-domain primary: FED-STD-H28/7A (1984).; pipe OD / tpi / L1 / L4: ASME B1.20.1 pipe OD / tpi / L1 / L4 via Wikipedia National pipe thread (CC BY-SA 4.0); OD and tpi cross-checked against FreeCAD FeatureHole.cpp NPT block (LGPL-2.1+) - **Method**: computed+compiled - **Verified against**: Wikipedia National pipe thread standard-sizes table (CC BY-SA 4.0) — 98 checks passed to 1e-5 in ### Assumptions - NPT threads are not a seal on their own — the standard assumes a sealing compound or tape on the joint. - Dataset scope: L1 is tabulated, not derivable (data-sourcing.md §3.3). EXCLUDED: NPTF crest/root truncation limits (single-sourced), NPSM/NPSC/NPSF straight pipe threads, BSPT/BSPP. **Data last verified**: August 1, 2026 ## Common Questions About 3/8-18 NPTF ### What is the tap drill size for 3/8-18 NPTF? The tap drill for 3/8-18 NPTF is 37/64 — 0.5781 in (14.684 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape. ### What is the pitch diameter of 3/8-18 NPTF? The basic pitch diameter of 3/8-18 NPTF is 0.6270 in (15.926 mm). ### What is the minor diameter of 3/8-18 NPTF? The basic minor diameter of 3/8-18 NPTF is 0.5826 in (14.798 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill. ### What is the thread pitch of 3/8-18 NPTF? 3/8-18 NPTF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm). ### What is the hand-tight engagement length (L₁) of 3/8-18 NPTF? The hand-tight engagement length L₁ for 3/8-18 NPTF is 0.2400 in (6.096 mm), on a pipe outside diameter of 0.6750 in (17.145 mm). ## Related pages - Smaller in NPTF Dryseal Pipe Thread: [1/4-18 NPTF](https://shopmath.org/threads/nptf/1-4.md) — tap drill 7/16 - Larger in NPTF Dryseal Pipe Thread: [1/2-14 NPTF](https://shopmath.org/threads/nptf/1-2.md) — tap drill 23/32 - All NPTF Dryseal Pipe Thread sizes: [NPTF Dryseal Pipe Thread thread chart](https://shopmath.org/threads/nptf.md) - Every thread series: [Screw thread reference](https://shopmath.org/threads.md) - Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md) --- *Source: [ShopMath](https://shopmath.org/threads/nptf/3-8) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*