What is the tap drill size for 3 1/2-8 NPT?

The tap drill for 3 1/2-8 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 3 1/2-8 NPT?

The basic pitch diameter of 3 1/2-8 NPT is 3.8888 in (98.776 mm).

What is the minor diameter of 3 1/2-8 NPT?

The basic minor diameter of 3 1/2-8 NPT is 3.7888 in (96.236 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 1/2-8 NPT?

3 1/2-8 NPT has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).

What is the hand-tight engagement length (L₁) of 3 1/2-8 NPT?

The hand-tight engagement length L₁ for 3 1/2-8 NPT is 0.8210 in (20.853 mm), on a pipe outside diameter of 4.0000 in (101.600 mm).

NPT Taper Pipe Thread · 3 1/2-8 NPT

3 1/2-8 NPT Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/npt/3-1-2 · data verified August 1, 2026

The tap drill for 3 1/2-8 NPT taper pipe thread is 1 — 1.0000 in (25.400 mm). Basic pitch diameter is 3.8888 in (98.776 mm) and basic minor diameter is 3.7888 in (96.236 mm).

Series: NPT Taper Pipe Thread
Designation: 3 1/2-8 NPT
TPI: 8
Standard: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A

Tap drill · taper pipe tap

Drill Ø: 1.0000in
Drill Ø: 25.400mm

NPT and NPTF holes are drilled straight and cut with a taper tap. The standard assumes a sealing compound or tape on the joint.

Basic Thread Dimensions for 3 1/2-8 NPT

Basic thread dimensions — 3 1/2-8 NPT
Property	Inch	Millimetre
Basic major diameter	4.0000	101.600
Threads per inch	8	—
Pitch	0.1250	3.175
Basic pitch diameter	3.8888	98.776
Basic minor diameter	3.7888	96.236
Sharp V thread height (H)	0.1082	2.750

Threads per inch has no millimetre counterpart — metric threads are specified by pitch instead. Basic dimensions are the theoretical values before any tolerance class is applied.

Taper Pipe Thread Geometry for 3 1/2-8 NPT

3 1/2-8 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.

Taper pipe thread geometry — 3 1/2-8 NPT
Property	Inch	Millimetre
Pipe outside diameter	4.0000	101.600
Taper on diameter, per foot	0.7500	19.050
Pitch Ø at end of pipe (E₀)	3.8375	97.472
Pitch Ø at hand-tight plane (E₁)	3.8888	98.776
Pitch Ø at effective plane (E₂)	3.9156	99.457
Hand-tight engagement (L₁)	0.8210	20.853
Effective thread length (L₂)	1.2500	31.750
Overall external thread length (L₄)	1.6837	42.766
Thread height (h)	0.1000	2.540
Sharp V height (H)	0.1082	2.750
Minor Ø at hand-tight plane	3.7888	96.236

ASME B1.20.1 publishes these dimensions in inches only; the millimetre column is an exact conversion (1 in = 25.4 mm by definition). L₁ is the hand-tight engagement length and is tabulated in the standard, not derived.

Sources and Method for 3 1/2-8 NPT

Basic dimensions · Taper pipe geometrycomputed+compiled

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+)

Verified against Wikipedia National pipe thread standard-sizes table (CC BY-SA 4.0) — 98 checks passed to 1e-5 in

Part computed from the standard's formulas, part compiled from agreeing public sources.

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

How this data is built and checked →

How to Cite This 3 1/2-8 NPT Reference

Citation

ShopMath. "3 1/2-8 NPT Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/npt/3-1-2

BibTeX

@misc{shopmath-threads-npt-3-1-2,
  title        = {3 1/2-8 NPT Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/npt/3-1-2}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/npt/3-1-2

Common Questions About 3 1/2-8 NPT

What is the tap drill size for 3 1/2-8 NPT?: The tap drill for 3 1/2-8 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 3 1/2-8 NPT?: The basic pitch diameter of 3 1/2-8 NPT is 3.8888 in (98.776 mm).
What is the minor diameter of 3 1/2-8 NPT?: The basic minor diameter of 3 1/2-8 NPT is 3.7888 in (96.236 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 1/2-8 NPT?: 3 1/2-8 NPT has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).
What is the hand-tight engagement length (L₁) of 3 1/2-8 NPT?: The hand-tight engagement length L₁ for 3 1/2-8 NPT is 0.8210 in (20.853 mm), on a pipe outside diameter of 4.0000 in (101.600 mm).

All NPT Taper Pipe Thread sizes · Every thread series