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AS/NZS 1393: dimensional standard for metric coach screws

AS/NZS 1393:1996 (R2016) is the dimensional standard for M6 to M20 metric coach screws with ISO hexagon heads in Australia and NZ. Specs, markings, lengths.

By Chalkline. Last updated 16 May 2026. Verified 16 May 2026.

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AS/NZS 1393:1996, Coach screws, metric series with ISO hexagon heads, is the joint Australian and New Zealand dimensional standard for metric coach screws used in structural timber connections. The standard was reconfirmed in 2016 (R2016) and remains current at May 2026 (verified 2026-05-16, Standards New Zealand).

AS/NZS 1393 sets the shape, dimensions, and tolerances of the coach screw itself. Mechanical properties (tensile and yield strength, marking) are set by AS 4291.1:2015. Together, the two standards define what a compliant Australian or New Zealand coach screw is.

Scope

AS/NZS 1393 covers (verified 2026-05-16, Standards Australia store listing):

  • Diameter range: ISO preferred series from M6 to M20 inclusive.
  • Length range: up to 200 mm.
  • Head type: ISO hexagon head only.
  • Shank: either full body (shank diameter equals thread diameter) or reduced diameter shank.
  • Use: timber structural connections.

Larger coach screws (over M20 or longer than 200 mm) and imperial-thread coach screws are outside the AS/NZS 1393 scope. Specifying anything outside the standard means a proprietary product with no national dimensional reference.

What the standard specifies

The standard prescribes:

ElementSpecified
Hexagon head size (across flats)Per ISO metric series for each diameter
Head heightPer ISO metric series
Shank length and diameterFull-body or reduced-diameter options
Thread (gimlet point)Wood-screw type for driving into timber
MaterialCarbon steel, class 4.6 per AS 4291.1
Tolerance classPer ISO standards referenced in the document
MarkingManufacturer identification

A reduced-diameter shank version has the shank turned down to slightly less than the thread root, reducing splitting risk in tight timber connections at the cost of bending resistance.

How AS/NZS 1393 applies on a residential site

Coach screws are the standard tie-down and bolt-down fastener for many timber connections in residential construction:

  • Subfloor bearer to stump connection: M12 or M16 coach screws into bottom plate.
  • Wall plate to slab edge (where a stud-to-slab bolt is impractical).
  • Pergola post to plate through a metal post base.
  • Hold-down brackets (top-plate-to-stud and stud-to-bottom-plate) where the bracket is screw-fixed rather than bolt-fixed.

For each of these, the engineer’s design tie-down schedule will specify the diameter (M10, M12, M16), the length (typically 75 mm to 150 mm), the property class (usually 4.6, sometimes 8.8 for higher-load applications), and the corrosion coating (HDG for treated pine and external exposure).

A coach screw conforming to AS/NZS 1393 with property class 4.6 markings per AS 4291.1 is what the engineer assumes.

Common builder issues

  • Length undersized. The thread must engage the underlying timber by at least the engineer-specified depth. Picking a coach screw that’s too short leaves the timber engagement below design.
  • Reduced-shank vs full-body confusion. Engineers sometimes specify a full-body shank for shear connections; substituting reduced-shank shortens the load path through bending. Read the bolting schedule carefully.
  • Pilot hole undersized or skipped. AS/NZS 1393 doesn’t mandate a pilot hole, but most timber substrates split if a coach screw is driven without one. Match the pilot to the unthreaded shank diameter (full-body) or to thread root diameter (reduced-shank).
  • Non-compliant offshore products. Imported coach screws may have ISO hex heads that don’t match the AS/NZS 1393 across-flats dimensions, fouling sockets and inspection. Check the manufacturer’s compliance statement on delivery.
  • Wrong corrosion coating. AS/NZS 1393 doesn’t cover corrosion protection; the specifier must add a coating requirement (HDG, mechanical galvanising, stainless 304/316) per AS 3566 or similar. Plain zinc-plated coach screws in external or treated-pine applications corrode fast.

References

See also

Last updated: 2026-05-16. Verified: 2026-05-16.

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