AS 4291.1: property-class system for steel fasteners
AS 4291.1:2015 sets the property class system (4.6, 8.8, 10.9) for steel bolts, screws, and studs in Australia. Strength values, markings, and on-site checks.
Ask Chalkline about this →AS 4291.1:2015, Mechanical properties of fasteners made of carbon steel and alloy steel, Bolts, screws and studs, sets the property-class system every metric bolt, screw, and stud sold in Australia is rated against. It adopts ISO 898-1:2013 and supersedes the AS 4291.1:2000 edition (verified 2026-05-15, Standards Australia Store, AS 4291.1:2015).
The standard is the reason a bolt head is stamped “4.6” or “8.8”. That stamping is your on-site check against engineer-specified strength.
Scope
AS 4291.1:2015 covers:
- Fastener types: bolts, screws, and studs made of carbon steel and alloy steel.
- Thread profile: ISO metric coarse and fine pitch threads.
- Diameter range: up to and including M39.
- Property classes: 3.6 to 12.9.
The companion parts of the AS 4291 series cover nuts (AS 4291.2) and set screws (AS 4291.3). Stainless-steel fasteners are covered separately by AS/NZS 1252.1 (high-strength structural bolts) and AS/NZS 1559 (hot-dip galvanised threaded fasteners), not by AS 4291.1.
What the property-class number means
A property class is a two-digit code stamped on the fastener head:
| Property class | Minimum tensile strength | Minimum yield (or proof) strength |
|---|---|---|
| 3.6 | 300 MPa | 180 MPa |
| 4.6 | 400 MPa | 240 MPa |
| 4.8 | 400 MPa | 320 MPa |
| 5.6 | 500 MPa | 300 MPa |
| 5.8 | 500 MPa | 400 MPa |
| 6.8 | 600 MPa | 480 MPa |
| 8.8 | 800 MPa | 640 MPa |
| 9.8 | 900 MPa | 720 MPa |
| 10.9 | 1000 MPa | 900 MPa |
| 12.9 | 1200 MPa | 1080 MPa |
The decoding rule (verified 2026-05-15, Standards Australia Store, AS 4291.1:2015):
- First digit × 100 = minimum nominal tensile strength (MPa).
- Both digits multiplied × 10 = minimum yield (or proof) strength (MPa).
So a class 8.8 bolt has 800 MPa tensile and 0.8 × 800 = 640 MPa yield. A class 4.6 bolt has 400 MPa tensile and 0.4 × 600 (read as 0.6 × 400) = 240 MPa yield.
Marking requirements
Property class identification must be permanently marked on the bolt head, the screw head, or one end of the stud. The marking must include:
- The property-class designation (e.g. “8.8”).
- The manufacturer’s identification mark.
Hexagon-head products of class 4.6 and above with a thread diameter of M5 or larger must carry the marking visibly on the head. Smaller fasteners and certain countersunk types use a side marking or are marked by container labelling instead.
How AS 4291.1 applies on a residential site
Most residential structural connections specify the property class on the drawings or in the engineer’s bolting schedule. Common applications:
| Connection | Typical property class | Standard |
|---|---|---|
| Coach screws into timber framing | 4.6 | AS/NZS 1393 (dimensions) + AS 4291.1 (mechanical) |
| Standard commercial hex bolts | 4.6 | AS 1111.1 + AS 4291.1 |
| Structural high-strength bolt assemblies (HSB) | 8.8 or 10.9 | AS/NZS 1252.1 (also covers 8.8 mechanical) |
| Hold-down anchors (chemical / mechanical) | 8.8 typical | AS 5216 + AS 4291.1 |
| Threaded rod (allthread) | 4.6 or 8.8 | AS 1111.1 / AS 4291.1 |
See bolts (M-series metric) for product-level selection and the matching anchor and dimensional standards.
Common builder failures
- Wrong property class on site. 4.6 and 8.8 bolts are the same size and look identical from a distance. The head stamp is the only on-site check. Always confirm the marking on delivery, not in the wall.
- Mixed classes in a single assembly. Pairing a class 4.6 bolt with a class 8.8 nut wastes the bolt strength and may fail engineering review.
- Substituting class 4.6 where 8.8 is specified. Most common on hold-downs and structural plate-to-frame fixings. The bolt passes a visual inspection but fails at a lower load.
- No marking visible. Unmarked or worn fasteners cannot be confirmed against AS 4291.1 and should be rejected at delivery.
- Galvanised class 10.9 bolts: AS 4291.1 cautions against hot-dip galvanising class 10.9 and 12.9 bolts due to hydrogen embrittlement risk. Use mechanically galvanised or zinc-flake coatings instead.
References
- AS 4291.1:2015, Mechanical properties of fasteners made of carbon steel and alloy steel - Bolts, screws and studs (verified 2026-05-15)
- ISO 898-1:2013, Mechanical properties of fasteners made of carbon steel and alloy steel - Bolts, screws and studs with specified property classes (adopted as AS 4291.1:2015)
Related
See also
Last updated: 2026-05-15. Verified: 2026-05-15.