--- type: "thread" name: "18-8 NPT" designation: "18-8 NPT" series: "npt" series_name: "NPT Taper Pipe Thread" system: "pipe" size_label: "18" tpi: 8 pitch_in: 0.125 pitch_mm: 3.175 basic_major_dia_in: 18 basic_major_dia_mm: 457.2 basic_pitch_dia_in: 17.875 basic_pitch_dia_mm: 454.025 basic_minor_dia_in: 17.775 basic_minor_dia_mm: 451.485 tap_drill: "1" tap_drill_dia_in: 1 tap_drill_dia_mm: 25.4 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/npt/18 --- # 18-8 NPT Thread — Dimensions, Tap Drill & Clearance The tap drill for 18-8 NPT taper pipe thread is 1 — 1.0000 in (25.400 mm). Basic pitch diameter is 17.8750 in (454.025 mm) and basic minor diameter is 17.7750 in (451.485 mm). - **Series**: NPT Taper Pipe Thread - **Designation**: 18-8 NPT - **Threads per inch**: 8 - **Tap drill**: 1 - **Standard**: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A (1984) ## Basic Thread Dimensions for 18-8 NPT | Property | Inch | Millimetre | | --- | --- | --- | | Basic major diameter | 18.0000 | 457.200 | | Threads per inch | 8 | — | | Pitch | 0.1250 | 3.175 | | Basic pitch diameter | 17.8750 | 454.025 | | Basic minor diameter | 17.7750 | 451.485 | | Sharp V thread height (H) | 0.1082 | 2.750 | _Basic dimensions are the theoretical values before any tolerance class is applied._ ## Taper Pipe Thread Geometry for 18-8 NPT 18-8 NPT 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 | 18.0000 | 457.200 | | Taper on diameter, per foot | 0.7500 | 19.050 | | Pitch Ø at end of pipe (E₀) | 17.7500 | 450.850 | | Pitch Ø at hand-tight plane (E₁) | 17.8750 | 454.025 | | Pitch Ø at effective plane (E₂) | 17.9156 | 455.057 | | Hand-tight engagement (L₁) | 2.0000 | 50.800 | | Effective thread length (L₂) | 2.6500 | 67.310 | | Overall external thread length (L₄) | 3.0837 | 78.326 | | Thread height (h) | 0.1000 | 2.540 | | Sharp V height (H) | 0.1082 | 2.750 | | Minor Ø at hand-tight plane | 17.7750 | 451.485 | _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._ ## Sources and Method for 18-8 NPT ### 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+) - **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 18-8 NPT ### What is the tap drill size for 18-8 NPT? The tap drill for 18-8 NPT is 1 — 1.0000 in (25.400 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape. ### What is the pitch diameter of 18-8 NPT? The basic pitch diameter of 18-8 NPT is 17.8750 in (454.025 mm). ### What is the minor diameter of 18-8 NPT? The basic minor diameter of 18-8 NPT is 17.7750 in (451.485 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 18-8 NPT? 18-8 NPT has 8 threads per inch, a pitch of 0.1250 in (3.175 mm). ### What is the hand-tight engagement length (L₁) of 18-8 NPT? The hand-tight engagement length L₁ for 18-8 NPT is 2.0000 in (50.800 mm), on a pipe outside diameter of 18.0000 in (457.200 mm). ## Related pages - Smaller in NPT Taper Pipe Thread: [16-8 NPT](https://shopmath.org/threads/npt/16.md) — tap drill 1 - Larger in NPT Taper Pipe Thread: [20-8 NPT](https://shopmath.org/threads/npt/20.md) — tap drill 1 - All NPT Taper Pipe Thread sizes: [NPT Taper Pipe Thread thread chart](https://shopmath.org/threads/npt.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/npt/18) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*