G Thread vs NPT: Differences in Dimensions, Uses, Standard Specs

2024.10.21

G thread (also known as BSP) and NPT thread are two common pipe thread standards, which are widely used in pipe and pipe fitting connections, each offering unique features suited to different uses. Learn the key differences between G thread (BSP) and NPT thread, including their features, applications, and dimensional charts. Understand how these pipe thread standards differ in sealing mechanisms, thread angle, and industrial usage, helping you choose the best option for your pipe fitting needs.

1. What is G Thread?

G thread, commonly referred to as British Standard Pipe (BSP) thread, is a 55° thread angle cylindrical pipe thread. It belongs to the Whitworth thread family and is used primarily in Europe and Asia. G threads are non-tapered, meaning both the external and internal threads are cylindrical. This type of thread is also known as a “non-sealing” pipe thread because the thread itself does not create a seal.

To achieve effective sealing, G threads rely on sealing materials such as PTFE tape or sealing rings. They are typically used in low-pressure applications, where higher precision in sealing might not be necessary.

NPT Thread Pros & Cons

Advantages:

Disadvantages:

G Thread Mark Symbols

NameSymbolFormula / Description
External DiameterdMajor diameter of the external thread
Mean Diameterd₂Average of major and minor diameters of the thread
Minor Diameterd₁Smallest diameter of the external thread
PitchPDistance between thread peaks (mm)
Thread HeightHVertical height of the thread (H = 0.866 × P)
Tooth Heighth₁Tooth height for G threads (h₁ = 0.8 × t)
Effective Lengthl₁Effective thread engagement length
Base Lengthl₂Distance from pipe end to thread base
Taper AngleφAngle of the thread taper, if applicable
Taper Ratio2tanφTaper ratio for tapered threads (commonly 1:16)
Internal DiameterdtInternal diameter of the pipe end
This table provides an overview of the most commonly used symbols in G thread measurements, along with their respective formulas or descriptions. Adding this to your guide will help readers understand the technical terminology and calculations related to G threads.

G Thread Dimensions (Sizes) Chart

G thread sizes follow a standardized chart based on nominal diameters. Here’s a quick reference for common G thread sizes and their equivalents in millimeters:

G Thread Dimensions (Sizes) Chart

This table provides a clear overview of the dimensions and tolerances for common G thread sizes. Adding this to the article will enhance the understanding of G thread specifications for the readers.

G Thread Application

G threads are typically applied in water supply, drainage systems, and construction industries. They are widely used in European and Asian markets due to their standardized dimensions and relatively easy machining process. However, because G threads do not inherently provide a seal, they are less suitable for high-pressure systems without additional sealing components.


2. What is NPT Thread?

NPT (National Pipe Thread) is a 60° tapered pipe thread primarily used in North America. The NPT thread is designed with a taper, meaning its diameter increases toward the base of the external thread and decreases toward the tip of the internal thread. This taper allows for the deformation of the threads as they are tightened, forming a tight, leak-resistant connection.

Unlike G threads, NPT threads inherently provide sealing through the thread’s shape and taper. However, in practice, PTFE tape or other sealing materials are often used to enhance the seal, especially in high-pressure applications.

G Thread Pros & Cons

Advantages:

Disadvantages:

NPT Thread Mark Symbols

NameSymbolFormulaNameSymbolFormula
External DiameterdTooth Heighth₁h₁ = 0.8t
Mean Diameterd₂Distance from Pipe End to Basel₂
Inner Diameterd₁Effective Lengthl₁
Pipe End Internal Diameterdtdt = d₁ – 12/16Angleφφ = 1°47’24”
Thread PitchtTaper2tanφ2tanφ = 1:16
Theoretical HeightHH = 0.866t
This table provides a clear overview of the key symbols and formulas used in NPT thread calculations. Adding this to the article will help readers better understand the technical aspects of NPT threads.

NPT Thread Dimensions (Sizes) Chart

NPT thread sizes are based on nominal pipe size and follow strict regulations for pitch and taper. Below is a size conversion chart for commonly used NPT thread sizes:

NPT Thread Dimensions (Sizes) Chart

This table provides detailed information about the thread length, thread diameter, pipe end internal diameter, and tooth height for different sizes of NPT threads. Including this chart in the article will offer readers a clear reference for the dimensions of various NPT thread sizes.

NPT Thread Application

NPT threads are widely used in high-pressure systems in industries such as oil, natural gas, chemical processing, and automotive. Due to their excellent sealing performance, NPT threads are particularly suitable for fluid transmission systems, ensuring leak-free connections in heavy-duty applications.


3. What is the Difference Between G Thread vs NPT Thread?

Thread Type

Shape Characteristics

Applications

Standard Specifications

NPT and G threads have some differences in their standard specifications. Below is a comparison of the key specification parameters for NPT and G threads:

Specification ParameterNPT ThreadsG Threads
Basic Thread Diameter (inches)1.5, 2.5, 3.5, 4.50.410, 0.625, 0.850, 1.063
Threads Per Inch1.5, 2.5, 3.5, 4.50.410, 0.625, 0.850, 1.063
External Diameter (inches)24, 11, 8, 636, 11.5, 6, 4
Thread Angle60°60°
Thread Length (inches)36, 11.5, 6, 424, 11, 8, 6
These standard parameters define the size, angle, and thread count for each type of thread, which in turn determines their application range and characteristics.

Advantages and Disadvantages

G thread and NPT thread serve similar purposes but differ in their design, sealing mechanisms, and applications. G threads are more common in Europe and Asia, where they are used in water systems and construction, while NPT threads are more prevalent in North America, particularly in industries requiring secure, high-pressure sealing.

FeatureG ThreadNPT Thread
Sealing PerformanceRequires sealing materials for tightnessProvides natural sealing through thread taper
Machining DifficultyEasier to machineMore complex due to taper and thread count
StrengthStrong connection but relies on external sealingHigh torque and side pressure resistance
Application SuitabilityBest for low to medium pressure systemsIdeal for high-pressure piping systems

4. Are NPT and G Threads Compatible?

No, NPT and G threads are not inherently compatible due to differences in their design, angle, and sealing mechanisms. Let’s break down the reasons why these two thread types are not interchangeable:

1. Thread Angle

2. Taper vs. Parallel

3. Sealing Mechanism

4. Size Compatibility

Even though NPT and G threads may have similar diameters for certain sizes, the differences in thread angle, taper, and sealing mechanisms make them incompatible in practice. Forcing NPT and G threads together may damage the threads and lead to leaks or connection failure.

When choosing between G thread and NPT thread, consider the specific application requirements – whether you need enhanced sealing for high-pressure systems or a more straightforward, standardized connection for low-pressure systems.