Metric vs Imperial: UNC/UNF and where it is still used in industry

Metric and imperial are the two dominant thread standards in global industry. In Norway both are active in oil and gas, offshore, mining and energy infrastructure, often at the same facility. A maintenance team can encounter both standards in the course of the same working day. This guide covers the differences, where each standard applies, conversion between them and which standards govern them.

What are metric threads?

Metric threads are measured in millimetres and defined by diameter and pitch. An M12 x 1,75 bolt has a 12 mm diameter and 1,75 mm between each thread. The applicable standards are ISO 68-1 for thread form and DIN 13 for European applications. The thread angle is 60 degrees with strict ISO tolerance classes, typically 6g for external and 6H for internal threads.

Metric threads are the global standard and are used in:

• New offshore and energy installations in Europe
• OEM equipment from European and Asian manufacturers
• Process pipelines, pressure vessels and structural steel connections
• Modern mining equipment and heavy machinery manufactured after the 1990s

What are imperial threads?

Imperial threads are measured in inches and defined by diameter and threads per inch (TPI). A 1/2-13 UNC bolt has a half-inch diameter and 13 threads per inch. The applicable standard is ASME B1.1. Within the imperial system there are two series: UNC (Unified National Coarse) is the coarse thread series, UNF (Unified National Fine) is the fine thread series.

Imperial threads are used in:

• Older offshore installations on the Norwegian Continental Shelf built between the 1970s and the 1990s
• Subsea wellhead systems and older compressor skids in oil and gas
• American and Canadian manufactured rotating machinery, drivelines and hydraulic systems
• Military and defence equipment as well as older turbine installations in the energy sector

Reference table: UNC vs UNF — threads per inch

For the most common imperial sizes, here is the difference between UNC and UNF in threads per inch:

Note: “approximate metric equivalent” refers to diameter only — the threads have different pitch, flank geometry and tolerance class, and cannot replace one another without engineering approval.

What is the difference between UNC and UNF in practice?

UNC threads have fewer threads per inch, assemble faster and tolerate incomplete or contaminated hole conditions better. They suit general structural connections and applications with primarily static loading.

UNF threads have more threads per inch, tighter manufacturing tolerance and higher tensile strength (typically 8-12 % higher static tensile strength at the same nominal diameter). They are the correct specification for high-load, vibration-exposed and precision connections where preload retention is critical.

In compressor stations, turbine mountings and flange connections, incorrectly specifying UNC where UNF is required gives lower clamping force and faster preload loss under dynamic loading. This is a specification error that is not visible at installation — it only shows up under operational load, often after the damage has already been done.

Strength classes: SAE Grade vs ISO 898-1

Imperial bolts are classified according to SAE standards (SAE J429 for grades). Metric bolts are classified according to ISO 898-1. The table below shows approximate equivalents for comparison:

These are approximate equivalents for comparison purposes. Bolts cannot be substituted between systems without engineering approval. The SAE standards test under different conditions than ISO 898-1, and the chemical requirements for the steel are not identical.

Can metric and imperial threads be interchanged?

No, not without documented engineering approval. Although M12 and 1/2-UNC are close to each other in diameter (12 mm vs 12,7 mm), they have different pitch, flank geometry and tolerance class. Using one instead of the other gives incompatible thread engagement that weakens the joint under load.

In DNV-classified installations, replacing UNC with UNF, or substituting imperial with metric without documented engineering approval, is a formal non-conformity. NORSOK standards (in particular NORSOK M-650 for qualification of manufacturers) require full traceability of the material’s standard and dimension.

Visual identification between metric and imperial is not reliable where diameters overlap. The difference between UNC and UNF is only measurable with an instrument (thread gauge). Every purchase order must contain the complete thread designation including diameter, pitch, standard and strength class.

Checklist: before you order imperial or metric

• Is the thread standard explicitly stated (ISO, DIN, UNC or UNF) — not just the diameter?
• Is the pitch (metric) or threads per inch (imperial) stated together with the diameter?
• Is the strength class stated according to the applicable standard (ISO 898-1 metric, SAE J429 imperial)?
• Are the material selection and coating system documented against the operating environment?
• Is the thread pitch verified with a calibrated thread gauge at goods receipt?
• Is there documented engineering approval if the standard deviates from the original drawing?

Frequently asked questions about UNC and UNF

What is the main difference between UNC and UNF?

UNC has fewer threads per inch (coarse thread), assembles quickly and suits general applications. UNF has more threads per inch (fine thread), gives higher clamping force and better preload retention, and is used where vibration or high precision is critical.

Can I convert a UNC bolt to metric?

No. Although the diameter is close (example: 1/2-UNC ≈ M12), the pitch and the geometry are different. Conversion requires documented engineering approval and verification against the original design requirements.

Which Norwegian industries still use imperial fasteners?

Oil and gas on the Norwegian Continental Shelf (older installations and subsea equipment), machine shops with American-manufactured equipment, the energy sector (turbine components), and the defence industry. Imperial is still in active use where conversion to metric is impractical or not approved.

What is ASME B1.1?

ASME B1.1 is the American standard that defines Unified Inch Screw Threads, including UNC, UNF and UNEF. It covers dimensions, tolerances and designations for imperial fasteners.

Conclusion

Metric and imperial threads exist side by side in Norwegian heavy industry, and both have consequences when they are incorrectly specified. The difference between coarse threads and fine threads is not administrative — it directly determines preload capacity and joint behaviour under load. Verify the standard with a calibrated thread gauge at goods receipt, specify in full on every order, and never replace one standard with another based on diameter alone.

INDUSTRISALG holds one of Norway’s broadest stocks of imperial fasteners in UNC and UNF — including NORSOK-approved stud bolts in L7 and Super Duplex — and combines 70+ years of specialist expertise with security of supply from our own warehouse. When critical deadlines demand that the specification is right the first time, this is what we deliver on.

Related reading

• How to specify correctly: size, thread, length, material, coating, standard, strength — the complete 7-parameter framework for correct specification
• Thread types 101: Machine threads vs thread-forming vs thread-cutting vs wood screws — thread types and where they are used

Get in touch

Send us the specification — diameter, thread standard, strength class, material and coating — and we will confirm stock status and delivery time the same day. If you are unsure whether UNC or UNF is right for your application, we will help you clarify before ordering.