Metric vs American vs British Threads — and the Australian Standards That Govern Them
In maintenance workshops and brownfield sites, one of the most common hidden problems is not bolt strength — it is thread identification.
Equipment imported from the USA, Europe and the UK often ends up assembled together on Australian sites.
The bolts may look identical.
They may even screw together.
But they are not interchangeable.
Incorrect thread matching damages load capacity, prevents correct preload, and leads to loosening, fatigue cracking and eventual failure.
This guide explains the major fastening thread systems encountered in Australia (excluding pipe threads), how to recognise them, and the Australian Standards that apply.
1. The Three Fastener Thread Systems
There are three main fastening thread families encountered in mechanical and structural equipment:
| System | Origin | Thread Angle | Typical Location |
|---|---|---|---|
| Metric ISO | Australia / Europe / modern equipment | 60° | Most modern machinery |
| Unified (UNC/UNF) | USA | 60° | Mining & imported plant |
| Whitworth (BSW/BSF/BA) | UK / older Commonwealth | 55° | Older equipment & legacy machinery |
Even though UNC and Metric share a 60° angle, the pitch is different — therefore they are not compatible.
Whitworth threads are particularly problematic because they will partially screw into metric or UNC holes before binding.
2. Metric Threads (ISO Metric — Australian Standard Fasteners)
These are the primary fastening threads used in Australia.
(Coarse pitch series)
| Size | Major Diameter | Pitch | Minor Diameter (approx) |
|---|---|---|---|
| M6 | 6.0 mm | 1.0 | 4.8 mm |
| M8 | 8.0 mm | 1.25 | 6.5 mm |
| M10 | 10.0 mm | 1.5 | 8.2 mm |
| M12 | 12.0 mm | 1.75 | 9.9 mm |
| M16 | 16.0 mm | 2.0 | 13.8 mm |
| M20 | 20.0 mm | 2.5 | 17.3 mm |
| M24 | 24.0 mm | 3.0 | 20.8 mm |
Fine pitch versions also exist for vibration and adjustment applications.
Typical Uses
- Structural steel connections
- Machinery assembly
- Guards and access platforms
- General engineering
3. Unified American Threads (UNC / UNF)
Common on imported mining and mobile equipment.
UNC – Coarse
| Size | Major Diameter | Pitch |
|---|---|---|
| 1/4-20 | 6.35 mm | 1.27 mm |
| 3/8-16 | 9.53 mm | 1.59 mm |
| 1/2-13 | 12.70 mm | 1.95 mm |
| 3/4-10 | 19.05 mm | 2.54 mm |
| 1-8 | 25.40 mm | 3.18 mm |
UNF – Fine
Used where vibration resistance is required.
Key Characteristic
UNC bolts will often start threading into metric holes but will not achieve correct preload.
4. British Threads (Whitworth Form)
Recognised by their 55° thread angle.
BSW – Coarse
| Size | Major Diameter | Pitch |
|---|---|---|
| 1/4 BSW | 6.35 mm | 1.34 mm |
| 3/8 BSW | 9.53 mm | 1.59 mm |
| 1/2 BSW | 12.70 mm | 2.12 mm |
| 3/4 BSW | 19.05 mm | 2.54 mm |
BSF – Fine
Used historically in machinery.
BA Threads
Small instrumentation and electrical fasteners.
Typical Location
- Pre-1980 plant
- UK imported machinery
- Electrical equipment
Why Incorrect Thread Matching Causes Failures
Threads do not primarily carry shear load — they generate preload.
If pitch or angle differs:
- preload is reduced
- flank contact is uneven
- joint loosens under vibration
- fatigue cracking begins
Many failures blamed on vibration are actually incorrect thread engagement.
Field Identification Tips
| Observation | Likely Thread |
|---|---|
| Marked M12 | Metric |
| Fraction size (1/2, 3/4) | UNC/UNF or Whitworth |
| Smooth but tight engagement | Wrong pitch |
| Binds after 2 turns | Whitworth vs Metric |
Thread gauge confirmation is always recommended.
Australian Standards Relating to Fastener Threads
Metric Thread Geometry
AS 1721 — General purpose metric screw threads
AS 1275 — Metric screw threads for fasteners
Fastener Product Standards
AS 1110 — Metric hex bolts and screws
AS 1111 — Commercial hex bolts and screws
AS 1112 — Hexagon nuts
AS 1420 — Socket head cap screws
Mechanical Properties
AS/NZS 4291.1 — Mechanical properties of bolts, screws and studs
AS/NZS 4291.2 — Mechanical properties of nuts
ISO 898-1 / ISO 898-2 — Adopted strength properties
ISO 3506 — Stainless steel fasteners
Structural Bolting
AS/NZS 1252 — High strength structural bolting assemblies
AS 4100 — Steel structures design
AS/NZS 5131 — Fabrication and erection of structural steel
Coatings and Fit Allowances
AS/NZS 1214 — Galvanised coatings on threaded fasteners
AS/NZS 4680 — Hot dip galvanising
AS 2312.2 — Corrosion protection guide
AS 1897 — Electroplated coatings