Coach Bolts

About Coach Bolts

Coach bolts, also known as carriage bolts, are specialised fasteners designed for heavy-duty applications where a smooth, rounded head and secure fixing are essential. Distinguished by their characteristic mushroom-shaped head and square section beneath it, coach bolts provide a self-locking mechanism that prevents the bolt from rotating during tightening. This makes them ideal for timber-to-timber joints, timber-to-metal connections, and various outdoor construction projects where both strength and a neat finish are required.

Our range of coach bolts includes products from trusted brands like TIMCO, Sealey, and Sirius, available in various materials and finishes to suit different applications. Whether you need stainless steel coach bolts for outdoor weatherproofing or zinc plated options for general construction work, you'll find coach bolts and nuts supplied together for convenience. These fasteners are manufactured to DIN 603 standards, ensuring consistent quality and reliable performance across demanding applications.

Coach bolts are particularly valued in joinery, fencing, decking, and structural timber work where they provide superior holding power compared to standard bolts. The rounded head creates a tamper-resistant fixing whilst the square shoulder section bites into the timber, preventing the bolt from spinning when the nut is tightened from the opposite side.

Jargon Buster

DIN 603

The German industrial standard specification for coach bolts, defining dimensions, tolerances, and material grades to ensure consistent quality and compatibility across manufacturers.

Square Shoulder

The squared section of the shank directly beneath the mushroom head that grips into timber or softer materials, preventing the bolt from rotating during tightening.

Zinc Plated

A protective coating applied to steel bolts through electroplating, providing moderate corrosion resistance suitable for indoor applications and covered outdoor use.

Stainless Steel (A2/A4)

Corrosion-resistant steel grades used for coach bolts in exposed outdoor applications. A2 offers good weather resistance, whilst A4 provides superior protection in marine or chemical environments.

Mushroom Head

The distinctive rounded, dome-shaped head of a coach bolt that sits flush or slightly proud of the material surface, providing a smooth finish and tamper resistance.

Thread Length

The portion of the bolt shank featuring screw threads, typically covering only part of the total bolt length on coach bolts, with the remainder being a smooth shank.

Who Uses Coach Bolts?

Coach bolts are essential fasteners for a wide range of trades and users, including:

• Carpenters and joiners - for structural timber connections, heavy-duty furniture assembly, and workshop projects
• Fencing contractors - securing fence posts, rails, and panels in both domestic and agricultural settings
• Decking specialists - fastening structural elements, posts, and joists where strength and weather resistance are critical
• Landscapers - constructing pergolas, garden structures, retaining walls, and outdoor features
• Agricultural workers - maintaining gates, stables, barns, and field equipment
• Groundsworkers - installing play equipment, outdoor furniture, and recreational structures
• Property maintenance teams - general repairs and installations requiring robust timber fixings
• DIY enthusiasts - building garden furniture, sheds, gates, and home improvement projects
• Shopfitters - securing fixtures and fittings where a neat, tamper-resistant finish is required

How to Choose the Right Coach Bolts

Selecting the appropriate coach bolts for your project requires consideration of several key factors:

Material and Finish: Choose stainless steel coach bolts for outdoor applications, coastal environments, or where maximum corrosion resistance is needed. Zinc plated options from TIMCO, Sealey, or Sirius offer cost-effective protection for covered outdoor use or interior applications. Consider the expected exposure to moisture and chemicals when making your selection.

Size and Length: Measure the total thickness of materials being joined and add sufficient length to accommodate the nut and washers. The bolt should protrude at least 10-15mm beyond the nut when fully tightened to ensure secure fixing. Coach bolts are available in various diameters and lengths to suit different applications.

Load Requirements: Consider the weight and forces the fixing will bear. Larger diameter bolts provide greater shear and tensile strength for structural applications. Always check that your chosen bolt size meets or exceeds the load requirements for your specific application.

DIN 603 Compliance: Products manufactured to DIN 603 standards ensure dimensional accuracy and consistent quality. Brands like Sealey offer DIN 603 certified coach bolts that guarantee reliable performance and compatibility.

Quantity and Packaging: Coach bolts are typically supplied with matching nuts included. Consider whether you need additional washers for load distribution, particularly when working with softer timbers or to prevent the nut from embedding into the material surface.

Popular Accessories

• Washers - flat washers distribute load and prevent damage to timber surfaces; essential for professional installations
• Spanners and socket sets - quality tools from brands like Bahco and Wera for proper tightening
• Wood drill bits - for creating pilot holes to prevent timber splitting and ensure accurate positioning
• Auger bits - ideal for drilling clearance holes for the bolt shank through thicker timber sections
• Ratchet sets - speed up installation when fitting multiple coach bolts
• Cordless impact drivers - from Makita, DeWalt, or Milwaukee for efficient tightening on larger projects
• Measuring and marking tools - ensuring accurate spacing and alignment of fixings
• Timber treatment products - protecting cut ends and drilled holes from moisture ingress

Safety Information

When installing coach bolts, always wear appropriate PPE including safety glasses to protect against wood splinters and metal fragments. Ensure timber is adequately supported during drilling and bolt installation to prevent movement or kickback.

Pre-drill clearance holes to the correct size - the square shoulder section requires a hole slightly larger than the bolt diameter to seat properly without splitting the timber. For the nut-side clearance, drill a hole matching the bolt diameter to ensure the square shoulder grips effectively.

Avoid over-tightening, which can crush timber fibres, reduce holding power, and potentially shear the bolt. Tighten nuts firmly but stop when resistance increases significantly. For structural applications, always follow relevant building regulations and manufacturer guidance on loading and spacing.

Frequently Asked Questions

What's the difference between coach bolts and standard bolts?

Coach bolts feature a rounded mushroom head and a square shoulder section beneath it that bites into timber to prevent rotation during tightening. Standard bolts have hexagonal heads and require holding with a spanner on both ends. Coach bolts are specifically designed for timber applications where only one-sided access is practical.

Do I need to use washers with coach bolts?

Whilst coach bolts function without washers, using a washer under the nut is highly recommended, especially with softer timbers. Washers distribute the load over a larger area, prevent the nut from embedding into the material, and make future removal easier. For structural applications, washers are essential.

Can coach bolts be used in metal-to-metal applications?

Coach bolts are primarily designed for timber applications where the square shoulder can bite into the softer material. For metal-to-metal fixing, standard hexagonal head bolts are more appropriate as they provide better purchase and don't rely on material deformation for the anti-rotation feature.

What size pilot hole should I drill for coach bolts?

For the head side, drill a clearance hole approximately 1-2mm larger than the bolt diameter to allow the square shoulder to seat. The nut-side hole should match the bolt diameter exactly, allowing the square shoulder to grip as the bolt is driven through. Always test on scrap material first to avoid splitting.

Are stainless steel coach bolts worth the extra cost?

For outdoor applications, exposed locations, or coastal environments, stainless steel coach bolts provide significantly superior corrosion resistance and longevity. Whilst zinc plated options are adequate for covered locations, the modest additional investment in stainless steel prevents rust staining, maintains structural integrity, and eliminates premature failure in demanding conditions.