British and Unified inch thread systems explained
BSW, BSF, UNC and UNF are all inch-based thread systems, but they do not use exactly the same thread shape, angle or pitch.
BSW and BSF are British thread standards based on the Whitworth thread form. UNC and UNF belong to the Unified inch thread system, which is commonly used in the United States and in many other inch-based applications.
Even when two threads have the same nominal diameter, they should not automatically be considered compatible.
What do BSW, BSF, UNC and UNF mean?
| Abbreviation | Full name | Thread type | System |
|---|---|---|---|
| BSW | British Standard Whitworth | Coarse thread | British Whitworth |
| BSF | British Standard Fine | Fine thread | British Whitworth |
| UNC | Unified National Coarse | Coarse thread | Unified inch |
| UNF | Unified National Fine | Fine thread | Unified inch |
The main differences at a glance
| Feature | BSW and BSF | UNC and UNF |
|---|---|---|
| Thread family | British Whitworth | Unified inch |
| Thread angle | 55° | 60° |
| Crest and root shape | Rounded Whitworth profile | Unified profile with specified crest and root truncation; practical roots may be flat or rounded depending on the thread form |
| Coarse series | BSW | UNC |
| Fine series | BSF | UNF |
| Typical use today | Older British equipment, replacement parts and specialist applications | Modern inch-based machinery, equipment and fasteners |
Difference in thread angle
One of the easiest ways to distinguish the two systems is the angle between the sides of the thread.
BSW and BSF: 55°
UNC and UNF: 60°
Difference in thread profile
The shape of the thread crests and roots is also different. Whitworth threads have a recognisably rounded profile, while Unified threads are based on a different 60° profile.
BSW and BSF profile
Theoretical UNC and UNF profile
Practical Unified thread profile
Can BSW or BSF be replaced by UNC or UNF?
Not as a direct replacement. The thread angle, profile and pitch can be different, so a BSW or BSF nut should not be installed on a UNC or UNF bolt simply because the stated diameter appears similar.
In a few sizes, the threads per inch may appear to match. However, the difference between the 55° Whitworth form and the 60° Unified form still means that the parts are not correctly matched.
Did metric threads replace BSW and BSF?
Metric threads have become more common in modern manufacturing, but BSW and BSF have not disappeared. They are still needed for existing machinery, older British equipment, restoration projects and replacement parts.
Metric threads are also not one single thread type. Like the British and Unified systems, metric threads are available with different diameters and pitches, including coarse and fine versions.
When repairing an existing assembly, it is normally better to identify the original thread correctly rather than replacing it with the nearest-looking metric or Unified size.
Where are BSW and BSF threads still used?
BSW and BSF threads are less common in new general-purpose designs, but they can still be found in:
- Older British machinery and industrial equipment
- Classic cars, motorcycles and restoration projects
- Replacement and spare parts for existing assemblies
- Older agricultural and workshop equipment
- Specialist applications that continue to specify Whitworth threads
Auto-Lok Nuts for British, Unified and metric threads
Auto-Lok locking nuts are available with several thread systems, including:
- BSW and BSF British threads
- UNC and UNF Unified inch threads
- Metric thread sizes
View the available double-slotted Class 8 Auto-Lok Nuts .
How do you identify the correct thread?
Check the outside diameter and count the number of threads per inch. A thread gauge can then be used to compare the pitch and thread form.
The component drawing, original part number or equipment manual may also state whether the required thread is BSW, BSF, UNC or UNF.
When the original thread is damaged or difficult to identify, provide the measurements and application details so that the correct repair part can be selected.