NPTF Thread Size Chart: Dimensions, TPI & Thread Guide

NPTF threads are widely used in hydraulic, pneumatic, fuel, lubrication, and general fluid-handling systems where a tapered pipe thread is required to create a pressure-tight connection. NPTF stands for National Pipe Taper Fuel, although the thread form is more commonly associated with the Dryseal pipe thread system defined by ASME standards.

At first glance, NPTF threads look very similar to standard NPT threads because both use a 60° thread angle and the same basic 1:16 taper. However, the sealing principle is different. NPTF threads are designed so that the thread crests and roots interfere more closely when tightened. This controlled interference reduces the spiral leakage path that can remain in a conventional tapered pipe-thread connection.

Because nominal pipe thread sizes do not directly match the measured outside diameter of the thread, identifying an unknown NPTF fitting by measurement alone can be confusing. For example, a fitting identified as 1/4 NPTF has a male thread diameter considerably larger than 1/4 inch.

An NPTF Thread Size Chart therefore provides a practical reference for comparing nominal thread size, threads per inch (TPI), pitch, thread diameter, and related dimensions. It is particularly useful when selecting fittings, checking replacement components, inspecting threaded ports, or distinguishing NPTF threads from NPT, BSPT, and other tapered thread systems.

1. What Is an NPTF Thread?

NPTF is a tapered pipe thread designed primarily to provide a pressure-tight mechanical seal through controlled interference between the mating thread surfaces.

The designation NPTF is commonly expanded as:

NPTF = National Pipe Taper Fuel

NPTF threads belong to the family of Dryseal pipe threads covered by ASME B1.20.3. The word “Dryseal” refers to the thread geometry being designed to minimize leakage through the thread itself rather than relying entirely on a sealing compound to fill the clearance between the male and female threads.

Basic NPTF Thread Geometry

NPTF threads use the same general thread profile as other American tapered pipe threads:

• Thread angle: 60°
• Taper: 1 in 16 on diameter
• Taper per side: approximately 1° 47′
• Thread form: symmetrical V-thread
• Sizing system: nominal pipe size
• Pitch designation: threads per inch (TPI)

The 1:16 taper means that the thread diameter changes by approximately 1 inch for every 16 inches of axial thread length.

Because both male and female threads are tapered, tightening the fitting causes increasing contact and interference between the two thread forms.

How NPTF Creates a Seal

A conventional NPT connection can contain small clearances between the thread crests and roots. These clearances may form a continuous spiral path through which gas or liquid can potentially leak.

NPTF modifies the thread tolerances so that greater interference occurs at the crest and root areas.

As the fitting is tightened:

Male thread + Female thread → Increasing interference → Crest/root deformation → Reduced leakage path

The resulting metal-to-metal contact helps create a tighter seal.

This is the fundamental reason NPTF threads are classified as Dryseal threads.

However, “Dryseal” does not necessarily mean that thread sealant must never be used. Sealant or lubricant may still be specified depending on system pressure, fluid compatibility, fitting material, manufacturer recommendations, and assembly requirements.

Typical NPTF Applications

NPTF threads are commonly encountered in:

• Hydraulic fittings
• Pneumatic fittings
• Fuel systems
• Lubrication systems
• Compressors
• Pumps
• Valves
• Pressure gauges
• Instrument connections
• Automotive fluid systems
• Industrial machinery

They are particularly useful where compact threaded connections are preferred and the thread itself contributes significantly to the sealing mechanism.

Example NPTF Designations

Typical thread callouts include:

1/8-27 NPTF

This means:

• Nominal size: 1/8 inch
• Thread density: 27 threads per inch
• Thread type: NPTF

Another example is:

1/2-14 NPTF

Meaning:

• Nominal size: 1/2 inch
• Thread density: 14 TPI
• Tapered Dryseal pipe thread

The nominal size should not be interpreted as the actual thread outside diameter.

For example, a 1/2-inch NPTF male thread has an outside diameter substantially larger than 0.500 inch.

2. NPTF Thread Dimensions and Terminology

Understanding an NPTF thread size chart requires familiarity with several thread dimensions. These dimensions describe the geometry of the tapered thread and help engineers identify the correct fitting or port.

Nominal NPTF Size

NPTF threads are designated using nominal pipe sizes, such as:

• 1/16″
• 1/8″
• 1/4″
• 3/8″
• 1/2″
• 3/4″
• 1″
• 1-1/4″
• 1-1/2″
• 2″

The nominal size is a designation rather than a direct physical measurement of the thread.

For example:

1/4 NPTF does not mean the thread OD is 0.250 inch.

The actual male thread diameter is much larger.

This is one of the most important points when identifying NPTF threads in the field.

Threads per Inch – TPI

NPTF threads are normally specified using threads per inch (TPI).

TPI represents how many complete thread pitches occur within one inch of axial length.

Typical examples include:

NPTF Size	TPI
1/16	27
1/8	27
1/4	18
3/8	18
1/2	14
3/4	14
1	11.5

TPI is one of the easiest characteristics to check with a thread pitch gauge.

Thread Pitch

Thread pitch is the axial distance between corresponding points on adjacent threads.

For inch-based threads:

Pitch = 1 / TPI

For example, an 18 TPI NPTF thread has a pitch of approximately:

1 / 18 = 0.0556 in

Converted to millimeters:

0.0556 × 25.4 ≈ 1.411 mm

A 14 TPI thread has a pitch of approximately:

1 / 14 = 0.0714 in

or:

0.0714 × 25.4 ≈ 1.814 mm

Major Diameter

The major diameter is the largest diameter of an external male thread.

On an NPTF thread, the diameter changes continuously along the threaded length because the thread is tapered.

Therefore, the measured major diameter depends on where along the thread the measurement is taken.

This is different from a parallel thread such as BSPP or SAE straight thread, where the thread diameter remains essentially constant.

Minor Diameter

The minor diameter is the smallest diameter of the thread profile.

For an external thread, it is measured near the thread roots.

For an internal thread, it corresponds to the diameter near the thread crests.

Because NPTF is tapered, the minor diameter also changes along the thread axis.

Pitch Diameter

The pitch diameter is an imaginary diameter at which the width of the thread ridge is approximately equal to the width of the thread groove.

It is one of the most important dimensions for controlling thread fit and engagement.

For NPTF threads, correct pitch diameter is essential because the sealing action depends on controlled interference between the male and female threads.

Thread Angle

NPTF threads use a:

60° included thread angle

This is the same general included angle used by NPT and Unified threads.

However, thread angle alone cannot be used to distinguish NPTF from NPT because both use a 60° profile.

Thread Taper

NPTF uses a standard taper of:

1:16 on diameter

This is equivalent to approximately:

3/4 inch diameter change per foot of thread length

The taper causes the connection to become progressively tighter as the male fitting is screwed into the female port.

Male and Female NPTF Threads

An external NPTF thread is normally found on:

• Male adapters
• Nipples
• Male connectors
• Plugs
• Valve connections

An internal NPTF thread is normally found in:

• Female adapters
• Valve ports
• Manifolds
• Pump ports
• Equipment housings

Correct sealing requires compatible external and internal thread forms with appropriate thread tolerances and engagement.

When identifying an unknown NPTF connection, the most useful measurements are typically the thread OD or ID, TPI, and confirmation that the thread is tapered.

3. NPTF Thread Standards and Specifications

NPTF threads are standardized under ASME B1.20.3 – Dryseal Pipe Threads (Inch). This standard defines the thread form, taper, dimensions, tolerances, engagement requirements, and gauging methods used for Dryseal pipe threads.

The purpose of the NPTF design is to produce a tighter thread fit than a conventional NPT connection. This is achieved by controlling the relationship between the thread crests, roots, and flanks so that the mating threads create greater interference when tightened.

ASME B1.20.3 Dryseal Pipe Threads

ASME B1.20.3 covers several types of Dryseal pipe threads, including:

• NPTF – National Pipe Taper Fuel
• PTF-SAE Short – Dryseal SAE short taper thread
• NPSF – Dryseal American Standard Fuel Internal Straight Pipe Thread
• NPSI – Dryseal American Standard Intermediate Internal Straight Pipe Thread

Among these, NPTF is one of the most commonly encountered tapered Dryseal thread forms in hydraulic, pneumatic, automotive, and general industrial applications.

NPTF Thread Angle

NPTF threads use a:

60° included thread angle

The thread profile is based on a symmetrical V-shaped form.

This is similar to standard NPT threads, which is one reason the two thread types can be difficult to distinguish visually.

NPTF Thread Taper

NPTF threads use a taper of:

1:16 on diameter

This means the thread diameter changes by approximately 1 inch over 16 inches of axial length.

The same taper can also be expressed as:

0.75 inch diameter change per foot

or approximately:

1° 47′ taper angle per side

This taper causes increasing interference as the male thread is screwed deeper into the female thread.

L1, L2, and L3 Thread Dimensions

NPTF thread specifications commonly use several reference lengths to describe thread engagement.

L1 – Hand-Tight Engagement Length

L1 represents the approximate axial length of engagement when the male and female threads are assembled by hand.

At this point, the connection has begun to develop interference but has not yet reached its final tightened condition.

L2 – External Thread Length

L2 is associated with the length of the external tapered thread required to provide sufficient wrench engagement beyond the hand-tight position.

It helps define the useful threaded length on the male component.

L3 – Internal Thread Length

L3 is associated with the internal thread length required to accommodate proper engagement of the external thread.

These dimensions vary depending on the nominal NPTF size and thread pitch.

Internal and External NPTF Threads

NPTF threads can be classified as:

External NPTF thread

Used on male fittings such as adapters, connectors, nipples, and plugs.

Internal NPTF thread

Used in valve bodies, manifolds, pumps, cylinders, and other equipment ports.

The two tapered threads mate together to create progressively increasing interference.

NPTF Gauging

Because NPTF sealing performance depends strongly on thread geometry and engagement, inspection is normally performed using standardized thread gauges.

Common inspection methods include:

• L1 plug gauges
• L1 ring gauges
• Six-step gauges
• Crest and root inspection
• Functional gauging

A simple caliper can help identify the nominal size, but it cannot confirm whether a manufactured thread fully complies with ASME B1.20.3.

For precision inspection, proper NPTF gauges should be used.

---

4. Complete NPTF Thread Size Chart

The following NPTF thread size chart provides a practical reference for common NPTF sizes.

Because tapered thread diameters vary along the thread length, the outside diameter shown below should be treated as an approximate identification value rather than a manufacturing inspection dimension.

Nominal NPTF Size	TPI	Pitch (in)	Pitch (mm)	Approx. Male Thread OD (in)	Approx. Male Thread OD (mm)
1/16	27	0.0370	0.941	0.313	7.94
1/8	27	0.0370	0.941	0.405	10.29
1/4	18	0.0556	1.411	0.540	13.72
3/8	18	0.0556	1.411	0.675	17.15
1/2	14	0.0714	1.814	0.840	21.34
3/4	14	0.0714	1.814	1.050	26.67
1	11.5	0.0870	2.209	1.315	33.40
1-1/4	11.5	0.0870	2.209	1.660	42.16
1-1/2	11.5	0.0870	2.209	1.900	48.26
2	11.5	0.0870	2.209	2.375	60.33

Important Note About Thread Diameter

The measured outside diameter of an NPTF male fitting is always larger than the nominal thread size.

For example:

• 1/4 NPTF has an OD of approximately 0.540 in
• 1/2 NPTF has an OD of approximately 0.840 in
• 1 NPTF has an OD of approximately 1.315 in

Therefore, measuring a thread diameter of approximately 0.84 inch does not mean the fitting is an 0.84-inch thread.

It normally corresponds to approximately:

1/2 NPTF

Why NPTF Diameter Is Not Equal to Nominal Size

Nominal pipe thread sizes originated from traditional pipe sizing systems rather than direct thread diameter measurements.

As a result:

Nominal size ≠ measured thread diameter

This is also true for NPT threads.

For field identification, always compare at least:

• Nominal size
• Measured outside diameter
• Threads per inch
• Thread taper

Using diameter alone can easily lead to incorrect identification.

Approximate NPTF Tap Drill Sizes

Tap drill dimensions can vary depending on material, thread engagement, tap type, and manufacturing recommendations.

Typical drill sizes are approximately:

NPTF Size	Typical Tap Drill Size
1/16-27	15/64 in
1/8-27	21/64 in
1/4-18	7/16 in
3/8-18	37/64 in
1/2-14	23/32 in
3/4-14	59/64 in
1-11.5	1-5/32 in

These values are useful as general machining references, but production components should always be manufactured according to the applicable standard, tooling manufacturer’s recommendations, and required thread class.

---

5. NPTF Thread Pitch and TPI Chart

Threads per inch, or TPI, is one of the easiest ways to identify an unknown NPTF thread.

TPI describes the number of thread pitches within one inch of axial thread length.

The relationship between thread pitch and TPI is:

Thread Pitch = 1 / TPI

For metric comparison:

Pitch in mm = 25.4 / TPI

These simple formulas are useful because NPTF threads are commonly specified by TPI rather than millimeter pitch.

NPTF TPI Quick Reference Chart

NPTF Size	TPI	Pitch (in)	Pitch (mm)
1/16	27	0.0370	0.941
1/8	27	0.0370	0.941
1/4	18	0.0556	1.411
3/8	18	0.0556	1.411
1/2	14	0.0714	1.814
3/4	14	0.0714	1.814
1	11.5	0.0870	2.209
1-1/4	11.5	0.0870	2.209
1-1/2	11.5	0.0870	2.209
2	11.5	0.0870	2.209

Example: 1/4-18 NPTF

A thread marked:

1/4-18 NPTF

means:

• Nominal size: 1/4 inch
• Thread density: 18 TPI
• Thread pitch: approximately 0.0556 inch
• Metric-equivalent pitch: approximately 1.411 mm
• Thread form: tapered Dryseal pipe thread

Example: 1/2-14 NPTF

A designation of:

1/2-14 NPTF

means:

• Nominal size: 1/2 inch
• 14 threads per inch
• Pitch: approximately 0.0714 inch
• Pitch: approximately 1.814 mm

Using a Thread Pitch Gauge

A thread pitch gauge is one of the fastest tools for checking NPTF thread pitch.

To identify the thread:

1. Select a likely TPI blade.
2. Place it against the thread.
3. Check whether the teeth fit evenly into the thread grooves.
4. Repeat until there is no visible gap.
5. Compare the measured TPI with an NPTF size chart.

For example, if a male tapered thread measures approximately:

0.54 in OD

and the thread gauge shows:

18 TPI

the likely thread size is:

1/4-18 NPTF

If the thread measures approximately:

0.84 in OD

with:

14 TPI

the likely size is:

1/2-14 NPTF

TPI Alone Is Not Enough

Several NPTF sizes share the same TPI.

For example:

• 1/16 and 1/8 both use 27 TPI
• 1/4 and 3/8 both use 18 TPI
• 1/2 and 3/4 both use 14 TPI
• 1, 1-1/4, 1-1/2, and 2 commonly use 11.5 TPI

Therefore, thread pitch should always be checked together with the approximate thread diameter.

A reliable identification method is:

Measure OD + Check TPI + Confirm taper = Determine NPTF size

This combination greatly reduces the risk of confusing NPTF with another pipe or hydraulic thread type.

6. NPTF vs. NPT Threads: What Is the Difference?

NPTF and NPT threads look very similar and share the same nominal sizes, thread pitch, 60° thread angle, and 1:16 taper. However, they are designed with different sealing characteristics.

The main difference is how the thread crests and roots interact when the connection is tightened.

NPT Thread Sealing

NPT stands for National Pipe Taper.

NPT threads rely primarily on interference along the thread flanks. Small clearances can remain between the crest of one thread and the root of the mating thread.

These clearances may create a spiral leakage path through the connection.

For this reason, NPT connections normally require a suitable thread sealant such as:

• PTFE tape
• Pipe-thread sealing compound
• Anaerobic thread sealant
• Manufacturer-approved liquid sealant

The sealant fills the microscopic clearance between the mating threads and helps prevent leakage.

NPTF Dryseal Thread Sealing

NPTF threads are designed to create additional interference between the thread crests and roots.

When properly assembled, the mating threads deform slightly and reduce the clearance that can form a spiral leakage path.

This produces a more complete metal-to-metal sealing condition.

The basic sealing principle can be summarized as:

Tapered thread engagement → Increasing interference → Crest and root contact → Reduced leakage path

This characteristic is why NPTF is commonly called a Dryseal pipe thread.

NPT vs. NPTF Comparison

Feature	NPT	NPTF
Full Name	National Pipe Taper	National Pipe Taper Fuel
Primary Standard	ASME B1.20.1	ASME B1.20.3
Thread Angle	60°	60°
Taper	1:16	1:16
Nominal Sizes	Same basic pipe sizes	Same basic pipe sizes
TPI	Generally same by size	Generally same by size
Crest/Root Interference	Limited	Greater controlled interference
Sealing Principle	Flank interference + sealant	Dryseal crest/root interference
Sealant Normally Used	Yes	May not always be required
Typical Applications	General piping	Fuel, hydraulic, pneumatic, industrial systems

Can NPT and NPTF Be Connected?

Because the basic thread dimensions are closely related, an NPT male thread may sometimes physically screw into an NPTF female thread and vice versa.

However:

Physical fit does not automatically mean the connection meets the intended sealing or specification requirements.

The crest and root geometry and tolerance controls are different.

For critical systems, always use the thread form specified by the component manufacturer or engineering standard.

An NPT fitting should not automatically be treated as an NPTF replacement simply because it appears to fit.

Does NPTF Require Thread Sealant?

The term Dryseal sometimes leads to the assumption that NPTF connections must always be assembled completely dry.

That is not necessarily correct.

Depending on the application, a lubricant or sealant may still be used to:

• Reduce assembly friction
• Prevent galling
• Improve sealing reliability
• Protect threads against corrosion
• Facilitate future disassembly

Always follow the fitting manufacturer’s assembly instructions.

---

7. How to Measure and Identify an NPTF Thread

Correctly identifying an NPTF thread requires more than measuring its outside diameter.

Because NPTF sizes are nominal and the threads are tapered, the best identification method is to combine several measurements.

The most useful parameters are:

• Male thread outside diameter
• Female thread inside diameter
• Threads per inch
• Thread taper
• Thread form

Tools Required

A basic NPTF identification can usually be performed using:

• Vernier caliper
• Digital caliper
• Thread pitch gauge
• Steel ruler
• NPT/NPTF reference chart

For production inspection or quality control, dedicated thread gauges should be used.

Step 1: Determine Whether the Thread Is Male or Female

First determine whether the component has an external or internal thread.

An external thread is commonly found on:

• Male adapters
• Nipples
• Connectors
• Plugs

An internal thread is commonly found in:

• Valve ports
• Manifolds
• Pump bodies
• Female adapters
• Equipment housings

For a male thread, measure the approximate outside diameter.

For a female thread, measure the approximate internal opening and compare it with a suitable reference chart.

Step 2: Measure the Male Thread OD

Use a caliper to measure across the thread crests.

Because NPTF is tapered, the measured diameter changes depending on the measurement location.

For field identification, measure near the larger end of an external tapered thread and compare the result with the approximate NPTF thread OD.

Typical values include:

Approx. Male Thread OD	Likely NPTF Size
0.405 in / 10.3 mm	1/8 NPTF
0.540 in / 13.7 mm	1/4 NPTF
0.675 in / 17.1 mm	3/8 NPTF
0.840 in / 21.3 mm	1/2 NPTF
1.050 in / 26.7 mm	3/4 NPTF
1.315 in / 33.4 mm	1 NPTF

These are approximate identification dimensions rather than precision inspection values.

Step 3: Measure the Threads per Inch

Place a thread pitch gauge against the thread.

The gauge teeth should fit completely into the thread grooves without visible gaps.

Common NPTF pitches include:

• 27 TPI
• 18 TPI
• 14 TPI
• 11.5 TPI

For example:

Approx. OD = 13.7 mm + 18 TPI

This strongly indicates:

1/4-18 NPTF

Another example:

Approx. OD = 21.3 mm + 14 TPI

This strongly indicates:

1/2-14 NPTF

Step 4: Confirm That the Thread Is Tapered

NPTF is a tapered thread.

The diameter should become progressively larger toward the base of an external thread.

If the thread remains essentially the same diameter along its entire length, it may be a straight-thread system rather than NPTF.

Examples of straight threads include:

• BSPP
• SAE ORB
• NPSF
• Metric parallel threads

This is an important distinction because tapered and parallel threads often use completely different sealing methods.

Step 5: Check the Thread Angle

NPTF uses a:

60° included angle

This helps distinguish it from BSP threads, which use a 55° thread angle.

However, distinguishing 55° and 60° thread forms by eye can be difficult.

A thread profile gauge or known reference fitting provides a more reliable result.

Practical Identification Example

Suppose an unknown male fitting has the following measurements:

• Outside diameter: approximately 17.1 mm
• Thread pitch: 18 TPI
• Thread diameter increases toward the fitting body

Comparing these values with the NPTF chart gives:

3/8-18 NPTF

Another fitting measures:

• Outside diameter: approximately 26.7 mm
• Pitch: 14 TPI
• Tapered thread

The likely identification is:

3/4-14 NPTF

NPT vs. NPTF Identification Limitation

A caliper and pitch gauge can usually determine the nominal pipe-thread size, but they may not reliably distinguish NPT from NPTF.

This is because both can have:

• The same nominal size
• The same TPI
• A 60° thread angle
• A 1:16 taper

For positive NPTF verification, thread geometry must be checked using the appropriate gauges and inspection procedures.

---

8. NPTF Thread Engagement, Sealing, and Installation

Proper thread engagement is essential to the sealing performance and mechanical strength of an NPTF connection.

Because NPTF is tapered, the connection becomes progressively tighter as the male thread advances into the female port.

Hand-Tight Engagement

The first stage of assembly is normally called hand-tight engagement.

The male thread is rotated into the female thread without applying significant wrench torque.

At this point:

• Several threads are engaged
• The connection becomes progressively tighter
• Initial thread interference begins
• The fitting is not yet fully installed

The theoretical hand-tight engagement is associated with the L1 reference dimension used in pipe-thread specifications.

Wrench-Tight Engagement

After reaching hand-tight engagement, additional rotation is applied using a suitable wrench.

This is known as wrench-tight engagement.

During this stage:

• Thread interference increases
• Crest and root contact increases
• Localized thread deformation can occur
• The leakage path is reduced
• Mechanical retention increases

Correct wrench engagement is important because both insufficient and excessive tightening can cause problems.

NPTF Dryseal Principle

In a standard tapered thread, small clearances may remain between mating thread roots and crests.

NPTF is designed so that additional interference develops in these areas.

The sealing mechanism can be represented as:

Thread engagement → Flank contact → Crest/root interference → Local deformation → Pressure-tight connection

This controlled interference is the primary difference between NPTF Dryseal threads and conventional NPT threads.

Effective Thread Length

Only a portion of the total thread length contributes effectively to the assembled connection.

The thread must provide sufficient engagement to achieve:

• Mechanical strength
• Proper interference
• Required sealing performance

Too little engagement can result in:

• Leakage
• Weak mechanical connection
• Insufficient thread interference
• Thread pull-out under load

Excessive engagement can cause:

• Port cracking
• Thread distortion
• Fitting damage
• Excessive installation stress

Proper Tightening

There is no single universal number of wrench turns that applies to every NPTF fitting.

Required tightening depends on factors such as:

• Nominal thread size
• Fitting material
• Port material
• Thread condition
• Surface finish
• Lubrication
• Sealant
• Manufacturer specification

Therefore, assembly torque or wrench-turn recommendations provided by the fitting manufacturer should take priority over general rules.

Avoid Over-Tightening

Tapered pipe threads generate radial forces in the female component.

As the male fitting is tightened deeper into the port, the male thread acts like a wedge.

Excessive tightening can therefore generate high hoop stress in the female component.

Possible consequences include:

• Cracked valve body
• Split manifold port
• Deformed female thread
• Damaged fitting
• Permanent dimensional distortion

This risk is particularly important with relatively brittle materials or thin-wall ports.

Thread Lubrication and Sealant

Although NPTF is a Dryseal thread form, some applications may still specify thread lubricant or sealant.

Possible reasons include:

• Preventing galling
• Reducing tightening friction
• Improving leak tightness
• Protecting against corrosion
• Improving assembly consistency

Stainless steel threaded fittings require particular care because stainless-to-stainless threaded connections can be susceptible to galling.

Always use products that are compatible with:

• System fluid
• Pressure
• Temperature
• Fitting material
• Process cleanliness requirements

Can NPTF Threads Be Reused?

Repeated assembly and disassembly can alter the thread surfaces because NPTF sealing depends partly on controlled interference and deformation.

Reuse can therefore affect:

• Thread geometry
• Engagement position
• Sealing performance
• Required tightening torque

Before reusing an NPTF connection, inspect it for:

• Flattened thread crests
• Damaged roots
• Galling
• Corrosion
• Cross-threading
• Excessive deformation

For critical pressure systems, replacement may be preferable when thread condition is questionable.

Good Installation Practices

For reliable NPTF assembly:

• Confirm the correct thread type and size.
• Inspect male and female threads before assembly.
• Remove dirt, metal chips, and damaged sealant.
• Avoid cross-threading.
• Use compatible lubricant or sealant when required.
• Follow the manufacturer’s tightening recommendation.
• Do not force a fitting that shows abnormal resistance.
• Perform the required pressure or leak test after assembly.

Proper identification and controlled installation are essential because an NPTF fitting can appear mechanically secure even when the thread combination or engagement is incorrect.

9. NPTF vs. Other Pipe and Hydraulic Threads

NPTF is only one of several thread systems commonly used in hydraulic, pneumatic, process, and industrial fluid systems. Correct identification is important because threads that appear similar may have different angles, tapers, sealing methods, and dimensional standards.

Using the wrong combination can result in:

• Poor thread engagement
• Leakage
• Damaged threads
• Cracked ports
• Reduced pressure capability
• Unsafe connections

The most common thread systems that may be confused with NPTF include NPT, BSPT, BSPP, SAE ORB, JIC, and metric threads.

NPTF vs. NPT

NPT and NPTF are very similar.

Both normally use:

• 60° thread angle
• 1:16 taper
• Inch-based nominal pipe sizes
• Similar TPI values for corresponding sizes

The main difference is the sealing geometry.

NPT typically relies on thread interference plus sealant, while NPTF is designed with greater crest and root interference to reduce the spiral leakage path.

Feature	NPTF	NPT
Thread Angle	60°	60°
Taper	1:16	1:16
Sealing Method	Dryseal interference	Thread interference + sealant
Main Standard	ASME B1.20.3	ASME B1.20.1
Crest/Root Interference	Greater	Less
Sealant	May be used	Normally used

Because their basic dimensions are closely related, NPT and NPTF fittings may sometimes physically engage. However, they should not automatically be considered equivalent for design or inspection purposes.

NPTF vs. BSPT

BSPT stands for British Standard Pipe Taper.

Like NPTF, BSPT uses a tapered thread, but the thread geometry is different.

The most important difference is the thread angle:

NPTF = 60°

BSPT = 55°

BSPT also uses the Whitworth thread form with rounded crests and roots.

Feature	NPTF	BSPT
Thread Angle	60°	55°
Thread Type	Tapered	Tapered
Thread Form	American pipe thread	Whitworth
Typical Standard	ASME B1.20.3	ISO 7-1 / BSP system
Sealing	Thread interference	Thread interference

Although some NPTF and BSPT sizes may appear close, they should not be intentionally connected.

Different pitch and thread profiles can create partial engagement without producing a reliable seal.

NPTF vs. BSPP

BSPP stands for British Standard Pipe Parallel.

The primary difference is that BSPP is a straight thread, while NPTF is tapered.

NPTF: tapered

BSPP: parallel

BSPP connections usually seal with a separate sealing element, such as:

• Bonded washer
• O-ring
• Gasket
• Metal sealing surface

The thread primarily provides mechanical retention.

NPTF, in contrast, uses the tapered thread itself as an important part of the sealing mechanism.

NPTF vs. SAE ORB

SAE ORB stands for SAE O-Ring Boss.

SAE ORB connections use straight threads and an elastomeric O-ring to provide the pressure seal.

This gives ORB a fundamentally different sealing principle from NPTF.

Feature	NPTF	SAE ORB
Thread	Tapered	Straight
Sealing Element	Thread itself	O-ring
Typical Standard	ASME B1.20.3	SAE J1926
Reusability	More limited	Generally better
Orientation	Limited	Easier with adjustable fittings

SAE ORB connections are commonly preferred in modern hydraulic systems because the seal does not depend on thread deformation.

NPTF vs. JIC

JIC fittings commonly use a 37° flare connection.

The thread is a straight UN/UNF thread, but the thread does not create the pressure seal.

Instead, sealing occurs between two metal flare surfaces.

The basic principle is:

JIC thread = mechanical retention

37° flare = sealing surface

NPTF is different because the tapered thread itself contributes directly to the seal.

For this reason, a JIC fitting should never be identified only by measuring its thread diameter.

The sealing surface must also be inspected.

NPTF vs. Metric Threads

Metric fittings can use either parallel or tapered threads depending on the standard.

Metric threads are generally specified using:

Nominal diameter × pitch

For example:

M18 × 1.5

This means:

• Nominal diameter = 18 mm
• Pitch = 1.5 mm

NPTF threads, by contrast, are identified using nominal pipe size and TPI.

For example:

1/2-14 NPTF

means:

• Nominal pipe size = 1/2 inch
• Threads per inch = 14
• Thread form = NPTF

This difference in naming convention is useful when identifying an unknown connection.

Quick Thread Comparison Chart

Thread Type	Angle	Tapered / Parallel	Main Sealing Method
NPTF	60°	Tapered	Thread interference
NPT	60°	Tapered	Thread + sealant
BSPT	55°	Tapered	Thread interference
BSPP	55°	Parallel	Washer / O-ring / sealing face
SAE ORB	60° thread form	Parallel	O-ring
JIC	60° UN/UNF thread	Parallel	37° flare
Metric Parallel	60°	Parallel	O-ring / washer / sealing face

How to Avoid Thread Misidentification

When identifying a fitting, do not rely on diameter alone.

A better method is to check:

Diameter + Pitch/TPI + Taper + Thread angle + Sealing method

For example, if a fitting has:

• Approximately 21.3 mm outside diameter
• 14 TPI
• Tapered thread
• 60° thread form

it is likely a:

1/2-14 NPT or NPTF

Additional inspection or gauging is then required to distinguish NPT from NPTF.

If the thread is approximately the same diameter but remains parallel, it should not be classified as NPTF.

---

Conclusion

An NPTF Thread Size Chart is a useful reference for identifying and selecting Dryseal tapered pipe threads used in hydraulic, pneumatic, fuel, lubrication, and industrial fluid systems.

NPTF threads are typically defined by:

• A 60° thread angle
• A 1:16 taper
• Nominal pipe size
• Threads per inch
• Controlled crest and root interference
• ASME B1.20.3 requirements

One of the most important points when identifying NPTF threads is that the nominal size does not equal the actual measured thread diameter.

For example:

1/4-18 NPTF

has a male thread outside diameter of approximately:

0.540 in or 13.7 mm

rather than 0.250 inch.

For reliable field identification, use:

Measured OD + TPI + Taper

and compare the results with an NPTF size chart.

When greater certainty is required, particularly for manufacturing or quality inspection, use the appropriate NPTF plug or ring gauges rather than relying only on a caliper.

It is also important to distinguish NPTF from similar systems such as NPT, BSPT, BSPP, SAE ORB, JIC, and metric threads. Threads that physically engage are not necessarily compatible or capable of producing a safe pressure-tight connection.

Using the correct thread size, standard, sealing method, and installation procedure helps ensure reliable performance and reduces the risk of leakage, thread damage, and component failure.

JIC Thread Size Chart: Dash Sizes, Threads & Dimensions

SAE Thread Size Chart: Complete SAE, JIC & ORB Thread Sizes

ORB Fitting Size Chart: Thread Sizes, Dash Sizes & Dimensions