Thread Standards: Metric, UNC, Whitworth, Trapezoidal
> In brief > - Metric, UNC, Whitworth and trapezoidal threads are not interchangeable. > - The thread form is the part of the specification most often left vague. > - Write the form and the fit class on the order.
A thread designation carries four pieces of information: the system, the nominal diameter, the pitch, and the tolerance class. Reading it correctly prevents the most expensive formwork mistake there is — ordering rods and nuts that do not mate.
Our own production and test data are available on request — see how our factory works and contact the engineering team for the documents behind any figure quoted here.
Thread designation is the notation that specifies the system, nominal diameter, pitch, and tolerance class of a thread. Flank angle refers to the angle between the flanks of a thread, which must match for threads to mate properly. Tolerance class means the degree of precision in the fit between mating threads, specified to ensure proper engagement. Galling is the wear and material transfer that occurs when two metal surfaces are in contact under high pressure, leading to seizing or stripping.
The four systems you will meet.
METRIC (ISO 60°): written M20 × 2.5, meaning 20 mm nominal diameter and 2.5 mm pitch. Coarse pitches are the default; fine pitches such as M20 × 1.5 are used where vibration loosening or thin-wall sections matter. Dominant in Europe, China, the Middle East and most of Asia.
UNC / UNF (Unified, 60°): written 1/2-13 UNC, meaning 1/2 inch nominal with 13 threads per inch. UNC is coarse, UNF is fine. Common on North American equipment, older climbing systems and US-sourced props.
WHITWORTH (BSW/BSP, 55°): still found on legacy UK and Commonwealth scaffolding and on pipe threads. The 55° flank angle is not interchangeable with 60° systems even at the same nominal size.
TRAPEZOIDAL (Tr): a 30° thread used for power transmission rather than fastening — jacking screws, high-axial-load tie rods, some climbing spindles. Written Tr 40 × 7.
Standard basis: ISO 965-1
Why mixing systems fails.
Two threads only mate when system (flank angle), diameter, pitch and tolerance class all agree. A 1/2 inch UNC nut on an M12 rod will start by hand and then bind: the flank angles differ by 5°, so contact occurs on a line instead of a surface. Torque rises, galling starts, and either the nut seizes or the thread strips under load.
Partial engagement is the dangerous case, because it can hold at low load and let go when the pour adds pressure.
Checks before ordering.
- Measure pitch, not just diameter. Use a pitch gauge, or measure ten pitches with a caliper and divide. Diameter alone cannot separate M12 × 1.75 from M12 × 1.25.
- Confirm tolerance class. 6g/6H is the general-purpose metric pairing; 4g/4h is tighter and less tolerant of coating thickness. Hot-dip galvanizing adds 45-85 µm per flank, enough to make a tight-class nut unusable — galvanized assemblies normally use 6g/6H or a tapped-oversize nut.
- Order rod and nut as a matched set from one source when the joint is safety-related. If they come from different suppliers, put the standard and class in writing on both purchase orders.
- Check the thread form on the drawing. Rolled threads outperform cut threads for fatigue because grain flow follows the profile instead of being severed.
Standard basis: ISO 68-1
Quick reference.
Metric coarse: M10 × 1.5, M12 × 1.75, M16 × 2.0, M20 × 2.5, M24 × 3.0. Unified coarse: 1/2-13, 5/8-11, 3/4-10, 1-8. Formwork tie rods use DW15 (15 mm root, 17 mm OD, 7.5 mm pitch) and Rd20 in metric markets; inch-pattern systems appear mainly on imported climbing hardware.
A five-minute gauge check at goods-in is cheaper than a stopped pour.
| Thread System | Tolerance Class | Consequence for Mismatch |
|---|---|---|
| Metric (M20 x 2.5) | 6g/6H | General-purpose, reliable fit |
| Metric (M20 x 2.5) | 4g/4h | Tighter fit, may seize with coating |
| UNC (1/2-13) | 2A/2B | Standard fit, common in North America |
| UNC (1/2-13) | 3A/3B | Tighter fit, less tolerant of coatings |
| Whitworth (BSW) | 6g/6H | Legacy systems, not interchangeable |
| Trapezoidal (Tr 40 x 7) | 6g/6H | For power transmission, not fastening |
References and standards.
- EN 14399-1 — High-strength structural bolting assemblies
- ISO 16047 — Hexagon head bolts and hex nuts
- High-strength bolting guidance
- EN 1090 — Execution of steel structures
- EN 10204:2004 — Types of inspection documents
- ISO 898-1 — Mechanical properties of fasteners made of carbon steel
About us: This article covers thread standards and their correct application in formwork. Our production and test data are available on request. Our contact page provides access to the engineering team for detailed inquiries, and the about page documents how our factory works. This article was reviewed by the Qianheng Formwork Engineering team.