BSP Thread Size Chart: BSPP and BSPT Dimensions

Contents

1 1. What Is a BSP Thread?
2 2. BSPP vs. BSPT Threads
- 2.1 BSPP Threads
- 2.2 BSPT Threads
3 3. BSP Thread Terminology and Dimensions
4 4. BSP Thread Size Chart
5 5. BSPP Thread Size Chart
- 5.1 How BSPP Connections Seal
- 5.2 BSPP External Thread Tolerance Classes
6 6. BSPT Thread Size Chart
7 7. How to Measure and Identify BSP Thread Sizes
8 8. BSP Threads vs. NPT Threads
9 9. BSP Thread Applications, Sealing Methods, and Selection
10 Conclusion

British Standard Pipe (BSP) threads are widely used to connect pipes, tubes, valves, hydraulic components, pneumatic equipment, and process instrumentation. Although the system originated in the United Kingdom, BSP connections are now common throughout Europe, Asia, Australia, the Middle East, and many other international markets.

A common source of confusion is that a BSP nominal size does not represent the actual outside diameter of the thread. For example, a 1/2-inch BSP male thread has an outside diameter of approximately 20.955 mm, rather than 12.7 mm. Correct identification therefore requires checking the thread diameter, pitch, threads per inch, thread form, and whether the thread is parallel or tapered.

The BSP system includes two principal thread types:

BSPP, also identified by the letter G, uses a parallel thread and normally requires an O-ring, bonded seal, gasket, or metal sealing face.
BSPT, commonly identified by the letter R, uses a tapered thread and typically seals through thread interference with a suitable thread sealant.

This guide provides practical BSP thread size charts covering nominal sizes, major diameters, pitch, and threads per inch. It also explains how to distinguish BSPP from BSPT, measure an unknown BSP connection, select the appropriate sealing method, and avoid confusing BSP threads with similar-looking NPT connections.

1. What Is a BSP Thread?

What Is a BSP Thread?

BSP stands for British Standard Pipe. It is a standardized family of screw threads used to connect pipes, valves, fittings, hoses, hydraulic components, pneumatic equipment, and process instruments. BSP threads originated in the United Kingdom but are now widely used across Europe, Asia, Australia, Africa, and many other international markets.

BSP threads use the Whitworth thread form, which has a symmetrical 55-degree thread angle with rounded crests and roots. This geometry distinguishes BSP threads from NPT threads, which use a 60-degree thread angle and have a different pitch for several common sizes.

BSP thread dimensions are generally specified according to two standards:

ISO 228-1 covers parallel pipe threads where the threads do not create the pressure-tight seal.
ISO 7-1 covers pipe threads where pressure-tight joints are made through thread engagement.

An important point is that the BSP nominal size does not equal the measured thread diameter. For example, a 1/2-inch BSP male thread has an outside diameter of approximately 20.955 mm. The nominal designation originally referred approximately to the internal bore of the pipe rather than the actual diameter of the thread.

BSP connections are divided into two main types:

BSPP: British Standard Pipe Parallel
BSPT: British Standard Pipe Taper

Correctly identifying the type is essential because BSPP and BSPT connections use different sealing principles.

2. BSPP vs. BSPT Threads

The main difference between BSPP and BSPT threads is their geometry. BSPP threads remain the same diameter along their entire length, while BSPT threads gradually change diameter because they are tapered.

Feature	BSPP	BSPT
Full name	British Standard Pipe Parallel	British Standard Pipe Taper
Thread form	Parallel	Tapered
Common male designation	G	R
Common female designation	G	Rp or Rc
Thread angle	55°	55°
Taper	None	1:16
Sealing location	Gasket, O-ring or sealing face	Engaged threads
Typical sealant	Not used as the primary seal	Thread sealant or PTFE tape
Primary standard	ISO 228-1	ISO 7-1

BSPP Threads

BSPP threads, also called G threads, have a constant diameter. Tightening the male and female threads together does not normally create a pressure-tight seal because clearance remains between the thread crests and roots.

The connection therefore requires a separate sealing element, such as:

An O-ring
A bonded seal
A flat gasket or washer
A metal-to-metal sealing face
A cone or seat incorporated into the fitting

The thread provides the mechanical clamping force, while the sealing element prevents leakage. Applying additional thread sealant cannot reliably compensate for a missing or damaged BSPP sealing element.

BSPT Threads

BSPT threads are tapered at a ratio of 1:16, meaning the diameter changes by one unit for every 16 units of axial length. As the male and female threads are tightened, the thread flanks increasingly interfere with one another.

A BSPT male thread is normally designated by the letter R. Internal threads may be:

Rp: Parallel female thread designed to accept an R tapered male thread
Rc: Tapered female thread designed to accept an R tapered male thread

Thread engagement creates the basic joint, but a suitable sealing compound or PTFE tape is normally required to fill the small clearance paths between the thread surfaces. The sealant should be compatible with the process fluid, operating temperature, pressure, and fitting material.

BSPP and BSPT fittings should not be considered interchangeable merely because they have the same nominal size and pitch. The sealing surfaces, thread geometry, and connection design must all be checked before assembly.

3. BSP Thread Terminology and Dimensions

Understanding BSP terminology makes it easier to read a thread size chart, inspect a fitting, and select a compatible connection.

Nominal BSP Size

The nominal size is the trade designation used to identify a BSP thread, such as 1/8, 1/4, 3/8, 1/2, or 1 inch. It does not represent the actual outside diameter of the thread.

For example:

Nominal BSP size	Approximate male major diameter
1/8 BSP	9.728 mm
1/4 BSP	13.157 mm
3/8 BSP	16.662 mm
1/2 BSP	20.955 mm
3/4 BSP	26.441 mm
1 BSP	33.249 mm

Because the nominal and measured dimensions are different, a fitting should never be identified using diameter alone.

Major Diameter

The major diameter is the largest diameter of the thread. On a male BSP thread, it is measured across the outermost thread crests using a caliper.

For parallel threads, the diameter remains essentially constant along the threaded section. On tapered threads, the measured diameter changes depending on where the caliper is positioned.

Minor Diameter

The minor diameter is the smallest diameter of the thread:

On a male thread, it is measured across the thread roots.
On a female thread, it is measured across the innermost thread crests.

The minor diameter is especially important when machining internal threads and selecting an appropriate tap-drill size.

Pitch Diameter

The pitch diameter is an imaginary diameter at which the width of the thread ridge equals the width of the space between adjacent threads. It is one of the most important dimensions for controlling thread fit and engagement.

Unlike the major diameter, the pitch diameter is difficult to measure accurately with a standard caliper. Thread micrometers, gauges, or other specialized inspection equipment are normally used.

Threads per Inch

Threads per inch, abbreviated TPI, indicates the number of complete thread peaks within one inch of axial thread length. Common BSP pitches include:

28 TPI for many smaller sizes
19 TPI for several intermediate sizes
14 TPI for many larger sizes
11 TPI for certain large BSP sizes

Different nominal sizes can share the same TPI. Therefore, TPI must be evaluated together with the measured thread diameter.

Thread Pitch

Thread pitch is the axial distance from one thread crest to the next. For inch-based threads, it can be calculated as:

$P = TPI 25.4$

where:

$P$ = thread pitch in millimetres
25.4 = millimetres per inch
TPI = threads per inch

For example, a 14-TPI BSP thread has a pitch of:

$P = 1425.4 = 1.814 mm$

Thread Angle

Both BSPP and BSPT threads use a 55-degree included angle. Their rounded crests and roots are characteristic of the Whitworth thread profile.

This is different from NPT threads, which have a 60-degree included angle. Although some BSP and NPT sizes appear similar, differences in angle, pitch, and diameter prevent a correct connection.

Thread Taper

BSPT threads have a taper ratio of 1:16 measured on the diameter. This corresponds to an included taper angle of approximately 3.58 degrees, or about 1.79 degrees per side relative to the thread axis.

BSPP threads have no intentional taper and maintain a constant nominal diameter along their threaded length.

Male and Female Thread Designations

The following designations are commonly used when specifying BSP connections:

Designation	Thread description
G	Parallel male or female thread under ISO 228-1
R	Tapered male thread under ISO 7-1
Rp	Parallel female thread for an R male thread
Rc	Tapered female thread for an R male thread

For example, G 1/2 indicates a 1/2-inch parallel BSP thread, while R 1/2 indicates a 1/2-inch tapered BSP male thread.

4. BSP Thread Size Chart

BSP Thread Size Chart

The following chart lists the basic dimensions of commonly used BSP threads. The same nominal sizes, basic diameters, thread pitches, and Whitworth profiles apply to both BSPP and BSPT threads. The primary difference is that BSPP threads are parallel, while BSPT threads have a 1:16 taper.

Nominal BSP size	Basic major diameter (mm)	Basic major diameter (in.)	TPI	Pitch (mm)
1/16	7.723	0.3040	28	0.907
1/8	9.728	0.3830	28	0.907
1/4	13.157	0.5180	19	1.337
3/8	16.662	0.6560	19	1.337
1/2	20.955	0.8250	14	1.814
5/8	22.911	0.9020	14	1.814
3/4	26.441	1.0410	14	1.814
7/8	30.201	1.1890	14	1.814
1	33.249	1.3090	11	2.309
1 1/8	37.897	1.4920	11	2.309
1 1/4	41.910	1.6500	11	2.309
1 1/2	47.803	1.8820	11	2.309
1 3/4	53.746	2.1160	11	2.309
2	59.614	2.3470	11	2.309
2 1/4	65.710	2.5870	11	2.309
2 1/2	75.184	2.9600	11	2.309
2 3/4	81.534	3.2100	11	2.309
3	87.884	3.4600	11	2.309
3 1/2	100.330	3.9500	11	2.309
4	113.030	4.4500	11	2.309
4 1/2	125.730	4.9500	11	2.309
5	138.430	5.4500	11	2.309
5 1/2	151.130	5.9500	11	2.309
6	163.830	6.4500	11	2.309

The actual diameter measured with a caliper may differ slightly from the basic value because of manufacturing tolerances, surface coating, wear, thread damage, and measurement position. This difference is particularly important for BSPT threads because their diameter changes along the tapered section.

To identify an unknown BSP thread:

Measure the male thread’s outside diameter.
Determine the thread pitch with a pitch gauge.
Compare both measurements with the chart.
Check whether the thread is parallel or tapered.
Inspect the sealing arrangement before selecting a mating fitting.

For example, a male thread measuring close to 20.9 mm with 14 TPI is most likely a 1/2-inch BSP thread. It must then be checked for taper to determine whether it is G 1/2 or R 1/2.

5. BSPP Thread Size Chart

BSPP stands for British Standard Pipe Parallel. These threads are designated by the letter G and are standardized under ISO 228-1. Their diameter remains constant along the entire threaded length.

The dimensions below are basic reference values. Exact permissible dimensions depend on whether the thread is internal or external and on the applicable tolerance class.

BSPP designation	Nominal size	Male major diameter (mm)	TPI	Pitch (mm)	Common sealing method
G 1/16	1/16	7.723	28	0.907	Washer or sealing face
G 1/8	1/8	9.728	28	0.907	O-ring or bonded seal
G 1/4	1/4	13.157	19	1.337	O-ring or bonded seal
G 3/8	3/8	16.662	19	1.337	O-ring or bonded seal
G 1/2	1/2	20.955	14	1.814	Bonded seal or gasket
G 5/8	5/8	22.911	14	1.814	Gasket or sealing face
G 3/4	3/4	26.441	14	1.814	Bonded seal or gasket
G 7/8	7/8	30.201	14	1.814	Gasket or sealing face
G 1	1	33.249	11	2.309	O-ring, gasket or washer
G 1 1/8	1 1/8	37.897	11	2.309	Gasket or sealing face
G 1 1/4	1 1/4	41.910	11	2.309	O-ring or gasket
G 1 1/2	1 1/2	47.803	11	2.309	O-ring or gasket
G 1 3/4	1 3/4	53.746	11	2.309	Gasket or sealing face
G 2	2	59.614	11	2.309	O-ring or gasket
G 2 1/2	2 1/2	75.184	11	2.309	Gasket or flange face
G 3	3	87.884	11	2.309	Gasket or flange face
G 4	4	113.030	11	2.309	Gasket or flange face
G 5	5	138.430	11	2.309	Gasket or flange face
G 6	6	163.830	11	2.309	Gasket or flange face

How BSPP Connections Seal

The parallel threads primarily provide mechanical retention and clamping force. They do not normally create a pressure-tight joint through thread interference.

The actual seal may be formed by:

An elastomeric O-ring against a machined port surface
A bonded seal beneath the fitting shoulder
A flat gasket or sealing washer
A 60-degree cone or another internal sealing seat
A metal-to-metal sealing face

A BSPP fitting should therefore be selected by examining both its thread and its sealing interface. Two fittings marked G 1/2 may have compatible threads but incompatible sealing faces.

BSPP External Thread Tolerance Classes

ISO 228-1 includes two principal tolerance classes for external parallel threads:

Class A: Tighter tolerance
Class B: Wider tolerance

Internal G threads have a single tolerance class. The tolerance class affects thread fit but does not change the nominal size, pitch, or sealing method.

Adding PTFE tape to a BSPP thread does not convert it into a properly designed thread-sealing connection. If leakage occurs, the O-ring, gasket, bonded seal, or sealing face should be inspected.

6. BSPT Thread Size Chart

BSPT stands for British Standard Pipe Taper. BSPT connections are covered by ISO 7-1 and use a taper of 1:16 measured on the diameter.

A tapered external BSP thread is designated R. The corresponding internal thread can be parallel or tapered:

R: Tapered external thread
Rp: Parallel internal thread
Rc: Tapered internal thread

The diameter shown in a BSPT chart is a basic diameter at a specified gauge plane. Because the thread is tapered, it should not be interpreted as a constant outside diameter along the entire thread.

BSPT designation	Nominal size	Basic gauge-plane diameter (mm)	TPI	Pitch (mm)	Taper
R 1/16	1/16	7.723	28	0.907	1:16
R 1/8	1/8	9.728	28	0.907	1:16
R 1/4	1/4	13.157	19	1.337	1:16
R 3/8	3/8	16.662	19	1.337	1:16
R 1/2	1/2	20.955	14	1.814	1:16
R 3/4	3/4	26.441	14	1.814	1:16
R 1	1	33.249	11	2.309	1:16
R 1 1/4	1 1/4	41.910	11	2.309	1:16
R 1 1/2	1 1/2	47.803	11	2.309	1:16
R 2	2	59.614	11	2.309	1:16
R 2 1/2	2 1/2	75.184	11	2.309	1:16
R 3	3	87.884	11	2.309	1:16
R 3 1/2	3 1/2	100.330	11	2.309	1:16
R 4	4	113.030	11	2.309	1:16
R 5	5	138.430	11	2.309	1:16
R 6	6	163.830	11	2.309	1:16

BSPT Taper and Diameter Change

A 1:16 diameter taper means the thread diameter changes by 1 mm over an axial distance of 16 mm. The diameter change can be estimated using:

$Δ D = 16 L$

where:

$Δ D$ = change in thread diameter
$L$ = axial distance along the thread

For example, over a 16 mm threaded length, the diameter changes by approximately 1 mm. This is why measuring a BSPT thread at different positions produces different results.

BSPT Thread Combinations

Common ISO 7-1 thread combinations include:

Male thread	Female thread	Connection type
R	Rc	Tapered male into tapered female
R	Rp	Tapered male into parallel female

Both arrangements can form pressure-tight threaded joints when correctly manufactured, assembled, and sealed. However, their engagement behavior and tightening requirements differ.

Sealing BSPT Connections

BSPT connections seal through increasing contact and interference between the male and female threads. A suitable thread sealant is normally applied to fill the helical leakage paths that remain between the thread surfaces.

Common options include:

PTFE thread-sealing tape
Anaerobic pipe-thread sealant
Non-hardening pipe-joint compound
Application-specific sealing paste

The sealant must be compatible with the process fluid, temperature, pressure, fitting material, and system cleanliness requirements. Excess sealant should be avoided because it may enter the fluid system and contaminate valves, regulators, instruments, or other sensitive components.

Overtightening can crack a female port, distort the fitting, damage the threads, or make future disassembly difficult. Proper assembly should follow the fitting manufacturer’s recommended engagement and tightening instructions.

7. How to Measure and Identify BSP Thread Sizes

Correctly identifying a BSP thread requires more than measuring its outside diameter. The nominal BSP size, actual diameter, pitch, thread form, taper, and sealing method should all be checked.

Step 1: Determine Whether the Thread Is Male or Female

First, identify the thread location:

Male thread: Located on the outside of the fitting.
Female thread: Located inside a port, valve, or fitting.

Male threads are easier to measure directly with a caliper. Female thread measurements are less reliable because the caliper contacts the internal thread crests rather than the thread’s basic major diameter.

Step 2: Measure the Thread Diameter

Use a vernier or digital caliper to measure the thread diameter:

For a male thread, measure across the outer thread crests.
For a female thread, measure across the internal thread crests.

For a suspected BSPP male thread, measure at several positions along the threaded length. The readings should remain approximately constant.

For a suspected BSPT male thread, measure near the first complete thread and again near the fitting body. The diameter should increase toward the fitting body because BSPT threads have a 1:16 taper.

Do not expect the measured diameter to equal the nominal BSP size. Typical examples include:

Measured male diameter	Likely BSP size
Approximately 9.7 mm	1/8 BSP
Approximately 13.2 mm	1/4 BSP
Approximately 16.7 mm	3/8 BSP
Approximately 21.0 mm	1/2 BSP
Approximately 26.4 mm	3/4 BSP
Approximately 33.2 mm	1 BSP

These values are approximate. Manufacturing tolerance, plating, wear, damage, and the measurement position can affect the result.

Step 3: Measure the Thread Pitch

Place a thread pitch gauge against the thread and find the blade that fits completely into the thread profile without visible gaps.

Common BSP pitches are:

TPI	Pitch
28	0.907 mm
19	1.337 mm
14	1.814 mm
11	2.309 mm

If a thread gauge is unavailable, measure the axial distance across several threads and divide it by the number of thread intervals. Measuring multiple threads reduces the effect of measurement error.

For example, if ten thread intervals cover approximately 18.14 mm:

$P = 1018.14 = 1.814 mm$

The thread therefore has a pitch of approximately 1.814 mm, equivalent to 14 TPI.

Step 4: Check Whether the Thread Is Parallel or Tapered

The easiest preliminary method is to use a caliper:

Measure the diameter near the end of the male thread.
Measure again close to the fitting body.
Compare the two readings.

A nearly constant diameter suggests a BSPP thread. A diameter that increases toward the fitting body suggests a BSPT thread.

A visual check alone may not be sufficient, particularly on short threads. A proper thread-identification gauge or taper gauge provides a more reliable result.

Step 5: Examine the Thread Profile

BSP threads use the Whitworth profile, characterized by:

A 55-degree included thread angle
Rounded thread crests
Rounded thread roots

The thread profile can be checked using a dedicated 55-degree thread gauge, optical comparator, profile projector, or other suitable inspection equipment.

Step 6: Inspect the Sealing Location

The sealing arrangement often provides an important clue:

A washer, bonded seal, O-ring, cone, or flat sealing face normally indicates BSPP.
A tapered thread with sealant applied directly to the threads normally indicates BSPT.
A parallel thread without a visible sealing feature may be incomplete, incorrectly assembled, or part of another connection standard.

However, the sealing feature should support the identification rather than replace dimensional inspection.

Step 7: Confirm the Thread With a Gauge

For production, inspection, or safety-critical applications, confirm the thread using the appropriate calibrated gauges:

Ring gauge for male threads
Plug gauge for female threads
Limit gauge for checking acceptance
Tapered gauge for BSPT threads

ISO 7-2 covers verification using limit gauges for ISO 7 tapered and parallel mating threads. A fitting should not be identified by testing whether another fitting can be screwed onto it. Different thread standards may engage for several turns while remaining dimensionally incompatible.

BSP Identification Example

Suppose a male fitting has the following characteristics:

Outside diameter: approximately 20.9 mm
Thread pitch: 14 TPI
Diameter remains constant along the thread
Bonded seal located under the fitting shoulder

The size chart indicates a nominal 1/2-inch BSP thread. The constant diameter and separate bonded seal identify it as a G 1/2 BSPP connection.

8. BSP Threads vs. NPT Threads

BSP and NPT threads are widely used in piping, hydraulics, pneumatics, and instrumentation. They can look similar, but they use different thread profiles and are generally not interchangeable.

NPT stands for National Pipe Taper and is primarily associated with North American equipment. BSP threads are common in Europe, Asia, Australia, and many international industrial systems.

Feature	BSP	NPT
Full name	British Standard Pipe	National Pipe Taper
Common standards	ISO 7-1 and ISO 228-1	ASME B1.20.1
Thread angle	55°	60°
Thread profile	Rounded crests and roots	Flattened crests and roots
Parallel version	BSPP	NPS/NPSM and other forms
Tapered version	BSPT	NPT
Taper ratio	1:16	1:16
Common small-size pitches	28, 19 and 14 TPI	27, 18 and 14 TPI
Primary sealing method	Depends on BSPP or BSPT	Thread interference and sealant

Thread Angle and Profile

BSP threads use a 55-degree Whitworth thread form with rounded crests and roots. NPT threads use a 60-degree thread form with flatter crests and roots.

This five-degree difference prevents the thread flanks from contacting correctly when BSP and NPT components are mixed. The connection may feel tight but still have insufficient engagement or an unreliable sealing path.

BSP and NPT Pitch Comparison

Several common BSP and NPT sizes have different pitches:

Nominal size	BSP pitch	NPT pitch
1/8	28 TPI	27 TPI
1/4	19 TPI	18 TPI
3/8	19 TPI	18 TPI
1/2	14 TPI	14 TPI
3/4	14 TPI	14 TPI
1	11 TPI	11.5 TPI
1 1/4	11 TPI	11.5 TPI
1 1/2	11 TPI	11.5 TPI
2	11 TPI	11.5 TPI

The 1/2-inch and 3/4-inch BSPT and NPT sizes both have 14 TPI, making them particularly difficult to distinguish with a pitch gauge alone. Swagelok notes that positive identification of these sizes may require an optical comparator because their dimensions and appearance are very close. Swagelok Thread and End Connection Identification Guide

Why BSP and NPT Should Not Be Mixed

Attempting to connect BSP and NPT components can cause:

Limited thread engagement
Damaged or deformed thread flanks
Leakage under pressure
Reduced mechanical strength
Cracking of the female component
Inconsistent assembly torque
Difficulty during future maintenance
Contamination from excessive sealant

Adding more PTFE tape does not correct an incompatible thread profile. It may temporarily reduce visible leakage at low pressure, but it does not create a properly engineered connection.

Parker specifically states that BSPT threads are different from and should not substitute for NPT threads. Parker Installation Manual

Using BSP-to-NPT Adapters

When equipment has different connection standards, use a purpose-designed BSP-to-NPT adapter. Common configurations include:

Male BSPT to female NPT
Female BSPT to male NPT
Male BSPP to female NPT
Female BSPP to male NPT

The adapter must match not only the nominal sizes but also the sealing method at each end. A BSPP end may require an O-ring, bonded seal, washer, or compatible sealing face, while the NPT or BSPT end normally requires an approved thread sealant.

9. BSP Thread Applications, Sealing Methods, and Selection

BSP threads are used across a broad range of industrial and commercial systems. Selecting the correct connection requires consideration of the thread type, sealing design, operating pressure, temperature, fluid compatibility, material, and maintenance requirements.

Common BSP Thread Applications

BSP connections are frequently found in:

Hydraulic pumps, cylinders and control valves
Pneumatic cylinders and air preparation equipment
Pressure gauges, transmitters and switches
Process instrumentation tubing systems
Water and wastewater equipment
Air compressors and compressed-air networks
Pumps, filters and pressure regulators
Agricultural and construction machinery
Automotive and mobile hydraulic systems
Marine equipment
Chemical-processing systems
Laboratory and analytical equipment
Plumbing and irrigation systems

BSPP connections are common in hydraulic and instrumentation ports where an elastomeric or metallic sealing element provides a controlled, repeatable seal. BSPT connections are common in general pipework and equipment ports designed for tapered-thread sealing.

BSPP Sealing Methods

ISO 228-1 BSPP threads are intended primarily for mechanical assembly and are not designed to create a pressure-tight seal on the threads themselves. ISO 228-1

Common BSPP sealing arrangements include:

Bonded Seal

A bonded seal consists of a metal washer with an elastomeric sealing ring bonded to its inside diameter. It is compressed between the fitting shoulder and the flat surface surrounding the port.

The port face must be:

Flat and smooth
Perpendicular to the thread axis
Free from radial scratches
Large enough to support the entire seal

O-Ring Seal

An O-ring may be installed in a groove beneath the fitting shoulder or within a specially designed port. The O-ring material must be compatible with the system fluid and temperature.

Common O-ring materials include:

NBR for general hydraulic and pneumatic service
FKM for higher temperatures and many chemicals
EPDM for water, steam and selected chemicals
PTFE-based materials for specialized chemical service

Material compatibility must be verified for the actual operating conditions.

Flat Gasket or Washer

A soft gasket or sealing washer may be compressed between two flat surfaces. The gasket material can be elastomeric, polymeric, metallic, or a composite, depending on the application.

Cone or Metal Sealing Face

Some BSPP connections use a 30-degree or 60-degree cone, spherical seat, or another machined sealing surface. In these connections, the G thread only holds the mating surfaces together. Thread compatibility does not guarantee that the sealing faces are compatible.

Swagelok explains that ISO 228-1 parallel threads normally use an O-ring or gasket because they do not produce the interference found in tapered pipe threads. Swagelok

BSPT Sealing Methods

BSPT connections create increasing interference as the tapered male thread is tightened into the mating female thread. ISO 7-1 defines pipe threads intended for pressure-tight joints made through thread engagement. ISO 7-1

A sealant is normally used to fill the small spiral leakage paths between mating threads.

Suitable options may include:

PTFE thread-sealing tape
Anaerobic liquid thread sealant
Non-hardening pipe-joint compound
Process-specific sealing paste

Sealant selection should consider:

Process fluid
Operating pressure
Minimum and maximum temperature
Oxygen compatibility
Chemical resistance
System cleanliness
Future disassembly requirements
Fitting and port materials

PTFE tape should be wrapped in the tightening direction and kept away from the first thread to reduce the risk of fragments entering the system. The exact application method should follow the sealant and fitting manufacturers’ instructions.

Metal and Material Selection

BSP fittings are available in materials such as:

Carbon steel
Stainless steel
Brass
Bronze
Cast iron
Nickel alloys
Engineering plastics

Material selection must account for both external and internal operating environments. Galvanic corrosion can occur when dissimilar metals are connected in the presence of an electrolyte.

For example, installing a brass fitting directly into an incompatible aluminum or stainless-steel component may require additional corrosion evaluation. The fitting material must also provide sufficient mechanical strength for the pressure and temperature of the system.

Selecting the Correct BSP Connection

Use the following process when choosing a BSP fitting:

Identify the required nominal size.
Determine whether the port is BSPP or BSPT.
Confirm the thread designation: G, R, Rp or Rc.
Check the required sealing arrangement.
Confirm that the mating sealing faces are compatible.
Select a material suitable for the fluid and environment.
Verify pressure and temperature ratings.
Confirm seal and lubricant compatibility.
Follow the manufacturer’s assembly instructions.

BSP Thread Selection Guide

Application requirement	Preferred arrangement
Repeatable assembly and disassembly	BSPP with replaceable seal
Controlled sealing position	BSPP with O-ring or bonded seal
Compact general pipe connection	BSPT with approved sealant
Adjustable elbow orientation	BSPP adjustable stud with O-ring
High-purity or clean service	BSPP face-sealing design or another dedicated clean connection
Frequent maintenance	BSPP with an accessible sealing element
Existing tapered BSP port	R male into compatible Rp or Rc female
Equipment with an NPT port	Purpose-designed BSP-to-NPT adapter

Common Selection and Installation Mistakes

Frequent BSP connection problems include:

Confusing nominal size with measured diameter
Connecting BSP threads to NPT threads
Using thread sealant as the primary seal on a BSPP connection
Installing an R male thread into an incompatible female port
Failing to identify the BSPP sealing-face geometry
Reusing a damaged O-ring or bonded seal
Applying excessive PTFE tape
Overtightening tapered threads
Selecting an incompatible elastomer
Ignoring fitting pressure and temperature ratings
Measuring a BSPT diameter at the wrong axial position

A reliable BSP connection depends on the entire joint design, not only the thread designation. Thread dimensions provide mechanical compatibility, while the sealing element, fitting material, port geometry, assembly procedure, and operating conditions determine whether the completed connection will perform safely.

Conclusion

BSP threads are widely used in hydraulic, pneumatic, process instrumentation, plumbing, and industrial piping systems. Correct identification requires checking the actual thread diameter, pitch, thread profile, taper, and sealing arrangement because the nominal BSP size does not correspond directly to the measured diameter.

BSP connections are divided into two primary types. BSPP, identified by the letter G, uses parallel threads and normally seals with an O-ring, bonded seal, gasket, or machined sealing face. BSPT uses tapered threads—designated R, Rp, or Rc—and typically forms a pressure-tight joint through thread engagement combined with a suitable sealant.

BSP and NPT fittings should not be treated as interchangeable. Although some sizes have similar diameters and pitches, their thread angles and profiles are different. When connecting equipment that uses different standards, a purpose-designed adapter should be used.

The BSP thread size charts in this guide provide a practical reference for nominal size, major diameter, pitch, and threads per inch. However, final selection should always be confirmed against the applicable standard, manufacturer specifications, pressure and temperature ratings, material compatibility, and required sealing method.

UNC Thread Size Chart: Dimensions, TPI & Tap Drill Sizes

Metric Thread Size Chart: Dimensions, Pitch & Tap Drill Sizes

UN Thread Size Chart: UNC, UNF, UNEF Dimensions

Contents1 1. What Is a UN Thread?1.1 Basic UN Thread Profile1.2 UN Thread Size Designation1.3 UN Threads and Unified National Threads1.4 Key Characteristics of UN Threads2 2. UN Thread Terminology and Dimensions2.1 Nominal Thread Size2.2 Threads per Inch2.3 Thread Pitch2.4 Major Diameter2.5 Minor Diameter2.6 Pitch Diameter2.7 Thread Angle2.8 Thread Height2.9 Crest, Root, and Flank2.10 Lead2.11 […]

UNC Thread Size Chart: Dimensions, TPI & Tap Drill Sizes

Contents1 1. What Is a UNC Thread?1.1 UNC thread characteristics1.2 Understanding a UNC thread designation1.3 External and internal UNC threads1.4 Advantages of UNC threads1.5 Limitations of UNC threads1.6 Common UNC thread applications2 2. UNC Thread Terminology and Dimensions2.1 Nominal thread size2.2 Major diameter2.3 Minor diameter2.4 Pitch diameter2.5 Threads per inch2.6 Thread pitch2.7 Lead2.8 Thread angle […]