DIN 603 Carriage Bolt — Stainless Steel A2 (SUS304) | Grade A2-70 | FULLERKREG
FULLERKREG DIN 603 Stainless Steel A2 Carriage Bolts combine the classic round-head square-neck design of DIN 603 with the inherent corrosion resistance of A2 austenitic stainless steel (SUS304). Finished with acid washing and passivation to maximize the natural oxide layer, these bolts deliver a clean, bright appearance and reliable corrosion resistance for indoor, outdoor, food-adjacent, and light marine applications — without any applied coating that can chip, peel, or wear away.
Product Overview
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Standard: DIN 603
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Common Name: Carriage Bolt / Coach Bolt / Round Head Square Neck Bolt
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Material: A2 Stainless Steel (SUS304 / 1.4301)
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Property Class / Grade: A2-70 (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.
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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.
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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: A2 Stainless Steel (SUS304 / 1.4301)
A2 stainless steel — designated SUS304 in JIS, 1.4301 in EN, and commonly known as 18-8 stainless steel — is the most widely used austenitic stainless steel in the world. Its combination of chromium and nickel content provides excellent corrosion resistance through the formation of a self-repairing passive oxide layer on the surface.
Chemical Composition (Typical)
| Element |
Content (%) |
| Chromium (Cr) |
17.5 – 19.5 |
| Nickel (Ni) |
8.0 – 10.5 |
| Carbon (C) |
≤ 0.07 |
| Manganese (Mn) |
≤ 2.0 |
| Silicon (Si) |
≤ 1.0 |
| Phosphorus (P) |
≤ 0.045 |
| Sulfur (S) |
≤ 0.015 |
| Iron (Fe) |
Balance |
Key Material Properties
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Corrosion Resistance: Excellent in atmospheric, freshwater, and mild chemical environments. The chromium content forms a self-repairing passive oxide layer (Cr₂O₃) that regenerates when damaged.
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Non-Magnetic: Austenitic structure — non-magnetic in the annealed condition (may become slightly magnetic after cold working)
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Hygienic: Smooth, non-porous surface — suitable for food-adjacent and pharmaceutical 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
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Work Hardening: Moderate — cold working increases strength but reduces ductility
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Recycling: 100% recyclable — environmentally responsible material choice
Mechanical Properties — Grade A2-70
Per ISO 3506-1, the grade designation for stainless steel fasteners uses a material class (A2) combined with a minimum tensile strength indicator (70 = 700 MPa). A2-70 is the standard grade for austenitic stainless steel bolts and screws.
| Property |
Specification |
Standard Reference |
| Material |
A2 Stainless Steel (SUS304 / 1.4301) |
ISO 3506-1 |
| Property Class |
A2-70 |
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 — no coating required; self-repairing passive layer |
ISO 3506-1 |
Surface Finish: Acid Washing + Passivation
FULLERKREG A2 stainless steel DIN 603 carriage bolts are finished with a two-stage acid washing and passivation process. This is not a coating — it is a surface treatment that enhances the inherent corrosion resistance of the stainless steel itself.
Stage 1 — Acid Washing (Pickling)
The bolts are immersed in a controlled acid solution (typically a mixture of nitric and hydrofluoric acid) to:
- Remove heat tint, scale, and oxide discoloration from the manufacturing process
- Dissolve any free iron contamination on the surface that could initiate rust spots
- Restore a uniform, bright, clean stainless steel surface
- Expose a fresh, chromium-rich surface layer ready for passivation
Stage 2 — Passivation
Following acid washing, the bolts undergo passivation — a chemical treatment (typically nitric acid or citric acid solution) that:
- Accelerates and thickens the natural chromium oxide (Cr₂O₃) passive layer on the surface
- Maximizes the corrosion resistance of the A2 stainless steel
- Removes any remaining free iron or contaminants that could cause rust staining
- 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 to unfinished or electroplated fasteners
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No Coating Thickness: Zero added coating — thread dimensions remain at nominal tolerance, no oversize tapping required
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Self-Repairing: If the surface is scratched, the passive layer regenerates automatically in the presence of oxygen
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No Peeling or Flaking: Unlike zinc coatings, there is no applied layer to chip, peel, or wear away
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Food-Safe: Passivated A2 stainless steel is suitable for food contact applications
Corrosion Resistance: A2 vs Other Finishes
| Finish / Material |
Corrosion Mechanism |
Salt Spray Resistance |
Outdoor Service Life |
Self-Repairing |
| A2 Stainless (Passivated) |
Passive Cr₂O₃ layer — inherent |
≥ 500 hours (no red rust) |
20–50+ years (atmospheric) |
✅ Yes |
| Hot Dip Galvanized (HDG) |
Sacrificial zinc anode |
≥ 1,000 hours |
20–50+ years (structural) |
✅ Yes (zinc) |
| Zinc CR3 Electroplate |
Barrier + limited sacrificial |
24 – 48 hours (Grade 4.8) |
2 – 5 years (exposed) |
❌ No |
| Plain Carbon Steel |
None — rusts immediately |
< 24 hours |
Months (exposed) |
❌ No |
Note: A2 stainless steel is not suitable for chloride-rich marine environments (seawater, salt spray zones). For marine applications, A4 (SUS316) stainless steel is recommended.
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
🏗️ Construction & Architectural Timber
- Exposed timber frame connections where aesthetics matter — visible bolt heads in architectural woodwork
- Interior structural timber joints in public buildings, hotels, and retail spaces
- Hardwood decking and cladding fastening where staining from carbon steel rust is unacceptable
- Staircase, handrail, and balustrade structural connections
- Glazing and curtain wall timber sub-frame fastening
🌿 Outdoor & Landscape (Non-Marine)
- Garden furniture, pergolas, and outdoor seating structures
- Playground equipment in urban and suburban environments
- Outdoor signage and display structure mounting
- Fence and gate hardware in residential and commercial settings
- Bicycle rack and street furniture fastening
🍽️ Food, Beverage & Pharmaceutical
- Food processing equipment structural fastening
- Commercial kitchen and catering equipment assembly
- Brewery, winery, and dairy facility structural connections
- Pharmaceutical and cleanroom equipment fastening
- Cold storage and refrigeration unit structural joints
🏥 Healthcare & Hygienic Environments
- Hospital and medical facility furniture and equipment fastening
- Laboratory bench and equipment structural connections
- Hygienic wall panel and partition fastening
⚡ Electrical & Electronics
- Electrical enclosure and panel assembly where non-magnetic fasteners are required
- Switchgear and control cabinet structural fastening
- Solar panel mounting structure connections (non-coastal)
- Telecommunications equipment enclosure fastening
Key Features & Design Advantages
- ✨ Inherent Corrosion Resistance — No Coating Required: A2 stainless steel resists corrosion through its passive oxide layer — not through an applied coating. There is nothing to chip, peel, or wear away, making it ideal for applications where coating integrity cannot be guaranteed.
- 🔒 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 with no tool access required on the bolt head side.
- 📏 Bright, Aesthetic Finish: Acid washing and passivation produce a uniform, bright silver-white surface — ideal for visible, architectural, and design-conscious applications where appearance matters.
- 🧪 Non-Magnetic: Austenitic A2 stainless steel is non-magnetic in the annealed condition — suitable for applications sensitive to magnetic interference.
- ♻️ Environmentally Responsible: No zinc, chromate, or other coating chemicals — 100% recyclable stainless steel with no hazardous surface treatment waste.
- 🌡️ Wide Temperature Range: Suitable from cryogenic temperatures (−196°C) to elevated service temperatures (+800°C intermittent) — far exceeding the range of zinc-coated carbon steel fasteners.
- ✅ Standard Thread Fit: No coating thickness — standard-tolerance nuts fit without modification. No oversize tapping required (unlike HDG).
Important Limitations — When NOT to Use A2
A2 stainless steel provides excellent corrosion resistance in most environments, but has known limitations that engineers and specifiers should be aware of:
- ⚠️ Chloride Environments: A2 (SUS304) is susceptible to pitting and crevice corrosion in chloride-rich environments — including seawater, coastal salt spray zones, and de-icing salt exposure. For these applications, A4 (SUS316) stainless steel is required.
- ⚠️ Galling Risk: Stainless steel fasteners are prone to galling (thread seizure) during tightening, especially at high speeds. Always lubricate threads with an anti-galling compound (e.g., molybdenum disulfide paste or PTFE-based lubricant) before assembly.
- ⚠️ Galvanic Corrosion: When A2 stainless steel contacts dissimilar metals (e.g., carbon steel, aluminium, copper) in the presence of an electrolyte, galvanic corrosion may occur. Use isolation washers or sleeves where necessary.
- ⚠️ High-Strength Applications: A2-70 (700 MPa) is lower in tensile strength than carbon steel Grade 8.8 (800 MPa) or Grade 10.9 (1040 MPa). For high-strength structural joints, verify that A2-70 meets the required load capacity.
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 to prevent galling and thread seizure.
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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: Use A2 stainless steel flat washer and hex nut to maintain material compatibility and avoid galvanic corrosion.
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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 A2-70 and the thread size.
Recommended Tightening Torque — A2-70
| 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 A2-70 bolts at 70% proof load with lubricated threads. Always use lubrication with stainless steel fasteners to prevent galling. Verify with your structural engineer for safety-critical applications.
Why Choose FULLERKREG A2 Stainless DIN 603 Carriage Bolts?
- ✅ Full DIN 603 & ISO 3506-1 Compliance: Every batch manufactured to DIN 603 dimensional standards and ISO 3506-1 A2-70 mechanical requirements.
- ✅ Acid Washed & Passivated: Superior surface finish maximizes the inherent corrosion resistance of A2 stainless steel — bright, clean, and consistent across every batch.
- ✅ Full Traceability: Material certificates (MTC), mechanical test reports, and chemical composition reports available per shipment.
- ✅ No Coating — No Compromise: Inherent stainless steel corrosion resistance — no coating to chip, peel, or fail. Ideal for applications where coating integrity is critical.
- ✅ Wide Size Range: M5 through M20, lengths from 16 mm to 200 mm — covering the full DIN 603 range in stainless steel.
- ✅ Matched A2 Nuts & Washers Available: Complete A2 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 (A2-70)
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EN 10088-1 — Stainless steels — list of stainless steels (1.4301 / A2)
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ASTM A967 / ASTM A380 — Standard practice for passivation of stainless steel
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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 (SUS304 material standard)
Ordering & Customization
FULLERKREG supplies DIN 603 A2-70 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 A2 nuts and washers are required
- Required quantity and packaging format (bulk / box / pallet)
- Certification requirements (material certificates, mechanical test reports, chemical composition reports)
Contact our technical sales team for datasheets, samples, and volume pricing.
FULLERKREG — Precision Stainless Steel Fasteners. Clean, Bright, and Built to Last.
