NPT Thread

What is NPT? National Pipe Thread

NPT stands for National Pipe Thread, a standard originating from the United States. It defines a family of screw threads commonly used for pipes, fittings, and adapters. The key difference between thread types lies in whether they are tapered or straight, which affects how they seal:

NPT – National Pipe Tapered: Tapered thread, seals by wedging and tightening into the female thread.
NPS – National Pipe Straight: Straight (parallel) thread, seals with a washer, gasket, or O-ring.

Other related variations include:

NPTF – National Pipe Tapered Fuel (dry-seal)
NPSF – National Pipe Straight Fuel
NPSM – National Pipe Straight Mechanical

1. NPT Threads Explained

The NPT thread is a tapered thread, meaning the diameter decreases slightly along the length of the thread. This taper (1°47′ or 3/4" per foot) causes the male and female threads to compress and form a mechanical seal. However, unlike some other thread standards, standard NPT threads typically require an additional thread sealant (e.g., PTFE tape or thread sealant compound) to ensure a leak-free connection.

Key Characteristics

Taper angle: 1°47′ (approx. 3/4" per foot)
Threads per inch (TPI): Defined by nominal size
Seal method: Mechanical interference + thread sealant
Applications: Plumbing, hydraulics, pneumatics, fuel systems, motorsport, industrial piping

2. Installation of NPT Threads

When installing tapered NPT threads, the male fitting will “bite” or lock into the female fitting as it is tightened. The number of turns can vary depending on tolerances and whether sealant is used:

Typically 2–3 turns by hand, followed by 2–3 turns with a wrench.
Using PTFE tape or liquid thread sealant reduces how far the fitting can be screwed in, as the material fills the gaps in the threads.

Sealants for NPT

PTFE tape (Teflon tape) – widely used, quick to apply.
Liquid sealants (e.g., Loctite 567, Loctite 577) – create a strong chemical bond, more resistant to vibration and temperature.

3. NPT Size Chart

The taper means the diameter changes along the thread. Therefore, both a maximum (large end) and minimum (small end) thread diameter are defined. Below is a simplified chart of common NPT sizes:

Nominal Size	Threads per Inch	Max. OD (Large End)	Min. OD (Small End)
1/16" - 27	27 TPI	0.3125" / 7.94 mm	7.40 mm
1/8" - 27	27 TPI	0.405" / 10.29 mm	9.88 mm
1/4" - 18	18 TPI	0.540" / 13.72 mm	13.11 mm
3/8" - 18	18 TPI	0.675" / 17.15 mm	16.27 mm
1/2" - 14	14 TPI	0.840" / 21.34 mm	20.60 mm
3/4" - 14	14 TPI	1.050" / 26.67 mm	25.90 mm
1" - 11½	11½ TPI	1.315" / 33.40 mm	32.50 mm
1¼" - 11½	11½ TPI	1.660" / 42.16 mm	41.30 mm

4. Identifying the Correct NPT Thread

If you are unsure which NPT thread you have, the above table can help:

Internal (female) thread: Measure the minor diameter deep inside the thread. Match it with the minimum OD in the table.
External (male) thread: Measure the outside diameter at the large end of the thread. Match it with the maximum OD in the table.

Tip: NPT threads can sometimes be mistaken for metric or UNF threads (e.g., M10 x 1.0 vs. 1/8" NPT). Always confirm with both diameter and TPI.

5. Tapping and Drilling for NPT Threads

To create an NPT thread, a tapered tap is required. Proper drilling size is critical — too small and the tap may break or the thread may be too tight, too large and the thread may not seal properly.

Nominal Size	Drill Size
1/16" - 27	6.2 mm
1/8" - 27	8.5 mm
1/4" - 18	11.0 mm
3/8" - 18	14.5 mm
1/2" - 14	17.8 mm
3/4" - 14	23.0 mm
1" - 11½	29.0 mm
1¼" - 11½	37.5 mm

6. NPT vs. NPTF

While both NPT and NPTF are tapered threads, there are important differences:

NPT (National Pipe Tapered)
Requires thread sealant to ensure leak-tightness. Widely used in aftermarket and general applications.
NPTF (National Pipe Tapered Fuel – Dryseal)
Designed to seal without additional sealant. Threads are manufactured with tighter tolerances, so that they deform and create a metal-to-metal seal during tightening.
- Advantages: Leak-free without sealant.
- Disadvantages: Cannot always be reused; once deformed, threads may not seal properly if refitted.

Important: Do not mix NPT and NPTF. While they appear visually similar, mixing them often results in leaks or mechanical failure.

7. Practical Tips & Common Mistakes

Always use proper sealant with NPT unless you are certain you have NPTF.
Do not overtighten — this can crack fittings or distort the threads.
Check for cross-threading before applying torque.
Remember that NPT sizes are nominal and do not correspond to the actual outside diameter of the pipe or fitting.
In motorsport and performance applications, NPT is often replaced with AN/JIC fittings for higher reliability — but NPT is still very common in fuel systems, cooling, and sensors.