Working with threaded parts can be challenging if your measurements are not precise. That’s why Ojiyas is a trusted name in the industry. OJIYA SEIKI Co., Ltd. (commonly known as Ojiyas) is a Japanese manufacturer of high-precision gauges, including thread gauges that meet major international standards such as JIS, ISO, and others. These gauges help ensure that threaded parts meet the required GO/NO-GO conditions.
In this guide, we explain the basics of threads, the most common thread standards, and the complete range of Ojiyas thread gauge products to help you choose the right one for your needs.
Basic of Threads
Threads are one of the most common elements in mechanical engineering, used in everything from fasteners to piping systems. Understanding the fundamentals of threads is essential for selecting the right thread type and ensuring proper fit and function.
A. Thread Gender
Threads always come in pairs: a male (external) thread and a female (internal) thread.
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Male thread: The external thread, like a screw or bolt.
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Female thread: The internal thread, such as a nut or a threaded hole.
These two components work together to create a secure connection.
B. Thread Handedness
Threads can twist in two directions:
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Right-hand thread: Tightens clockwise. This is the standard and most widely used globally.
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Left-hand thread: Tightens counter-clockwise, used for specific applications like rotating machinery or gas fittings.
C. Thread Design: Parallel vs. Tapered
Thread profiles can be parallel or tapered, depending on their purpose:
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Parallel threads: Maintain a constant diameter along the entire length. Used where alignment is important but sealing is not required.
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Tapered threads: Gradually decrease in diameter to create a tight, self-sealing fit. Common in piping systems where leak prevention is critical.
D. Thread Pitch and TPI
The spacing between threads is crucial for compatibility:
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Pitch (Metric Threads): The distance from one thread crest to the next, measured in millimeters.
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TPI / Threads Per Inch (Imperial Threads): Number of threads per inch, indicating thread density.
E. Thread Diameters
Three key diameters define a thread:
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Major diameter: Measured at the crests of the thread; the largest diameter.
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Minor diameter: Measured at the roots of the thread; the smallest diameter.
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Pitch diameter: The distance between opposing flanks; critical for ensuring proper engagement and fit.
Understanding these basic thread characteristics is essential for selecting the right gauges, tools, or fasteners in manufacturing, mechanical engineering, and quality control.
Most Common Thread Types
Threads come in several international standards. Below are the types most widely used in industry.
1. Metric ISO Thread (M)
The ISO Metric thread (M) is the most common thread type worldwide, using a 60° flank angle and metric measurements.
Key features:
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60° flank angle
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Denoted with M + nominal diameter
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All dimensions in millimeters
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Commonly used in Europe and globally for screws, nuts, and general fasteners.
2. Unified National Thread (UNC & UNF)
In the United States, UNC (Unified National Coarse) and UNF (Unified National Fine) threads are widely used. Both are defined in inches, with pitch measured in TPI (threads per inch). UNF threads have a finer pitch, providing increased strength and greater precision.
A. Unified National Coarse (UNC)
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60° flank angle
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Dimensions in inches
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Pitch specified as TPI
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Functionally similar to ISO Metric Coarse threads
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Widely used in the USA
B. Unified National Fine (UNF)
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Fine-pitch counterpart to UNC
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60° flank angle
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Dimensions in inches
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Used globally, including in Japan, Australia, and the USA
3. National Pipe Tapered (NPT & NPTF)
For piping systems, NPT and NPTF threads are commonly used. Both are tapered, but NPT requires sealant, while NPTF is designed for a dry-seal connection without additional sealing compound.
Key features:
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Commonly used in fluid, pneumatic, and piping systems
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NPT (National Pipe Taper): requires sealant
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NPTF (National Pipe Taper Fuel): dry-seal; no sealant needed
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Both appear similar but differ slightly in crest and root diameters
Ojiyas Thread Gauge Products
1. Wear-Resistant Plug Gauge Options
To accommodate various industrial needs, Ojiyas offers plug gauges with advanced materials and coatings for enhanced durability and performance.
a. DLC-Coated Thread Plug Gauges
DLC-coated gauges provide exceptional wear resistance and prevent material adhesion, making them ideal for softer materials such as aluminum, copper, and plastics.
b. Powder High-Speed Steel (HSS) Thread Plug Gauges
Powder high-speed steel (HSS) gauges offer high toughness and long service life. HSS ring gauges are also available for matched pair applications.
c. TiN-Coated Thread Plug Gauges
TiN-coated gauges increase durability and reduce wear during repeated inspections, making them suitable for applications requiring maximum hardness.
d. Carbide Thread Plug Gauges
Carbide gauges offer superior wear resistance, though they are comparatively more brittle than other materials.
2. Standard Thread Gauges
Ojiyas standard thread gauges consist of matched pairs of plug and ring gauges that replicate the basic profile of screw threads. Unlike limit gauges used in mass production, these standard gauges are ideal for inspecting small quantities of screws, ensuring precise and accurate engagement.
3. Gauges for Screw Pipe Threads
Limit gauges for screw threads are provided to match the specifications of the screw thread products and typically come as a pair: GO and NO-GO. In conventional JIS, gauges are classified for inspection and production use. However, in ISO-based JIS, this classification is no longer applied.
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4. Gauges for Parallel Pipe Threads
A. Parallel Pipe Threads (G)
JIS-specified parallel pipe threads (G) are used to connect fluid equipment, plumbing parts, and pipes where sealing is not required for the mechanical connection. Ring gauges and plug gauges are used as limit gauges to inspect and verify the dimensions of these threads.
B. Parallel Pipe Threads (PF)
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Similarly, JIS parallel pipe threads (PF) connect fluid equipment, plumbing parts, and pipes without requiring density resistance. Ring gauges and plug gauges serve as limit gauges to ensure accurate dimensional inspection.
5. Gauges for Taper Pipe Threads
A. Taper Pipe Threads (R)
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Taper pipe threads (R), specified in JIS, are primarily designed to enhance the sealing performance of screwed connections in fluid equipment, plumbing components, and piping systems. Ring gauges and plug gauges for R threads are used to inspect and verify their dimensions, ensuring proper fit and function.
B. Taper Pipe Threads (PT)
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Taper pipe threads (PT), specified in JIS, are used to connect fluid equipment, plumbing parts, and pipes where density and sealing are essential for mechanical connections. Ring gauges and plug gauges for PT threads are used to inspect and verify the dimensions of these threads.
C. Taper Pipe Threads (NPT）
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Taper pipe threads (NPT), specified in the American standard (ANSI), are used to connect pipes, plumbing parts, fluid equipment, and more, designed to enhance the density and sealing of threaded connections. Ring gauges and plug gauges for NPT threads are used to accurately measure and verify the dimensions of these threads.
D. Taper Pipe Threads（NPTF）
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Taper pipe threads NPTF, specified in the American standard (ANSI), are used to connect fluid equipment, plumbing parts, and pipes. The thread density can be ensured even without sealant. Ring gauges and plug gauges for NPTF threads are used to accurately check the dimensions of such threads.
Ojiyas Distributor in Singapore
LFC PTE LTD is the authorized distributor of Ojiyas thread plug and ring gauges in Singapore. We offer a variety of gauge sizes and types. Our sales staff will help you find the best gauge for your application. For more information about the products we offer, contact us to discuss your requirements.
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