Are you looking for high quality carriage bolts? What is a carriage bolt? So, next, learn about the definition, types, structure, materials, and uses of carriage bolts in our guide.
What Is a Carriage Bolt?
Carriage Bolt, also called Round Head Square Neck Bolt, Coach Bolt or DIN 603 Mushroom Head Square Neck Bolt. It is a fastener that can be made from a variety of materials and has a square step where the head meets the screw, so it is also called a step screw. Carriage bolts have a large, semi-circular and smooth head, a neck with a quadrangular design and a threaded stem. Used to join wood to wood or wood to metal. The square neck becomes embedded in the mating material when installed, preventing the bolt from turning when the nut and washer are tightened.
Structure of Carriage Bolts
- Head: Semi-circular top, smooth and beautiful.
- Neck: Quadrilateral neck to prevent rotation.
- Rod: fully threaded or half threaded.
Specifications for Carriage Bolts
| Materials | Carbon steel: Light-duty applications Alloy steel: High-stress scenarios Stainless steel: Corrosive environments |
| Nominal diameter | 5mm to 20mm |
| Length | 12mm to 300mm |
| Property Classes | Grade 4.6, 5.6, 8.8, 10.9, 12.9. |
| Surface treatment | Galvanized, chrome-plated, copper-plated, blackened surface, anodized |
| Standard | DIN 603, ASME B18.5, ISO 8677 |
- Common sizes: Imperial 1/4″, 3/4″, 5/16″, 3/8″, 1/2″, “5/8″; Metric M6, M8, M10, M12.
- Length calculation: Material thickness + washer thickness (if used) + nut thickness + thread protrusion (approximately 1/2 inch) = total length
Common Size Reference Chart:
| Diameter (inches) | Diameter (mm) | Common Lengths (inches) | Common Lengths (mm) |
|---|---|---|---|
| 1/4″ | 6.35 mm | 1″ – 6″ | 25 mm – 150 mm |
| 5/16″ | 7.94 mm | 1″ – 8″ | 25 mm – 200 mm |
| 3/8″ | 9.53 mm | 1.5″ – 10″ | 38 mm – 254 mm |
| 1/2″ | 12.7 mm | 2″ – 12″ | 50 mm – 305 mm |
| 5/8″ | 15.88 mm | 3″ – 12″ | 76 mm – 305 mm |
| 3/4″ | 19.05 mm | 4″ – 12″ | 102 mm – 305 mm |
Types of Carriage Bolts
Standard Round Head Square Neck Bolt
Round head + square neck, the most common carriage bolt type
- Material: Carbon steel
- Performance: 8.8
Short Square Neck Carriage Bolt
Suitable for thin plates or limited embedment depth
- Material: 304 Stainless Steel
- Performance: A2
Zinc-Plated Carriage Bolt
Other surface-treated carriage bolts are also available, such as hot-dip galvanized, Dacromet, black oxide, etc.
Performances : ASTM A307 Grade A
How Are Carriage Bolts Installed?
Prepare carriage bolts, hammer, drill, wrench, nuts and washers
- Drill holes (slightly larger than screw)
- Insert carriage bolt
- Tap head to embed square neck
- Install the washer and nut
- Use a torque wrench to tighten the nut according to the specified value, not too tight or too loose. For example, the recommended torque for carriage bolts of size M16 is 80 to 120 N-m (depending on the material).
Recommended Load:
| Light load | For example assembling garden furniture, smaller diameters such as 1/4″ or 5/16″ are recommended. |
| Medium load | Fixed deck railings, for example, use 3/8″ or 1/2″ diameter. |
| Heavy load | For structural applications (e.g., attaching a beam), choose the larger diameter 5/8″ or 3/4″. |
Recommended Bore Size:
| Bolt diameter | Diameter of hole |
|---|---|
| 1/4″ | 9/32″ (7.14 mm) |
| 5/16″ | 11/32″ (8.73 mm) |
| 3/8″ | 13/32″ (10.32 mm) |
| 1/2″ | 17/32″ (13.49 mm) |
| 5/8″ | 21/32″ (16.67 mm) |
| 3/4″ | 25/32″ (19.84 mm) |
Recommended Bolt Length:
| Material Thickness | Recommended Bolt Length |
|---|---|
| 1 inch | 1-1/4 inch |
| 2 inches | 2-1/4 inch |
| 3 inches | 3-1/4 inch |
Uses of Carriage Bolts
Carriage bolts provide a strong and tamper-proof connection.
- Wood Frame Construction: Fixed Roof Trusses, Beams & Posts
- Fences and Fence Systems: Park Fences, Ranch Fences
- Bridges and Municipal Facilities: Landscape Bridges, Viewing Platforms
- Agricultural Machinery: Livestock Equipment, Farm Equipment
- Amusement facilities: assembling swings, slides and other equipment
- Furniture Manufacturing: Outdoor furniture, garden furniture, round head concealed mounting to maintain wood surface integrity.
Carriage Bolts: FAQ
How do I know if the carriage bolts need to be replaced?
If there is obvious deformation, corrosion, substandard torque or the service life exceeds 5 years, it should be replaced immediately.
Can carriage bolts be reused?
If there is no obvious damage and the torque meets the requirements, secondary use can be considered, but should not be frequent.
Is a washer required when using carriage bolts?
It is recommended to use washers, as they distribute the load, prevent material crushing, and protect the surfaces being joined.
Carriage Bolt Supplier
BD Fastener offers carriage bolts in a wide range of standards, sizes, performance grades, and surface finishes, including both imperial and metric series. If you’re seeking a high-quality carriage bolt supplier or require selection support and custom bolts, feel free to contact us online for professional advice.
