DIN 603 / ISO 8678 Carriage Bolts (Round Head Square Neck)
Mushroom (round) head square neck carriage bolts per DIN 603, ISO equivalent ISO 8678 — a smooth domed head that cannot be gripped from above and a square neck that self-anchors in wood. M5–M20 metric line plus the inch-series round head square neck range; the timber and coach bolt.
Last updated: 2026-11
Specifications
The carriage bolt is built for exactly one installation direction: the smooth domed head sits proud of the surface and gives no wrench faces, so the bolt is tightened entirely from the nut side — while the square shoulder under the head bites into a square-cut hole in timber and resists the shaft turning as the nut is torqued. That combination (anti-vandal smooth head plus self-anchoring neck) is why it dominates wood-to-wood and wood-to-metal construction: fencing, trailer decks, play structures, agricultural equipment. DIN 603 is the metric designation and ISO 8678 its ISO equivalent; the collected stock program runs M5 to M20 with lengths up to 80 mm, and the inch-series round head square neck range is documented from #10 through 3/4 diameter. Order by designation and note that the hole must be square-cut or drilled square at the neck for the anti-turn feature to engage.
| Standard | DIN 603 — mushroom head square neck carriage bolt; ISO equivalent: ISO 8678 |
|---|---|
| Head | Smooth domed head — prevents tightening or loosening from above |
| Neck | Square shoulder below the head self-anchors in wood and resists turning while the nut is tightened |
| Metric size range | M5 – M20, lengths up to 80 mm (stock program) |
| Inch-series range (reference) | Round head square neck bolts tabulated from #10 through 3/4 in diameter |
| Exemplar dimension (1/4 in) | Body dia max 0.260 / min 0.237 in; head dia max 0.594 / min 0.563 in; head height max 0.145 / min 0.125 in |
| Installation | Drill square at the neck; drive the square shoulder into the hole, tighten from the nut side only |
| Typical applications | Timber construction, fencing, trailer and vehicle decks, play equipment, agricultural machinery |
Materials
- Steel
- Stainless steel
Ordering & documentation
Documents
Shipments are backed by Mill Test Certificate (MTC), Dimensional Report, Salt Spray Test Report on request, with full batch traceability from raw material to finished product. Quality management follows the owner-confirmed compliance list: ISO 9001:2015, RoHS, REACH. Exact document sets are confirmed on the order.
Quotation
- The fastener family (DIN 603 / ISO 8678 Carriage Bolts (Round Head Square Neck)) plus the standard designation from your drawing — DIN / ISO / JIS / ASTM / ANSI, or a drawing for custom parts.
- Size (diameter and length), thread type (metric or inch), property class or grade, and coating or finish.
- Quantity per size and your target delivery date.
- Destination country, and any required documents (for example a mill test certificate) so the quote includes them.
Sample & supply
Minimum order quantity is 1,000–10,000 pieces depending on size. Stock items run 3–7 days. High-strength and custom parts run 15–25 days. Quantities or variants outside the standard range are confirmed at quotation.
Storage & shipping
Parts are packed for export and shipped factory-direct to buyers in Europe, Asia, the Americas and the Middle East. Store fasteners dry and protected until installation, and keep coated parts in their packaging until use; specific packing and preservation requirements can be stated on the order and confirmed at quotation.
Frequently asked questions
Why does a carriage bolt spin, and is that a defect?
Below the domed head, a carriage bolt carries a square neck sized to bite into the material. In timber, the bolt is seated by striking the head so the square shoulder embeds; from then on the shaft will not turn and all torque goes to the nut. If the bolt spins during tightening, the hole was not squared into the neck (drilled round through metal or hardwood) or the neck has pulled out of soft timber — the fix is a square plate or washer arrangement at the head side, not a bigger hammer. The spinning feature is also the point on the exposed face: with no wrench faces, the smooth head cannot be backed off from outside.
What is the MOQ and lead time for this bolt grade?
The minimum order quantity is 1,000–10,000 pieces depending on size. Stock items ship in 3–7 days; high-strength and custom parts in 15–25 days. Common diameters in this grade are widely stocked; confirm the exact schedule against quantity and size mix at quotation.
What should an RFQ for this bolt grade state?
State the standard and grade (for example ISO 4017 Class 8.8 or ASTM F3125 Grade A325 Type 1), diameter and length, thread type (metric coarse or UNC/UNF), finish (plain, zinc, or hot-dip galvanized where the grade permits), required certificates such as a mill test report per EN 10204 3.1, and quantity per size — plus destination country.
Related knowledge
- What Bolt Property Classes 8.8, 10.9 and 12.9 Encode — and When High Strength Stops Being the Right Choice
- Bolt Grade Selection: ISO 898-1 8.8/10.9/12.9 vs ASTM A307 and A325
- 8.8 vs 10.9 vs 12.9 Bolts: Strength, Hardness and Cost — What the Evidence Settles
- Calculating Bolt Preload and Clamp Force: Torque, Friction and Tightening Method
- How to Determine Tightening Torque and Choose a Tightening Method for a Bolted Joint
- Fastener Failure Modes: Which Fatigue, Creep and SCC Controls the Evidence Supports
- Hydrogen Embrittlement in High-Strength Bolts: Which Grades Need Controls, Which Coatings and Baking Schedules Are Acceptable, and What to Require Before the RFQ
- Carriage Bolts (Coach Bolts) for Wood and Metal-to-Wood Joints: When the Square Neck Is the Right Call
- Bolt Head Types and Drive Styles: What the Evidence Can Settle Before You Specify
Request a Quote
Get factory-direct pricing from the manufacturer.
What to include in your inquiry
- ✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
- ✓ Sizes, lengths and thread type (metric or inch).
- ✓ Quantity per size and your target delivery date.
- ✓ Destination country, plus any coating or finish requirement.
- ✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
- ✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.
- ✓ Factory-direct pricing
- ✓ Standard and custom specifications
- ✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
- ✓ Full fastener family support