Imperial fasteners are specified by three quantities: diameter in inches, number of threads per inch and thread standard. It is the middle one of these three, the number of threads per inch (TPI), that distinguishes UNC threads from UNF threads at the same nominal diameter. A 1/2" bolt can be 1/2-13 UNC or 1/2-20 UNF. They look alike. They are not interchangeable. Knowing the difference is fundamental knowledge for everyone working with imperial fasteners in offshore, oil/gas and heavy industry.
What does TPI mean?
TPI stands for Threads Per Inch, the number of threads per inch. It states how many complete thread turns are contained within one inch of the thread length. A bolt with 13 TPI has 13 thread crests per inch, a bolt with 20 TPI has 20. The higher the TPI, the finer the thread and the closer the spacing between the thread crests. Diameter and TPI are always given together in an imperial fastener designation: 3/8-16 means 3/8 inch diameter and 16 threads per inch.
TPI is the imperial system's equivalent of pitch in the metric system. The pitch is measured in millimetres between the thread crests, while TPI counts the number of crests per inch. They are mathematically inversely proportional: a high TPI corresponds to a low pitch. An M12 x 1.75 bolt has a pitch of 1.75 mm, corresponding to approximately 14.5 TPI. A 1/2-13 UNC bolt has a pitch of 1.95 mm. They are not metric-to-imperial equivalents even though the diameters are close to each other.
What are UNC threads?
UNC threads are Unified National Coarse, the coarse thread series in the imperial standard system. UNC threads have fewer threads per inch compared with UNF threads at the same diameter. The coarse thread is assembled faster, is more tolerant of incomplete or contaminated holes and is easier to use under field conditions. UNC threads are the standard choice for general industrial use, structural connections with static load and applications where assembly speed matters.
A common misconception is that UNC threads are «weaker» than UNF threads. That is not correct without context. UNC threads and UNF threads of the same SAE Grade have the same material strength. The difference lies in preload capacity and vibration behaviour, not in the tensile strength of the material. UNC threads are standardised under ASME B1.1 and classified according to SAE J429 for strength classes.
What are UNF threads?
UNF threads are Unified National Fine, the fine thread series. UNF threads have more threads per inch than UNC threads at the same diameter. This gives higher preload at the same applied torque, better resistance to loosening under vibration and more precise adjustment. UNF threads are used in joints where controlled clamping force is critical: flange joints under pressure, compressor housings, turbine installations and high-load mechanical connections.
UNF threads are more demanding to install than UNC threads. They are more sensitive to incomplete holes and to contamination, and take longer to screw in. Under field conditions where the hole is not machined to high precision, and where assembly speed counts, UNC threads are the practical choice. Where the drawing requires UNF threads, substituting with UNC threads is not an option, even if the diameter is identical.
TPI reference table: UNC threads and UNF threads
The table below shows TPI for the most common imperial dimensions:
|
Diameter |
UNC threads (TPI) |
UNF threads (TPI) |
|
1/4" |
20 |
28 |
|
5/16" |
18 |
24 |
|
3/8" |
16 |
24 |
|
1/2" |
13 |
20 |
|
5/8" |
11 |
18 |
|
3/4" |
10 |
16 |
|
7/8" |
9 |
14 |
|
1" |
8 |
12 |
|
1 1/8" |
7 |
N/A |
|
1 1/4" |
7 |
N/A |
|
1 1/2" |
6 |
N/A |
How to read an imperial thread designation
A complete imperial thread designation is built up from three elements: diameter, TPI and standard. For example: 1/2-13 UNC, 3/4-10 UNC or 1/2-20 UNF. The diameter is always stated in inches as a fraction or a decimal. TPI is stated as a whole number. The standard states the thread series: UNC for coarse thread, UNF for fine thread. After this comes the strength class (SAE Grade 5, Grade 8, L7 etc.) and the surface treatment.
An order that states "1/2 bolt" is not a complete specification. It lacks TPI and thread standard. Without these two pieces of information it is not possible to verify that the fasteners received are correct for the application. The correct specification is "Hex bolt, 1/2-13 UNC X 2’’, SAE Grade 8, hot-dip galvanised" or "Stud bolt, 1/2-20 UNF X 2’’, L7, plain steel".
Field identification: how to tell UNC threads from UNF threads
UNC threads and UNF threads at the same diameter look almost identical. The only reliable method of identification in the field is a calibrated thread gauge. A thread gauge is a set of profiled blades, one for each TPI value. Hold the blade against the thread on the bolt and find the blade that gives full contact without play. The result is stated as diameter-TPI, for example 1/2-13 (UNC) or 1/2-20 (UNF).
Thread gauges for imperial fasteners are calibrated in TPI and are not compatible with metric thread gauges, which are calibrated in millimetres of pitch. Keep both types available at facilities where both standard systems occur. The marking on the bolt head states the SAE strength class (number of lines), not the thread series. The thread series can never be determined visually without a thread gauge.
Which standards apply?
UNC threads and UNF threads are both defined under ASME B1.1 (Unified Inch Screw Threads). Strength classes for imperial bolts are defined under SAE J429. For petroleum-related materials, such as the alloy steels B7 and L7, ASTM A193 and ASTM A320 apply. For the Norwegian continental shelf, NORSOK M-630 and NORSOK M-650 are the applicable requirements for material quality and supplier certification. A purchase order for imperial fasteners must always state: type, diameter, TPI, thread standard (UNC or UNF), length, SAE Grade or ASTM designation, surface treatment and certificate type.
Why are NORD-LOCK washers necessary?
NORD-LOCK wedge-locking washers are designed to maintain the clamping force mechanically, independently of friction in the thread surfaces. The wedge-locking principle uses wedge-shaped undersides which, under load, force the washers to increase the clamping force rather than yield, and actively counteract rotation under vibration and dynamic load. For UNC threads and UNF threads in offshore installations and rotating machinery, NORD-LOCK washers provide documented preload retention where friction-based solutions are not sufficient. INDUSTRISALG carries the full range of NORD-LOCK wedge washers in stock.
Conclusion
UNC threads and UNF threads are two separate thread series within the imperial standard system, defined by TPI at the same nominal diameter. UNC threads are the standard choice for general industrial use. UNF threads are used where controlled preload and vibration resistance are requirements. Identification in the field requires a calibrated thread gauge. A complete specification always states diameter, TPI and thread standard, never diameter alone.
Checklist: ordering imperial fasteners
- Are diameter, TPI and thread standard (UNC or UNF) stated on the purchase order?
- Is the SAE Grade or ASTM designation stated, not just the dimension?
- Is a thread gauge available at the facility for identification in the field?
- Has the thread series been verified with a thread gauge, not assumed on the basis of visual inspection?
- Are surface treatment and certificate type stated on the order?
Related reading
- Marking and traceability: what you should require from a fastener supplier
- How to specify fasteners correctly: size, thread, length, material, coating, standard, strength
Get in touch
Send us the specification with type, diameter, TPI, thread standard (UNC or UNF), length, SAE Grade and surface treatment, and we will confirm stock status and delivery time the same day. INDUSTRISALG holds over 13,000 items in stock, including a broad range of imperial fasteners in UNC and UNF with NORSOK-approved materials in L7 and Super Duplex. What is a special order elsewhere is often standard with us.