DIN 603 Carriage Bolt — Marine Stainless Steel A4 (SUS316) | Grade A4-70 | FULLERKREG
FULLERKREG DIN 603 Marine Stainless Steel A4 Carriage Bolts represent the highest level of corrosion resistance available in the DIN 603 carriage bolt range. Manufactured from A4 austenitic stainless steel (SUS316 / 1.4401), these bolts incorporate molybdenum into the alloy composition — the critical element that provides superior resistance to chloride-induced pitting and crevice corrosion that limits the performance of standard A2 (SUS304) stainless steel. Finished with acid washing and passivation, they deliver a bright, clean surface and maximum passive layer integrity for the most demanding marine, coastal, chemical, and industrial environments.
Product Overview
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Standard: DIN 603
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Common Name: Marine Carriage Bolt / Coach Bolt / Round Head Square Neck Bolt
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Material: A4 Marine Stainless Steel (SUS316 / 1.4401)
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Property Class / Grade: A4-70 (per ISO 3506-1)
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Surface Finish: Acid Washed + Passivated
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Thread: Metric coarse thread (ISO 68-1)
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Head Type: Round (domed) head with square neck
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Brand: FULLERKREG
What Is a DIN 603 Carriage Bolt?
The DIN 603 carriage bolt — also known as a coach bolt or round head square neck bolt — is a specialized fastener defined by three key geometric features:
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Smooth Domed Head: The low-profile, rounded head presents a clean, tamper-resistant appearance on the visible face of the joint. No drive recess is present — the bolt is tightened exclusively from the nut side, leaving a smooth, snag-free surface ideal for marine and outdoor environments.
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Square Neck (Collar): Immediately below the head, a short square section embeds into the timber substrate or a pre-punched square hole when the nut is tightened, preventing the bolt from rotating. This enables single-sided tightening with a wrench on the nut alone — critical in marine applications where access is often restricted.
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Threaded Shank: The shank carries a metric coarse thread accepting a standard hex nut or flange nut. Shorter lengths may be fully threaded; longer lengths feature an unthreaded grip section.
Material: A4 Marine Stainless Steel (SUS316 / 1.4401)
A4 stainless steel — designated SUS316 in JIS, 1.4401 in EN, and commonly known as marine-grade or 316 stainless steel — is the premier choice for fasteners in corrosive, chloride-rich, and chemically aggressive environments. The key differentiator from A2 (SUS304) is the addition of 2–3% molybdenum (Mo), which dramatically improves resistance to pitting and crevice corrosion in chloride environments such as seawater, coastal salt spray, de-icing salts, and chemical processing media.
Chemical Composition (Typical)
| Element |
A4 / SUS316 Content (%) |
A2 / SUS304 Content (%) |
Key Difference |
| Chromium (Cr) |
16.5 – 18.5 |
17.5 – 19.5 |
— |
| Nickel (Ni) |
10.0 – 13.0 |
8.0 – 10.5 |
Higher Ni in A4 |
| Molybdenum (Mo) |
2.0 – 3.0 |
None |
⭐ Critical for chloride resistance |
| Carbon (C) |
≤ 0.07 |
≤ 0.07 |
— |
| Manganese (Mn) |
≤ 2.0 |
≤ 2.0 |
— |
| Silicon (Si) |
≤ 1.0 |
≤ 1.0 |
— |
| Iron (Fe) |
Balance |
Balance |
— |
Key Material Properties
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Chloride Resistance: Excellent — molybdenum content provides superior resistance to pitting and crevice corrosion in seawater, coastal salt spray, and chloride-containing chemical environments
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Passive Layer: Self-repairing chromium oxide (Cr₂O₃) passive layer — regenerates automatically when damaged in the presence of oxygen
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Non-Magnetic: Austenitic structure — non-magnetic in the annealed condition
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Hygienic: Smooth, non-porous surface — suitable for food, pharmaceutical, and chemical processing environments
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Temperature Range: Suitable for continuous service from −196°C to +800°C (intermittent)
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Weldability: Excellent — all standard welding methods applicable; low carbon content minimizes sensitization risk
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Recycling: 100% recyclable — environmentally responsible material choice
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PREN (Pitting Resistance Equivalent Number): ~25–28 (vs ~18–20 for A2) — significantly higher pitting resistance
Mechanical Properties — Grade A4-70
| Property |
Specification |
Standard Reference |
| Material |
A4 Stainless Steel (SUS316 / 1.4401) |
ISO 3506-1 |
| Property Class |
A4-70 |
ISO 3506-1 |
| Minimum Tensile Strength (Rm) |
≥ 700 MPa |
ISO 3506-1 |
| 0.2% Proof Stress (Rp0.2) |
≥ 450 MPa |
ISO 3506-1 |
| Elongation after fracture |
≥ 20% |
ISO 3506-1 |
| Hardness (Vickers, max) |
≤ 250 HV |
ISO 3506-1 |
| Surface Finish |
Acid Washed + Passivated |
ASTM A967 / EN 2516 |
| Corrosion Resistance |
Inherent — self-repairing passive layer; superior chloride resistance vs A2 |
ISO 3506-1 |
Surface Finish: Acid Washing + Passivation
FULLERKREG A4 stainless steel DIN 603 carriage bolts are finished with a two-stage acid washing and passivation process that maximizes the inherent corrosion resistance of the SUS316 alloy.
Stage 1 — Acid Washing (Pickling)
- Removes heat tint, scale, and oxide discoloration from the manufacturing and cold-heading process
- Dissolves free iron contamination on the surface that could initiate rust spots
- Restores a uniform, bright, clean stainless steel surface across the entire batch
- Exposes a fresh, chromium- and molybdenum-rich surface layer ready for passivation
Stage 2 — Passivation
- Accelerates and thickens the natural chromium oxide (Cr₂O₃) passive layer
- Maximizes the corrosion resistance of the A4 stainless steel, particularly in chloride environments
- Removes any remaining free iron or contaminants
- Produces a consistent, bright, silver-white appearance across the entire batch
Result
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Appearance: Bright, uniform silver-white — clean and aesthetically superior
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No Coating Thickness: Zero added coating — thread dimensions remain at nominal tolerance, standard nuts fit without modification
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Self-Repairing: Passive layer regenerates automatically in the presence of oxygen
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No Peeling or Flaking: No applied layer to chip, peel, or wear away
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Food & Chemical Safe: Passivated A4 stainless steel is suitable for food contact and chemical processing environments
A4 vs A2: Why Molybdenum Makes the Difference
| Property |
A4 (SUS316) — This Product |
A2 (SUS304) |
| Molybdenum Content |
2.0 – 3.0% |
None |
| PREN (Pitting Resistance) |
~25 – 28 |
~18 – 20 |
| Seawater Resistance |
✅ Excellent — suitable for splash zone & immersion |
⚠️ Limited — pitting risk in seawater |
| Coastal Salt Spray |
✅ Excellent — suitable for direct coastal exposure |
⚠️ Moderate — may pit in heavy salt spray |
| De-icing Salt Exposure |
✅ Suitable |
⚠️ Risk of pitting |
| Chemical Processing |
✅ Suitable for many acids and chlorides |
⚠️ Limited chemical resistance |
| Swimming Pool Environments |
✅ Suitable (chlorinated water) |
⚠️ Not recommended |
| Food & Pharmaceutical |
✅ Preferred grade |
✅ Suitable |
| Tensile Strength (A4-70 / A2-70) |
≥ 700 MPa |
≥ 700 MPa |
| Price |
Higher (Mo content) |
Lower |
Corrosion Resistance Comparison — All Finishes
| Finish / Material |
Chloride Resistance |
Salt Spray (NSS) |
Outdoor Service Life |
Marine Suitability |
| A4 Stainless (This Product) |
⭐⭐⭐⭐⭐ Excellent |
≥ 1,000 hours |
30–50+ years |
✅ Yes — seawater & coastal |
| A2 Stainless (SUS304) |
⭐⭐⭐ Good (non-marine) |
≥ 500 hours |
20–50 years (non-coastal) |
⚠️ Splash zone only |
| Hot Dip Galvanized |
⭐⭐ Moderate |
≥ 1,000 hours |
20–50 years (structural) |
⚠️ Not for seawater immersion |
| Zinc CR3 Electroplate |
⭐ Poor |
24 – 48 hours |
2 – 5 years (exposed) |
❌ No |
Dimensional Reference — DIN 603 Key Parameters
| Thread Size |
Pitch (mm) |
Head Diameter dk (mm) |
Head Height k (mm) |
Square Neck Width f (mm) |
Square Neck Height m (mm) |
| M5 |
0.8 |
14.0 |
3.0 |
5.0 |
3.5 |
| M6 |
1.0 |
16.0 |
3.6 |
6.0 |
4.0 |
| M8 |
1.25 |
21.0 |
4.8 |
8.0 |
5.0 |
| M10 |
1.5 |
26.0 |
6.0 |
10.0 |
6.0 |
| M12 |
1.75 |
30.0 |
7.2 |
12.0 |
7.0 |
| M14 |
2.0 |
35.0 |
8.4 |
14.0 |
8.0 |
| M16 |
2.0 |
40.0 |
9.6 |
16.0 |
9.0 |
| M20 |
2.5 |
50.0 |
12.0 |
20.0 |
11.0 |
Standard lengths range from 16 mm to 200 mm depending on thread size. Full dimensional tables per DIN 603. Custom lengths available upon request.
Typical Application Sectors
⛵ Marine & Boating
- Boat deck hardware, cleats, and fitting fastening
- Teak deck and timber deck planking fastening
- Pontoon, jetty, and floating dock timber connections
- Mast, boom, and rigging hardware structural fastening
- Marine trailer and launching ramp structural connections
- Underwater and splash zone structural fastening (hull fittings, keel bolts)
🌊 Coastal & Offshore Infrastructure
- Coastal boardwalk and promenade timber decking
- Sea wall, breakwater, and coastal defence structure fastening
- Offshore platform and jetty structural connections
- Coastal signage, lighting, and street furniture fastening
- Tidal zone and splash zone structural timber connections
- Desalination plant and water treatment facility fastening
🏊 Swimming Pools & Aquatic Facilities
- Swimming pool surround and deck structural fastening
- Pool ladder, handrail, and diving board structural connections
- Aquatic centre and water park structure fastening
- Spa, hot tub, and hydrotherapy pool equipment fastening
🧪 Chemical & Industrial Processing
- Chemical plant structural fastening in chloride-containing environments
- Pulp and paper mill equipment connections
- Pharmaceutical and bioprocessing facility structural fastening
- Food and beverage processing plant — wash-down and high-humidity areas
- Wastewater treatment plant structural connections
🌿 Outdoor & Architectural (Coastal Zones)
- Coastal residential and commercial building structural timber connections
- Beachfront and waterfront development fastening
- Outdoor furniture and structures within 1 km of the coastline
- Bridge and pedestrian walkway structural connections in coastal environments
Key Features & Design Advantages
- 🌊 Superior Chloride & Marine Corrosion Resistance: The 2–3% molybdenum content in A4 (SUS316) provides dramatically better resistance to pitting and crevice corrosion in seawater, coastal salt spray, and chloride-containing environments compared to standard A2 (SUS304).
- 🔒 Anti-Rotation Square Neck: The square collar embeds into timber or a pre-punched square hole, preventing bolt rotation during nut tightening — enabling single-sided assembly, critical in marine environments where access is often restricted.
- ✨ Bright, Aesthetic Finish: Acid washing and passivation produce a uniform, bright silver-white surface — ideal for visible marine hardware and architectural applications.
- ♻️ No Coating — No Compromise: Inherent stainless steel corrosion resistance — no zinc, chromate, or paint coating to chip, peel, or fail in harsh marine environments.
- 🧪 Non-Magnetic: Austenitic A4 stainless steel is non-magnetic in the annealed condition — suitable for applications sensitive to magnetic interference.
- 🌡️ Wide Temperature Range: Suitable from cryogenic temperatures (−196°C) to elevated service temperatures (+800°C intermittent).
- ✅ Standard Thread Fit: No coating thickness — standard-tolerance A4 nuts fit without modification.
- 🍽️ Food & Chemical Safe: Passivated A4 stainless steel is the preferred grade for food processing, pharmaceutical, and chemical environments.
Installation Guide
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Lubricate the thread: Apply an anti-galling lubricant (molybdenum disulfide paste, PTFE tape, or anti-seize compound) to the thread before assembly. Stainless steel fasteners are prone to galling — lubrication is essential, especially in marine environments where fasteners may be tightened under difficult conditions.
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Prepare the substrate: Drill a clearance hole equal to the bolt diameter. For timber, the square neck will self-embed; for metal, punch or drill a square hole matching the neck dimensions.
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Insert the bolt: Push the carriage bolt through the hole from the front (visible) face.
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Embed the square neck: Tap the bolt head lightly with a mallet to seat the square neck fully into the timber or square hole.
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Apply washer and nut: Always use A4 stainless steel flat washer and hex nut to maintain material compatibility. Never mix A4 bolts with carbon steel or A2 nuts in marine environments — galvanic corrosion may occur at the contact interface.
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Tighten from the nut side: Tighten slowly with a wrench — avoid impact tools or high-speed tightening to minimize galling risk. Tighten to the recommended torque for A4-70 and the thread size.
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Periodic Inspection: In marine environments, inspect fasteners periodically for signs of crevice corrosion at the joint interface, particularly in timber connections where moisture can be trapped.
Recommended Tightening Torque — A4-70
| Thread Size |
Tightening Torque (Nm) — Dry |
Tightening Torque (Nm) — Lubricated |
| M5 |
4.0 |
3.0 |
| M6 |
6.8 |
5.1 |
| M8 |
16.5 |
12.4 |
| M10 |
33 |
25 |
| M12 |
57 |
43 |
| M14 |
90 |
68 |
| M16 |
140 |
105 |
| M20 |
272 |
204 |
Torque values are indicative for A4-70 bolts at 70% proof load. Always lubricate stainless steel threads before tightening to prevent galling. Verify with your structural or marine engineer for safety-critical applications.
Why Choose FULLERKREG A4 Marine Stainless DIN 603 Carriage Bolts?
- ✅ Full DIN 603 & ISO 3506-1 Compliance: Every batch manufactured to DIN 603 dimensional standards and ISO 3506-1 A4-70 mechanical requirements.
- ✅ Genuine A4 / SUS316 Material: Full material traceability with chemical composition certificates confirming molybdenum content — not A2 misrepresented as A4.
- ✅ Acid Washed & Passivated: Superior surface finish maximizes the inherent corrosion resistance of A4 stainless steel — bright, clean, and consistent across every batch.
- ✅ Full Traceability: Material certificates (MTC), mechanical test reports, and chemical composition reports (confirming Mo content) available per shipment.
- ✅ Wide Size Range: M5 through M20, lengths from 16 mm to 200 mm — covering the full DIN 603 range in marine stainless steel.
- ✅ Matched A4 Nuts & Washers Available: Complete A4 stainless steel fastener sets (bolt + nut + washer) available for simplified procurement and guaranteed material compatibility.
- ✅ Flexible Packaging: Bulk, box, and custom packaging options available.
Applicable Standards & References
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DIN 603 — Round head square neck bolts (metric coarse thread)
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ISO 3506-1 — Mechanical properties of corrosion-resistant stainless steel fasteners: bolts, screws and studs (A4-70)
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EN 10088-1 — Stainless steels — list of stainless steels (1.4401 / A4)
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ASTM A967 / ASTM A380 — Standard practice for passivation of stainless steel parts
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ISO 9227 — Corrosion tests in artificial atmospheres — neutral salt spray (NSS) test
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JIS G4303 — Stainless steel bars (SUS316 material standard)
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NORSOK M-630 — Material data sheets for piping (referenced for offshore/marine material selection)
Ordering & Customization
FULLERKREG supplies DIN 603 A4-70 marine stainless steel carriage bolts in standard and custom configurations. Please specify the following when placing an inquiry:
- Thread diameter and pitch (e.g., M8 × 1.25, M12 × 1.75)
- Nominal length (e.g., 30 mm, 60 mm, 100 mm)
- Fully threaded or partially threaded shank preference
- Whether matched A4 nuts and washers are required
- Required quantity and packaging format (bulk / box / pallet)
- Certification requirements (material certificates with Mo content confirmation, mechanical test reports, chemical composition reports)
Contact our technical sales team for datasheets, samples, and volume pricing.
FULLERKREG — Marine-Grade Fasteners for the World’s Most Demanding Environments. A4 Stainless Steel — Where Corrosion Cannot Compromise.
