NPT Weld Fittings Guide
NPT is the tapered pipe thread used on most fuel, hydraulic and water tank fittings. This guide covers how NPT is measured and sealed, how our machined bungs, half couplings and full couplings compare, and the measurements we advertise for each NPT fitting.
Part of the Weld Fittings Product Guide.
On this page: How Each Thread Seals | NPT Thread Explained | How We Check the Threads | Our NPT Fittings Compared | Our NPT Fittings | FAQ
How Each Thread Seals
NPT Thread Explained
NPT (American National Standard Taper Pipe Thread) is defined by ASME B1.20.1. It is the most common thread on fuel, hydraulic and water tank fittings.
The taper
- The diameter changes 1" for every 16" of length (3/4" per foot).
- Each side of the thread is angled 1°47′24″ (1.7899°) from the centerline. This is the "1.8° taper" in our listings.
- The thread form is 60°, with flattened crests and roots.
- Because both parts are tapered, the male wedges tighter the farther it goes in.
Why 1/2" NPT doesn't measure 1/2"
The nominal size names the pipe, not any dimension you can measure on the thread. It comes from old pipe sizing, and the thread is cut on the outside of the pipe, so a 1/2" NPT male measures about 0.840" across the threads and a 1/8" NPT male about 0.405". Our listings use the same rule: the NPT size refers to the pipe that threads into the fitting.
Key measurements
- E0: pitch diameter at the small end of the male thread.
- E1: pitch diameter at the L1 plane, where hand-tight engagement ends.
- L1, hand-tight engagement: how far a male thread goes into a female thread by hand.
- L2, effective thread length: the length of full-form male thread that does the sealing.
- L3, wrench makeup: after hand-tight, the joint is tightened with a wrench to wedge the thread flanks together. As a general guideline, ASME B1.20.1 allows about 3 more threads past hand-tight for 1/8" to 2" and 2 more threads for 2-1/2" to 4".
- Tap drill: the drill size used before tapping a female NPT thread without reaming.
| NPT size | TPI | Male OD (D) | E0 | L1 hand-tight | L2 effective | Tap drill |
|---|---|---|---|---|---|---|
| 1/8" | 27 | 0.405" | 0.36351" | 0.1615" (4.36 turns) | 0.2639" | R (0.339") |
| 1/4" | 18 | 0.540" | 0.47739" | 0.2278" (4.10 turns) | 0.4018" | 7/16" |
| 3/8" | 18 | 0.675" | 0.61201" | 0.2400" (4.32 turns) | 0.4078" | 37/64" |
| 1/2" | 14 | 0.840" | 0.75843" | 0.3200" (4.48 turns) | 0.5337" | 23/32" |
| 3/4" | 14 | 1.050" | 0.96768" | 0.3390" (4.75 turns) | 0.5457" | 59/64" |
| 1" | 11.5 | 1.315" | 1.21363" | 0.4000" (4.60 turns) | 0.6828" | 1-5/32" |
| 1-1/4" | 11.5 | 1.660" | 1.55713" | 0.4200" (4.83 turns) | 0.7068" | 1-1/2" |
| 1-1/2" | 11.5 | 1.900" | 1.79609" | 0.4200" (4.83 turns) | 0.7235" | 1-47/64" |
| 2" | 11.5 | 2.375" | 2.26902" | 0.4360" (5.01 turns) | 0.7565" | 2-7/32" |
| 2-1/2" | 8 | 2.875" | 2.71953" | 0.6820" (5.46 turns) | 1.1375" | 2-5/8" |
| 3" | 8 | 3.500" | 3.34062" | 0.7660" (6.13 turns) | 1.2000" | 3-1/4" |
| 4" | 8 | 4.500" | 4.33438" | 0.8440" (6.75 turns) | 1.3000" | 4-1/4" |
| 6" | 8 | 6.625" | 6.44609" | 0.9580" (7.66 turns) | 1.5125" | 6-11/32" |
Basic dimensions per ASME B1.20.1. Tap drill sizes are common no-ream sizes from handbook charts.
How NPT seals
NPT seals where the thread flanks are wedged together by the taper. A tiny spiral path can remain at the crests and roots, so NPT always needs a thread sealant or PTFE tape. NPTF (Dryseal) is a related fuel thread made to seal without sealant. Our fittings are NPT, so use sealant.
How to identify and measure an NPT thread
1Look at the thread. The threads get smaller toward the end of a male fitting. Straight threads (AN, ORB) stay the same size.
2Measure the OD. Measure the male OD across the first full threads with calipers. For a female port, measure the opening. Compare with the Male OD and Tap drill columns above.
3Count the threads per inch. Use a thread pitch gauge, or count the threads in 1/2" and double it. NPT is 27, 18, 14, 11.5 or 8 TPI.
4Match the size. The OD plus TPI gives you the nominal size from the table.
How We Check the Threads
Our female NPT threads are checked with an L1 taper thread plug gauge per ASME B1.20.1. The gauge is screwed in hand-tight. At basic size the gauge notch is flush with the fitting face; the thread passes when the face lands within ±1 turn of the notch.
Our NPT Fittings Compared
We make three styles of NPT weld fitting: machined flanged bungs, half couplings and full couplings. Each one has the same ASME B1.20.1 tapered thread profile. They differ in how they sit on the tank and how much thread length they give you.
Which one should I use?
| Machined NPT flanged bung | Half coupling | Full coupling | |
|---|---|---|---|
| Materials | 6061-T6 aluminum, 1018 steel, 304 stainless | 6061-T6 aluminum | 6061-T6 aluminum |
| Sizes | 1/8" to 3" aluminum; 1/8" to 2" steel and stainless | 1/8" to 6", plus 2" heavy wall | 1/8" to 4" |
| Thread | One female NPT thread through the bung | One end only. The large outside chamfer marks the top. | Both ends |
| How it sits | Shoulder (pilot) drops into the tank hole; flange rests on the tank and is welded at its edge | Welded on at the base | Inserted about halfway through the tank wall; welded from the outside only |
| Overall height | 0.406" to 1.0" (D) | 0.435" to approx. 1.75" (1/8" to 6") | 0.870" to 3.374" (1/8" to 4") |
| Wall / rating | Meets 150# rating minimum | SCH 40 or heavier, 150# rating | SCH 40 or heavier, 150# rating |
| Hole cut | Listed for each size (see chart) | Match the OD as tight as possible, plus slight clearance | Match the OD as tight as possible, plus slight clearance |
| Best for | A low-profile port where higher precision is needed | More thread length on one side of the tank | A thread inside and outside the tank, such as a pass-through |
- Thread length: in every size we list, the half coupling is taller than the machined bung, so it gives more room for the male thread. Every listed height is longer than the standard hand-tight engagement (L1) for its size.
- Orientation: half couplings thread from one end only. Check that the chamfered top faces out before you weld.
- Coupling hole cut: cut the hole to match the coupling OD as tight as possible. Allow a small tolerance for part variance so the coupling still fits.
- Stainless or steel: the machined flanged bung is the NPT style we make in all three metals. Couplings are aluminum only.
Our NPT Fittings
Measurements below are the ones we advertise on each product page. All are made in the USA.
NPT flanged weld-on tank fittings (stamped)
Stamped from 6061 aluminum sheet, 1018 CR steel flat bar or 316 stainless. Shop: Aluminum | Steel | Stainless
| NPT | TPI | ID | OD | Thickness | Installed height | Hole cut | Aluminum | Steel | Stainless |
|---|---|---|---|---|---|---|---|---|---|
| 1/8" | 27 | 0.35" | 1.75" | 0.27" | 0.22" | 0.750" | 21001 | 22001 | — |
| 1/4" | 18 | 0.46" | 1.75" | 0.27" | 0.22" | 0.750" | 21002 | 22002 | 23001 |
| 3/8" | 18 | 0.58" | 1.75" | 0.27" | 0.22" | 1" | 21003 | 22003 | 23002 |
| 1/2" | 14 | 0.73" | 1.75" | 0.27" | 0.22" | 1" | 21004 | 22004 | 23003 |
| 3/4" | 14 | 0.92" | 2.37" | 0.31" | 0.25" | 1.25" | 21005 | 22005 | 23004 |
| 1" | 11.5 | 1.17" | 2.37" | 0.31" | 0.25" | 1.5" | 21006 | 22006 | 23005 |
| 1-1/4" | 11.5 | 1.49" | 2.86" | 0.40" | 0.25" | 2" | 21007 | 22007 | 23006 |
| 1-1/2" | 11.5 | 1.75" | 3.39" | 0.437" | 0.31" | 2.25" | 21008 | 22008 | 23018 |
| 2" | 11.5 | 2.21" | 3.84" | 0.50" | 0.36" | 2.75" | 21009 | 22009 | — |
Machined NPT flanged weld bungs
CNC machined from 6061-T6 aluminum, 1018 steel or 304 stainless bar. Shop: Aluminum | Steel | Stainless
| NPT | (A) Max OD | (B) Shoulder | (C) Height | (D) Overall | Hole cut | Aluminum | Steel | Stainless |
|---|---|---|---|---|---|---|---|---|
| 1/8" | 1.490" | 0.875" | 0.246" | 0.406" | 0.875–1.0" | 21082 | 22012 | 23009 |
| 1/4" | 1.490" | 0.875" | 0.246" | 0.406" | 0.875–1.0" | 21083 | 22013 | 23010 |
| 3/8" | 1.490" | 1.190" | 0.309" | 0.469" | 1.190–1.25" | 21084 | 22014 | 23011 |
| 1/2" | 1.490" | 1.190" | 0.309" | 0.469" | 1.19–1.25" | 21085 | 22015 | 23012 |
| 3/4" | 1.749" | 1.312" | 0.340" | 0.500" | 1.312–1.375" | 21086 | 22016 | 23013 |
| 1" | 2.490" | 1.688" | 0.360" | 0.500" | 1.68–1.75" | 21087 | 22017 | 23014 |
| 1-1/4" | 2.490" | 1.938" | 0.403" | 0.563" | 1.93–2.0" | 21088 | 22018 | 23015 |
| 1-1/2" | 2.990" | 2.250" | 0.418" | 0.578" | 2.25–2.375" | 21089 | 22019 | 23016 |
| 2" | 3.490" | 2.625" | 0.434" | 0.594" | 2.62–2.75" | 21090 | 22020 | 23017 |
| 3" | 4.625" | 3.813" | 0.750" | 1.0" | 3.813–4.0" | 21103 | — | — |
Aluminum NPT half and full couplings
6061-T6, SCH 40 wall or heavier for 150# rating. Half couplings thread from one end; the large outside chamfer marks the top. Full couplings thread from both ends and are inserted about halfway through the tank wall, then welded from the outside. Hole cut: match the OD as tight as possible, plus a slight clearance for part variance. Shop: Half couplings | Full couplings
| NPT | TPI | ID | OD | Hole cut | Half height | Half SKU | Full height | Full SKU |
|---|---|---|---|---|---|---|---|---|
| 1/8" | 27 | 0.330" | 0.600" | Match OD (0.600") + slight clearance | 0.435" | 21027 | 0.870" | 21031 |
| 1/4" | 18 | 0.450" | 0.750" | Match OD (0.750") + slight clearance | 0.500" | 21011 | 1.000" | 21016 |
| 3/8" | 18 | 0.580" | 0.910" | Match OD (0.910") + slight clearance | 0.570" | 21012 | 1.140" | 21017 |
| 1/2" | 14 | 0.750" | 1.060" | Match OD (1.060") + slight clearance | 0.650" | 21013 | 1.300" | 21018 |
| 3/4" | 14 | 0.950" | 1.292" | Match OD (1.292") + slight clearance | 0.760" | 21014 | 1.520" | 21019 |
| 1" | 11.5 | 1.170" | 1.625" | Match OD (1.625") + slight clearance | 0.880" | 21015 | 1.760" | 21020 |
| 1-1/4" | 11.5 | 1.500" | 1.970" | Match OD (1.970") + slight clearance | 1.000" | 21035 | 2.000" | — |
| 1-1/2" | 11.5 | 1.750" | 2.235" | Match OD (2.235") + slight clearance | 1.120" | 21033 | 2.240" | 21064 |
| 2" | 11.5 | 2.250" | 2.750" | Match OD (2.750") + slight clearance | 1.250" | 21028 | 2.500" | 21055 |
| 2" heavy wall | 11.5 | 2.250" | 3.226" | Match OD (3.226") + slight clearance | 1.375" | 21028-HW | — | — |
| 2-1/2" | 8 | 2.635" | 3.300" | Match OD (3.300") + slight clearance | 1.410" | 21026 | 2.820" | — |
| 3" | 8 | 3.280" | 3.930" | Match OD (3.930") + slight clearance | 1.530" | 21036 | 3.060" | 21071 |
| 4" | 8 | 4.250" | 4.940" | Match OD (4.940") + slight clearance | 1.687" | 21034 | 3.374" | 21073 |
| 6" | 8 | Not listed | Not listed | Not listed | Approx. 1.75" | 21067 | — | — |
1-1/4" and 2-1/2" full coupling heights are from our chart; those sizes have no full coupling SKU listed (—). The 4" full coupling is twice the 4" half coupling height. Allow a small tolerance on the hole cut for part variance.
FAQ
Why doesn't my NPT fitting measure its size?
NPT sizes name the pipe, not the thread. A 1/2" NPT male measures about 0.840" OD. See the NPT table above.
What size hole do I cut in the tank?
Stamped flanged fittings and machined bungs: use the Hole cut column. Couplings: match the coupling OD as tight as possible and allow a small tolerance for part variance.
Which material should I use?
Match the tank: aluminum fittings on aluminum tanks, steel on steel, stainless on stainless. Couplings are aluminum only.
References: ASME B1.20.1 Pipe Threads, General Purpose (Inch); Machinery's Handbook (Industrial Press), pipe thread section; L1 gaging per ASME B1.20.1. NPT values were cross-checked against published B1.20.1 tables from PipingCalcPro and Osterman Mfg.