Aug 2025
Thread Standards Explained: Metric, BSW, BSF, UNC and UNF
Fasteners like bolts, nuts, and screws are used in almost every industry — from construction and automotive to oil & gas and machinery manufacturing. But have you ever wondered why the threads on fasteners are not the same everywhere? This is because different thread standards are followed around the world, depending on the application, region, and industry requirement.
For buyers, engineers, and manufacturers, understanding thread standards is very important. Choosing the wrong thread can cause fitting issues, weak joints, or even system failure. In this blog post, we at Pankaj International, a leading fastener manufacturer and exporter from India, explain the most widely used thread standards: Metric, BSW, BSF, UNC, and UNF.
What Are Thread Standards?
Thread standards define how the helical ridges (threads) on bolts, screws, and nuts are shaped and measured. These standards decide:
• The angle of the thread
• The distance between threads (pitch)
• The depth and shape of the groove
• Whether the threads are coarse or fine
Because of standardization, a nut produced in one part of the world can perfectly fit a bolt made somewhere else, as long as they follow the same thread standard.
1. Metric Threads
Overview
The Metric thread standard (ISO Metric) is the most widely used thread type in the world today. It is recognized internationally and has replaced many older national standards. Metric threads are defined by the International Organization for Standardization (ISO).
Key Features
• Thread angle: 60°
• Size is given in millimeters (mm). Example: M12 × 1.75 means a 12 mm diameter with a 1.75 mm pitch.
• Available in coarse and fine pitch versions.
• Coarse pitch is the default (commonly used in general applications).
Applications
• Automotive industry
• Construction and machinery
• Household appliances
• General engineering
At Pankaj International, we manufacture a wide range of fasteners with metric threads to meet ISO standards, ensuring global compatibility.
2. BSW (British Standard Whitworth)
Overview
The British Standard Whitworth (BSW) thread was one of the first standardized thread systems, introduced by Sir Joseph Whitworth in 1841. Although not as common today, it is still used in certain industries and in older equipment.
Key Features
• Thread angle: 55°
• Measured in inches, not millimeters
• Coarse thread series
• Strong and durable due to the rounded profile at the root and crest
Applications
• Heavy engineering
• Railway equipment
• Vintage British machinery and vehicles
BSW threads are often found in older equipment. Pankaj International provides custom fasteners with BSW threads for clients who need replacements or work with heritage machinery.
3. BSF (British Standard Fine)
Overview
The British Standard Fine (BSF) thread was developed as a companion to BSW. It has a finer pitch than BSW, meaning more threads per inch for the same diameter.
Key Features
• Thread angle: 55° (same as BSW)
• Fine pitch provides better strength in tension applications
• Measured in inches
Applications
• Automotive industry (especially classic British cars)
• Aerospace components
• Precision engineering
BSF threads are not as widely used today but remain important for industries that deal with older British designs.
4. UNC (Unified National Coarse)
Overview
The Unified Thread Standard (UN) was developed jointly by the USA, UK, and Canada during World War II to create a common standard. UNC (Unified National Coarse) is one of its types.
Key Features
• Thread angle: 60°
• Measured in inches
• Coarse pitch – fewer threads per inch compared to UNF
• Strong and quick to assemble, less likely to cross-thread
Applications
• Construction
• General-purpose fastening
• Automotive and machinery where high strength is needed
At Pankaj International, we manufacture fasteners with UNC threads for global clients, especially in North America.
| Thread Standard |
Unit |
Angle |
Pitch Type |
Common Applications |
| Metric (ISO) |
mm |
60° |
Coarse & Fine |
Automotive, Machinery, Global use |
| BSW |
inch |
55° |
Coarse |
Railways, Vintage British machinery |
| BSF |
inch |
55° |
Fine |
Aerospace, Classic cars |
| UNC |
inch |
60° |
Coarse |
Construction, Machinery |
| UNF |
inch |
60° |
Fine |
Aerospace, Automotive precision |
Choosing the Right Thread Standard
When selecting fasteners, always consider:
1.Application requirement – Does it need coarse or fine threads?
2.Industry standard – Which thread is commonly accepted in your industry?
3.Compatibility – Will the fastener fit existing components?
4.Regional preference – Metric threads dominate globally, but inch-based systems (UNC/UNF) are common in North America and the UK
Why Choose Pankaj International?
At Pankaj International, we specialize in manufacturing and exporting fasteners in all major thread standards, including Metric, BSW, BSF, UNC, and UNF. Our products are made with precision, tested for quality, and supplied to clients worldwide across industries like automotive, construction, oil & gas, and railways.
We understand that the right thread type ensures strength, safety, and durability in any project. That’s why we provide fasteners that meet international standards and customer-specific requirements.
Conclusion
Thread standards may look like a small detail, but they play a huge role in ensuring fasteners work reliably in different applications. Whether it is Metric, BSW, BSF, UNC, or UNF, each thread type has its own history, design, and application.
By understanding these standards, buyers and engineers can select the right fasteners for their projects. And with Pankaj International as your fastener partner, you can be assured of quality, precision, and timely supply for all types of thread standards.