If you have searched for coach bolts and ended up more confused than when you started — wondering whether you need a coach bolt or a coach screw, what clearance hole to drill, or why the supplier's catalogue calls them cup head bolts — this guide covers it all.
Coach bolts and coach screws are two different products that share a name and often share a shelf. Both are used in timber construction — decking, fencing, pergolas, structural framing, agricultural and playground equipment — but they work in fundamentally different ways and are not interchangeable. Understanding which one you need, in what size and finish, is the practical focus of this guide.
All specifications and standards referenced here are metric and Australian.
What Is a Coach Bolt?
A coach bolt (also called a cup head bolt in Australian supply catalogues, or a carriage bolt in US and sometimes New Zealand usage) is a through-bolt with a distinctive domed head and a short square neck immediately below it.
The dome is smooth — there is no slot, hex socket, or recess for a driver. You cannot tighten a coach bolt from the head side. Instead, the square neck bites into the timber surface as the nut on the reverse side is tightened, locking the bolt against rotation and allowing the joint to be fully tightened with a spanner or socket from one side only.
The bolt passes completely through all members being joined and is secured with a nut and flat washer on the reverse face. The washer spreads the clamping load to prevent the nut from pulling into the timber grain.
Coach Bolt — Key Characteristics
- Domed, smooth head (no drive recess)
- Square neck below head prevents rotation during tightening
- Passes through all members — requires access to both faces
- Secured with a nut and washer on the reverse
- Australian standard: AS 1390 (Cup Head Bolts — Metric Series)
- Typical grade: 4.6 mild steel (structural: 8.8 high tensile)
- Common finish: hot-dip galvanised for outdoor use
The smooth dome head is both a feature and a limitation. It gives a clean, tamper-resistant appearance on the visible face — useful for playground equipment and public structures — but means the bolt can only be tightened from the nut side. If you do not have access to the reverse face, a coach screw is the right choice instead.
What Is a Coach Screw?
A coach screw (also called a lag bolt or lag screw in US usage) is a large-diameter, heavy-duty fastener with a hexagonal head and a pointed, coarse self-tapping thread. Unlike a coach bolt, it does not require a nut — it threads directly into the timber and relies on thread engagement for its holding strength.
Coach screws are driven with a spanner or socket from one side only. They are faster to install than coach bolts in applications where through-access is not available or practical: fixing ledger boards to wall framing, attaching brackets to posts, connecting rails to timber uprights.
Coach Screw — Key Characteristics
- Hexagonal head — driven with spanner or socket wrench
- Coarse, self-tapping thread with pointed tip
- Screws into timber from one side only — no nut required
- Requires a pilot hole to prevent timber splitting
- No Australian-specific standard; commonly supplied to DIN 571 dimensions
- Typical grade: mild steel (not graded in the same system as bolts)
- Common finish: hot-dip galvanised for outdoor and treated timber use
The hex head means a coach screw looks superficially similar to a hex bolt, but the thread form is completely different. A hex bolt has a machine thread for use with a nut or tapped hole; a coach screw has a wood thread that cuts directly into timber. Do not use a coach screw in a tapped metal hole, and do not substitute a hex bolt for a coach screw in timber — the machine thread will not hold.
Coach Bolt vs Coach Screw: Key Differences
The confusion between these two products is the most common question in this product category. Here is the comparison in full:
| Feature | Coach Bolt | Coach Screw |
|---|---|---|
| Head style | Smooth dome — no drive | Hexagonal — spanner/socket |
| Thread type | Machine thread (metric coarse) | Wood thread (coarse self-tapping) |
| Fixing method | Through-bolt: nut + washer on reverse | Screws into timber — no nut |
| Side access | Both sides required | One side only |
| Pre-drilling | Clearance hole (bolt shank + 0.5–1mm) | Pilot hole (70–80% of shank dia.) |
| Shear strength | Higher — bears against both faces | Lower — thread engagement only |
| Removability | Fully removable — undo nut | Removable but timber thread degrades with repeated removal |
| Installation speed | Slower — drill, insert, nut, washer | Faster — drill pilot, drive in |
| Appearance (face side) | Clean dome — no drive marks | Hex head visible |
| Australian standard | AS 1390 | DIN 571 (no AS equivalent) |
Is a Coach Bolt the Same as a Carriage Bolt?
Yes. Coach bolt and carriage bolt refer to the same fastener — the difference is regional terminology. In Australia and the UK, the product is called a coach bolt. In the United States (and in some New Zealand catalogues), the same product is called a carriage bolt.
If you are working from an imported design, a US manufacturer's specification, or an international structural timber connection guide, "carriage bolt" means coach bolt. The dimensions are the same in metric (M8, M10, M12 etc.) and the approximate imperial equivalents most commonly encountered are: 3/8" ≈ M10, 1/2" ≈ M12, 5/16" ≈ M8.
You may also see "cup head bolt" in Australian supply catalogues — this is the same product again, named after the cup-shaped dome head, and it is what AS 1390 calls it officially.
Coach Bolt Sizes: Metric Dimensions
Coach bolts are specified by diameter × length. The length is measured from the underside of the head to the end of the thread. The square neck depth and head diameter vary by size but follow the proportions of AS 1390.
| Size | Shank Ø (mm) | Clearance Hole (mm) | Head Ø (approx) | Common Lengths (mm) | Nut Size (spanner) |
|---|---|---|---|---|---|
| M6 | 6 | 6.5 | 14mm | 20, 25, 30, 40, 50, 60, 75, 100 | 10mm |
| M8 | 8 | 8.5 | 18mm | 20, 25, 30, 40, 50, 60, 75, 100, 120, 150 | 13mm |
| M10 | 10 | 10.5 | 22mm | 25, 30, 40, 50, 60, 75, 100, 130, 150, 200 | 17mm |
| M12 | 12 | 13 | 27mm | 30, 40, 50, 60, 75, 100, 130, 150, 200, 250 | 19mm |
| M16 | 16 | 17 | 34mm | 40, 50, 60, 75, 100, 130, 150, 200, 250, 300 | 24mm |
| M20 | 20 | 21 | 40mm | 50, 60, 75, 100, 130, 150, 200, 250, 300 | 30mm |
How Long Should Your Coach Bolts Be?
The coach bolt must pass completely through all members plus allow enough thread engagement for the nut — a minimum of one full nut height (typically 1× bolt diameter) beyond the last member is the practical rule. For a 45mm decking post connection using M12 bolts, for example, total member thickness + 15–20mm for nut and washer gives your minimum bolt length.
Buying slightly longer than necessary is generally better than too short — you can always trim excess thread with an angle grinder, but a bolt that doesn't reach the nut face is useless. Standard stock lengths are typically in 10–25mm increments; non-standard lengths are often available to order for large structural projects.
For general reference on metric fastener dimensions, see our Fastener Reference Chart.
Coach Screw Sizes & Pilot Hole Chart
Coach screws are specified by diameter × length, measured from under the head to the tip. The pilot hole is critical — too small and you risk splitting the timber or snapping the screw; too large and the thread engagement is insufficient for the required holding load.
The general rule: pilot hole diameter ≈ 70% of the shank diameter for softwood (pine, treated pine), ≈ 80% for hardwood (ironbark, spotted gum, hardwood decking). Always drill a full-depth pilot hole — not just a starter hole.
| Size | Shank Ø (mm) | Pilot Hole — Softwood | Pilot Hole — Hardwood | Head Ø (across flats) | Common Lengths (mm) |
|---|---|---|---|---|---|
| M6 | 6 | 3.5mm | 4.5mm | 10mm | 30, 40, 50, 60, 75, 100 |
| M8 | 8 | 5mm | 6mm | 13mm | 30, 40, 50, 65, 75, 100, 120, 150 |
| M10 | 10 | 6mm | 7mm | 17mm | 40, 50, 65, 75, 100, 130, 150, 200 |
| M12 | 12 | 7mm | 9mm | 19mm | 40, 50, 65, 75, 100, 130, 150, 200, 250 |
| M16 | 16 | 10mm | 11mm | 24mm | 50, 65, 75, 100, 130, 150, 200, 250, 300 |
For drill bit sizing across all fastener types, see our Drill Bit Size Chart.
Coach Bolt Materials and Finishes
Choosing the right material and finish is as important as choosing the right size. An undersized bolt will be weak; a correctly sized bolt in the wrong finish will corrode and fail early — sometimes without visible warning from the surface.
Grade 4.6 vs Grade 8.8
Standard coach bolts (AS 1390 cup head bolts) are Grade 4.6 mild steel — minimum tensile strength 400 MPa, yield strength 240 MPa. This is adequate for most residential construction, fencing, playground equipment, and general timber joinery.
Grade 8.8 high tensile coach bolts are available for structural applications where higher clamping force or load capacity is required — heavy post connections, structural beams, agricultural equipment. Grade 8.8 has a minimum tensile strength of 800 MPa and yield strength of 640 MPa. For full bolt grade data and torque values, see our Bolt Grade Chart.
Hot-Dip Galvanised (HDG)
The standard finish for outdoor structural coach bolts in Australia. Hot-dip galvanising immerses the fastener in molten zinc at approximately 450°C, producing a metallurgically bonded coating typically 45–85 microns thick. This provides genuine long-term corrosion protection suitable for outdoor exposure, treated timber, and the Australian coastal environment.
HDG coach bolts will develop a dull grey patina over time as the zinc oxidises — this is normal and is the zinc sacrificially protecting the steel underneath. HDG is the correct specification for any exposed structural connection.
Electroplated Zinc
Electroplated (bright zinc) coach bolts have a thin coating (5–15 microns) applied electrically. This provides only light corrosion protection — adequate for indoor applications, sheltered conditions, and short-term outdoor use during construction. Electroplated coach bolts must not be used in permanently exposed outdoor connections, and must never be used in treated timber (see below).
Electroplated bolts are typically cheaper and have a brighter appearance than HDG. If you are unsure which you are purchasing, check the product specification — "galvanised" without qualification in Australian hardware retail often means electroplated rather than hot-dip.
Stainless Steel: A2 and A4
For coastal, marine, food processing, or chemical environments where zinc-coated fasteners will not provide adequate corrosion resistance, stainless steel coach bolts are specified. A2 (Grade 304) is suitable for most outdoor and mild marine environments. A4 (Grade 316) contains molybdenum and is specified for direct marine exposure, saltwater contact, and aggressive chemical environments.
Stainless coach bolts are significantly more expensive than galvanised and have lower tensile strength than high-tensile steel bolts — use them where corrosion resistance is the primary requirement, not where structural strength is marginal. For a full explanation of stainless fastener grades, see our Stainless Steel Fastener Grades Guide.
Treated Pine: The Critical Rule
CCA (copper chrome arsenate) and ACQ (alkaline copper quaternary) treated pine — the green-tinted structural and outdoor timber used for decking frames, fence posts, pergola posts, and landscaping — are chemically aggressive to electroplated zinc fasteners. The copper compounds in the treatment accelerate zinc corrosion significantly.
Electroplated coach bolts and coach screws installed in treated pine will typically corrode through within 2–5 years in outdoor conditions, causing structural failure that may not be visible from the surface until the connection is already seriously compromised.
When to Use Coach Bolts vs Coach Screws
The decision comes down to three factors: access, load type, and permanence.
| Situation | Use Coach Bolt | Use Coach Screw |
|---|---|---|
| Access to both sides of joint | ✅ Preferred for maximum strength | ✅ Also works, faster |
| One side only accessible | ❌ Not possible | ✅ Only option |
| High shear load (lateral force) | ✅ Stronger in shear | ⚠️ Adequate for light-moderate loads |
| Engineer-specified connection | ✅ Often specified for structural joints | ⚠️ Check drawing — do not substitute |
| Decking boards to joists | ⚠️ Overkill in most cases | ✅ Standard practice |
| Post to bearer / beam connection | ✅ Recommended for primary structure | ✅ Acceptable if properly sized |
| Appearance matters (face side) | ✅ Clean dome is less obtrusive | ⚠️ Hex head more visible |
| Playground or public structure | ✅ Dome head reduces snagging hazard | ⚠️ Hex head can snag clothing |
| Speed of installation is priority | ❌ Slower — nut and washer required | ✅ Drill and drive |
If an engineer or building certifier has specified the connection, always use exactly what is specified. Do not substitute coach screws for coach bolts on structural drawings without written engineering approval — the load calculations are based on the specified fastener type and quantity.
How to Install Coach Bolts
Coach bolt installation is straightforward but requires the square neck to seat correctly — if it does not, the bolt will spin when you tighten the nut and you will not be able to complete the joint.
- Mark and clamp the joint. Clamp both members together in their final position before drilling. Moving them after drilling will misalign the holes.
- Drill the clearance hole. Use a drill bit 0.5–1mm larger than the bolt shank (e.g. 10.5mm for M10). Drill through all members in a single pass if possible — this ensures the holes are aligned. Use a sharp bit and firm, steady pressure to avoid tearout at the exit face.
- Insert the coach bolt from the face side. Push the bolt through the hole with the dome head sitting on the surface. The square neck should be positioned in the hole at the entry face.
- Seat the square neck. Tap the dome head firmly with a hammer — 2–3 moderate blows — to drive the square neck into the timber at the entry face. The square neck must embed fully to prevent the bolt from rotating during tightening. If the wood is very hard, use a punch and hammer to square up the entry hole slightly, or use a spanner on the nut while holding the head still.
- Fit washer and nut. Slide a flat washer over the thread from the reverse side (washer distributes load across the timber grain and prevents the nut from pulling through). Thread the nut on by hand.
- Tighten with a spanner or socket. Tighten firmly to the specified torque. The dome head should not rotate — if it does, the square neck has not seated. Stop, re-seat by tapping the head, then continue tightening.
For the correct nut to use with metric coach bolts, see our Types of Nuts Guide. For washer selection, see our Types of Washers Guide.
How to Install Coach Screws
Coach screw installation is faster than coach bolts but requires a correctly sized pilot hole. Skipping or under-sizing the pilot hole is the most common cause of both splitting the timber and shearing the coach screw head on installation.
- Mark the fixing position. Mark centre points for each screw. Pre-drilling precisely on-centre is important — coach screws cannot be steered once started.
- Drill the pilot hole to full depth. Use the pilot hole sizes from the table above, matched to your timber species. Drill to the full penetration depth of the screw — not just a starter hole. A pilot hole that ends halfway means the final threads are forced through undrilled timber.
- Apply a small amount of wax or soap to the thread (optional but recommended). Running the thread lightly across a block of wax or a bar of soap reduces drive torque significantly in hardwood and reduces the risk of snapping the screw. Do not use oil-based lubricants — they can affect timber treatments.
- Start the screw by hand. Thread the screw into the pilot hole a few turns by hand to ensure it is running straight and not cross-threading.
- Drive with a socket wrench or impact driver (low speed). Use a socket matched to the hex head (see size table). If using an impact driver, use the lowest torque setting — impact drivers can easily snap M6–M8 coach screws in hardwood. For M10 and above in hardwood, a torque wrench at a controlled setting is preferable.
- Drive to depth. The screw is correctly seated when the head bears firmly against the timber surface (or a washer, if specified). Do not overtighten — coach screws in timber can strip the thread engagement if over-driven.
For help choosing the right screwdriver or socket for driving, see our Screwdriver Types Guide.
Common Applications in Australian Construction
Decking
Coach screws (M8 or M10, HDG or stainless, 65–100mm long depending on deck board and joist thickness) are the standard fastener for attaching deck boards to joists and for fixing ledger boards to wall framing. For post-to-bearer connections and primary structural joints under the deck frame, M12 or M16 coach bolts through both members are the stronger choice where access allows.
Always use HDG or stainless fasteners in treated pine decking frames. Specify the timber treatment type (CCA, ACQ, H3, H5) before purchasing fasteners — some timber treatment systems have specific fastener requirements beyond simple HDG.
Fencing
Coach screws (M8–M10, 65–100mm, HDG) are standard for attaching timber rails to posts. Coach bolts are used where maximum pull-out resistance is needed — gate hinge attachments, structural post-to-rail connections on farm and rural fencing, or anywhere the fence is subject to significant lateral load such as vehicle impacts or stock pressure.
Pergolas and Outdoor Structures
Both fastener types are used in pergola construction. Coach screws fix rafters to beams and beams to posts from the face side. Coach bolts are used for primary structural connections — post bases, beam splices, and any connection specified by the engineer or building certifier. Always use HDG in all outdoor timber-to-timber connections.
Playground and Public Equipment
Coach bolts with dome heads are preferred for exposed connections in playground equipment and public furniture because the smooth dome does not snag clothing and presents no projecting edges. Stainless steel (A2 or A4) is often specified for vandal resistance and the extended service intervals required in public infrastructure.
Agricultural and Rural Applications
Coach bolts (M12–M20, HDG) are widely used in stockyard construction, loading ramps, cattle crush assemblies, and farm shed framing. Sizes are larger and bolt lengths are longer than residential applications. Check whether the design uses metric or imperial specifications — older farm structures and some imported agricultural equipment use imperial carriage bolt sizes.
Frequently Asked Questions
What is a coach bolt used for?
Coach bolts are used to join timber members where both sides of the joint are accessible, such as decking posts, fence rails, playground equipment, gate hardware, and structural timber framing. The domed head sits on the face side while a nut and washer are tightened from the reverse, creating a strong through-bolted connection resistant to shear and pull-through forces.
What is the difference between a coach bolt and a coach screw?
A coach bolt has a smooth domed head with a square neck and requires a nut and washer on the reverse — it goes completely through all members. A coach screw has a hexagonal head and a self-tapping thread that screws directly into timber from one side with no nut. Coach bolts are stronger in shear; coach screws are faster and require single-side access only.
Is a coach bolt the same as a carriage bolt?
Yes. Coach bolt (Australia, UK) and carriage bolt (US, sometimes NZ) are the same fastener. If you are working from a US specification, carriage bolt means coach bolt. The metric dimensions are the same; approximate imperial equivalents are 5/16" ≈ M8, 3/8" ≈ M10, 1/2" ≈ M12.
Is a coach bolt the same as a cup head bolt?
Yes. Cup head bolt is the trade and catalogue name used in Australian fastener supply for the same product called a coach bolt on site. AS 1390 (the Australian standard) calls it a cup head bolt. All three names — coach bolt, carriage bolt, cup head bolt — refer to the same fastener with the same dimensions and specifications.
Do you need to pre-drill for a coach bolt?
Yes. Drill a clearance hole through all members using a bit 0.5–1mm larger than the bolt shank: M6 = 6.5mm, M8 = 8.5mm, M10 = 10.5mm, M12 = 13mm, M16 = 17mm. The square neck does not self-drill — it seats into the timber surface as the nut is tightened, preventing rotation during tightening.
What size drill bit do I need for a coach bolt?
Drill a clearance hole 0.5–1mm larger than the bolt diameter: M6 = 6.5mm, M8 = 8.5mm, M10 = 10.5mm, M12 = 13mm, M16 = 17mm, M20 = 21mm. Always fit a flat washer under the nut to distribute clamping load across the timber grain. See our drill bit size chart for full reference.
What is the pilot hole size for a coach screw?
Pilot hole size depends on timber species. Softwood (pine, treated pine): M6 = 3.5mm, M8 = 5mm, M10 = 6mm, M12 = 7mm, M16 = 10mm. Hardwood (ironbark, hardwood decking): M6 = 4.5mm, M8 = 6mm, M10 = 7mm, M12 = 9mm, M16 = 11mm. Always drill to the full screw penetration depth — not just a starter hole.
Are coach bolts stronger than screws?
Coach bolts are generally stronger in shear (lateral forces) than coach screws of the same diameter because the through-bolt bears against both faces of the joint. Coach screws rely on timber thread engagement, which can loosen with moisture cycling over time. For structural connections subject to significant lateral load — deck posts, pergola beams, gate hinges — coach bolts through both members are the more reliable choice where access allows.
What grade are coach bolts in Australia?
Standard Australian coach bolts (AS 1390 cup head bolts) are Grade 4.6 mild steel — minimum tensile strength 400 MPa, yield 240 MPa. Grade 8.8 high tensile coach bolts are available for structural applications. Stainless versions are Grade A2 (304) or A4 (316). For full bolt grade data, see our Bolt Grade Chart.
What is the difference between hot-dip and electroplated galvanising on coach bolts?
Hot-dip galvanised (HDG) has a thick zinc coating (45–85 microns) bonded at the molecular level — suitable for outdoor structural use, treated timber, and coastal environments. Electroplated zinc has a thin coating (5–15 microns) — adequate for indoor or sheltered use only. In outdoor and treated pine applications, always specify hot-dip galvanised.
Can coach screws be used in treated pine?
Yes, but only with hot-dip galvanised or stainless steel coach screws. CCA and ACQ treated pine are corrosive to electroplated zinc — electroplated coach screws in treated pine will corrode through within 2–5 years outdoors. Never use electroplated zinc fasteners in treated timber for any outdoor or structural application.
Can I use coach screws instead of coach bolts for decking?
For decking board-to-joist connections, coach screws are the standard and accepted practice. For primary structural connections — post-to-bearer, beam connections, or any joint specified by an engineer — check the structural drawings. If coach bolts are specified, do not substitute coach screws without written engineering approval. The load calculations are based on the specified fastener type.
Coach Bolts and Coach Screws at AIMS Industrial
AIMS Industrial stocks coach bolts and coach screws in metric sizes, in hot-dip galvanised mild steel and stainless steel. All fasteners meet Australian supply standards and are suitable for structural timber construction, decking, fencing, and outdoor applications.