DIN 603 Carriage Bolt — Grade 8.8 | Hot Dip Galvanized | FULLERKREG
FULLERKREG DIN 603 Grade 8.8 Hot Dip Galvanized Carriage Bolts combine the high clamping strength of Grade 8.8 with the unmatched long-term corrosion protection of hot dip galvanizing — making them the definitive choice for heavy structural timber connections, civil infrastructure, and outdoor load-bearing applications exposed to weather, moisture, and corrosive environments. Manufactured from Alloy Steel 10B21 (boron steel) or Carbon Steel 35K, these bolts deliver both the mechanical performance and the environmental durability demanded by the most challenging structural applications.
Product Overview
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Standard: DIN 603
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Common Name: High-Strength Carriage Bolt / Coach Bolt / Round Head Square Neck Bolt
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Material: Alloy Steel 10B21 (boron steel) or Carbon Steel 35K
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Property Class / Grade: 8.8 (per DIN EN ISO 898-1)
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Surface Treatment: Hot Dip Galvanized (HDG)
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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.
Why Grade 8.8? The Grade 8.8 variant delivers approximately double the tensile strength of the standard Grade 4.8 carriage bolt (800 MPa vs 420 MPa), making it the preferred choice for heavy structural timber connections, civil engineering joints, and load-bearing applications where higher clamping forces are required.
Why Hot Dip Galvanized? HDG provides a metallurgically bonded zinc coating of 45–85 µm — far thicker than electroplated alternatives — delivering decades of outdoor corrosion protection with self-healing cathodic protection properties. The combination of Grade 8.8 strength and HDG durability makes this the premium specification for permanent outdoor structural fastening.
Material Options
Option A — Alloy Steel 10B21 (Boron Steel)
10B21 is a boron-alloyed low-carbon steel widely used in high-volume structural fastener production. The boron addition (0.0005–0.003%) significantly improves hardenability, allowing Grade 8.8 mechanical properties to be achieved with excellent cold-forging characteristics and consistent lot-to-lot quality.
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Carbon Content: ~0.18–0.23% C
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Boron Addition: 0.0005–0.003% B — dramatically improves hardenability
- Superior hardenability vs plain carbon steel — consistent Grade 8.8 properties across large cross-sections
- Excellent cold-heading formability — ideal for high-volume production
- Low carbon content ensures good HDG zinc adhesion and minimizes risk of embrittlement during galvanizing
- Preferred material for structural OEM and Tier-1 supply chains
Option B — Carbon Steel 35K
35K is a medium-carbon steel offering a reliable balance of strength and toughness at an economical cost. Suitable for Grade 8.8 fasteners in general structural and civil engineering applications.
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Carbon Content: ~0.32–0.39% C (medium carbon)
- Good balance of tensile strength and ductility for Grade 8.8 requirements
- Cost-effective for high-volume structural fastener applications
- Suitable for non-critical structural joints and general heavy-duty assembly
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Note: Higher carbon content requires careful control of pickling and galvanizing process parameters to avoid hydrogen embrittlement and ensure coating adhesion
Mechanical Properties — Grade 8.8
| Property |
Specification |
Standard Reference |
| Material |
Alloy Steel 10B21 or Carbon Steel 35K |
DIN EN ISO 898-1 |
| Property Class |
8.8 |
DIN EN ISO 898-1 |
| Nominal Tensile Strength (Rm) |
≥ 800 MPa |
DIN EN ISO 898-1 |
| Proof Load Stress (Sp) |
600 MPa |
DIN EN ISO 898-1 |
| Yield Strength (Rp0.2) |
≥ 640 MPa |
DIN EN ISO 898-1 |
| Hardness (Vickers) |
250 – 320 HV |
DIN EN ISO 898-1 |
| Core Hardness (Rockwell) |
22 – 32 HRC |
DIN EN ISO 898-1 |
| Elongation after fracture |
≥ 12% |
DIN EN ISO 898-1 |
| Surface Treatment |
Hot Dip Galvanized (HDG) |
ISO 1461 / EN ISO 10684 |
| Zinc Coating Thickness |
≥ 45 µm (typical 50 – 85 µm) |
ISO 1461 |
| Corrosion Resistance (NSS) |
≥ 1,000 hours neutral salt spray |
ISO 9227 |
Grade Comparison: 4.8 vs 8.8 (HDG)
| Property |
Grade 4.8 HDG |
Grade 8.8 HDG (This Product) |
Improvement |
| Tensile Strength (Rm) |
≥ 420 MPa |
≥ 800 MPa |
+90% |
| Yield Strength (Rp0.2) |
≥ 340 MPa |
≥ 640 MPa |
+88% |
| Proof Load Stress (Sp) |
310 MPa |
600 MPa |
+94% |
| HDG Coating Thickness |
≥ 45 µm |
≥ 45 µm |
Equal |
| Outdoor Service Life |
20–50+ years |
20–50+ years |
Equal |
| Typical Use |
Light-to-medium structural timber |
Heavy structural, civil engineering, load-bearing |
— |
Hot Dip Galvanizing: Process & Performance
Hot dip galvanizing (HDG) is the most robust zinc coating process available for carbon and alloy steel fasteners. The steel is immersed in a bath of molten zinc at approximately 450°C, forming a series of zinc-iron alloy layers topped by a pure zinc outer layer — metallurgically bonded to the steel substrate, not merely adhered.
HDG Process Steps
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Degreasing: Removal of oils, grease, and organic contaminants
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Pickling: Acid cleaning to remove mill scale and rust — carefully controlled for Grade 8.8 to minimize hydrogen absorption
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Fluxing: Application of zinc ammonium chloride flux to prevent oxidation before galvanizing
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Galvanizing: Immersion in molten zinc bath at ~450°C for controlled dwell time
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Quenching & Inspection: Cooling, visual inspection, and coating thickness verification per ISO 1461
HDG Coating Structure
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Gamma layer (Γ): ~5 µm — zinc-iron alloy (innermost, highest iron content, hardest layer)
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Delta layer (δ): ~20 µm — zinc-iron alloy (intermediate, columnar structure)
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Zeta layer (ζ): ~15 µm — zinc-iron alloy (columnar crystals, visible spangle)
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Eta layer (η): ~10–20 µm — pure zinc (outermost, provides cathodic protection)
Key Performance Advantages
| Property |
Hot Dip Galvanized (HDG) |
Zinc CR3 Electroplate |
| Coating Thickness |
45 – 85 µm |
5 – 12 µm |
| Bond Type |
Metallurgical (alloy bond) |
Mechanical adhesion |
| Salt Spray Resistance |
≥ 1,000 hours |
72 – 120 hours (Grade 8.8) |
| Outdoor Service Life |
20 – 50+ years |
3 – 7 years (exposed) |
| Cathodic Protection |
✅ Yes — sacrificial zinc protects exposed steel |
Limited |
| Self-Healing |
✅ Yes — zinc oxide/carbonate forms at scratches |
❌ No |
| Thread Fit |
Requires oversize tapping (HDG nuts) |
Standard thread fit |
| Best For |
Outdoor, structural, heavy-duty, long-term |
Indoor, light-to-medium duty, precision fit |
⚠️ Important — Hydrogen Embrittlement in Grade 8.8 HDG: The pickling step in the HDG process can introduce hydrogen into high-strength steel. For Grade 8.8 fasteners (hardness 250–320 HV), FULLERKREG applies controlled pickling parameters and, where required, a post-pickling baking process to minimize hydrogen absorption before galvanizing. All Grade 8.8 HDG bolts are inspected per EN ISO 10684 requirements.
Dimensional Reference — DIN 603 Key Parameters
Note: Hot dip galvanizing adds 45–85 µm of coating per surface. Nuts must be tapped oversize (typically one tolerance class) to accommodate the coating thickness on threaded sections. Always specify HDG nuts when ordering.
| 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 20 mm to 300 mm depending on thread size. Custom lengths available upon request.
Typical Application Sectors
🏗️ Heavy Structural Timber & Civil Engineering
- Heavy timber frame and post-and-beam structural connections — permanent outdoor installations
- Glulam beam, LVL, and CLT structural joints in exposed or semi-exposed conditions
- Timber bridge decking, boardwalk, and pedestrian walkway structural fastening
- Retaining wall timber crib and sleeper connections under high lateral load
- Utility pole cross-arm and transmission tower timber connections
- Railway sleeper and track infrastructure structural fastening
🌳 Outdoor & Landscape Structures (Heavy-Duty)
- Heavy-duty decking, pergola, and outdoor pavilion structural connections
- Jetty, pontoon, and marine timber structure fastening (above waterline)
- Coastal boardwalk and promenade structural timber connections
- Heavy-duty playground and recreational equipment structural fastening
- Sports facility bleacher, grandstand, and spectator structure connections
- Outdoor signage and large-format display structure mounting
🌾 Agricultural & Rural Infrastructure
- Farm building and barn heavy structural connections
- Grain store, silo, and agricultural storage structure fastening
- Livestock pen, stable, and yard heavy fencing structural connections
- Irrigation system support structure fastening
- Agricultural machinery frame and implement structural connections
⚡ Industrial & Infrastructure
- Highway noise barrier and crash barrier structural joints
- Water treatment and wastewater facility structural fastening
- Mining and quarrying equipment structural connections
- Conveyor and material handling system heavy structural fastening
- Industrial storage rack and mezzanine floor structural connections
Key Features & Design Advantages
- 💪 High-Strength Grade 8.8 + Maximum Corrosion Protection: The premium combination — Grade 8.8 tensile strength (≥ 800 MPa) paired with HDG zinc coating (45–85 µm) for decades of outdoor structural performance. Neither strength nor durability is compromised.
- 🔒 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 even in difficult access conditions.
- ❤️ Cathodic (Sacrificial) Protection: Even if the zinc coating is scratched or damaged, the surrounding zinc acts as a sacrificial anode, protecting the exposed steel from rusting — critical for permanent structural joints that cannot be easily inspected or maintained.
- ⚙️ Metallurgical Bond: The HDG zinc-iron alloy layers are bonded at the atomic level — the coating cannot peel, flake, or delaminate under the high mechanical stresses generated by Grade 8.8 tightening torques.
- ✅ Dual Material Options: Alloy Steel 10B21 (boron steel, preferred for OEM and high-volume structural supply) or Carbon Steel 35K (cost-effective for general structural applications).
- 📏 Long Service Life: In rural atmospheric conditions, HDG Grade 8.8 fasteners typically provide 50+ years of maintenance-free structural service. In urban or industrial environments, 20–30 years is typical.
Installation Notes for Grade 8.8 HDG Carriage Bolts
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Nut and washer selection: Always use HDG nuts (tapped oversize) and HDG hardened washers with Grade 8.8 HDG bolts. Standard-tolerance nuts will not fit due to the additional coating thickness. Hardened washers (Grade 8 equivalent) are required under the nut to distribute the high clamping load.
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Verify substrate capacity: Grade 8.8 generates significantly higher clamping forces than Grade 4.8. Confirm that the timber species, metal thickness, or composite substrate is rated for the increased bearing stress at the bolt head and nut contact areas.
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Hole preparation: Drill clearance holes to the nominal bolt diameter. The HDG coating on the shank adds minimal bulk — standard clearance holes are generally acceptable.
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Square neck embedding: Insert the bolt from the front face and tap firmly with a mallet to seat the square neck. Grade 8.8 bolts require firm seating to prevent rotation under higher tightening torques.
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Tightening: Use a calibrated torque wrench. Tighten from the nut side only. Do not exceed the maximum recommended torque — Grade 8.8 fasteners are susceptible to torsional fracture if overtightened.
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Galvanic compatibility: When fastening aluminium or copper components, use isolation washers or sleeves to prevent galvanic corrosion between dissimilar metals.
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Periodic inspection: For permanent structural applications, inspect fastener condition and torque retention at regular intervals per the applicable structural design standard.
Recommended Tightening Torque — Grade 8.8
| Thread Size |
Tightening Torque (Nm) — Dry |
Tightening Torque (Nm) — Lubricated |
| M5 |
6.0 |
4.5 |
| M6 |
10.4 |
7.8 |
| M8 |
25 |
19 |
| M10 |
50 |
38 |
| M12 |
87 |
65 |
| M14 |
138 |
104 |
| M16 |
213 |
160 |
| M20 |
415 |
311 |
Torque values are indicative for Grade 8.8 bolts at 70% proof load. Use a calibrated torque wrench for all structural applications. Always verify with your structural engineer for safety-critical joints.
Why Choose FULLERKREG DIN 603 Grade 8.8 Hot Dip Galvanized?
- ✅ Full DIN 603, ISO 898-1 & ISO 1461 Compliance: Every batch manufactured to DIN 603 dimensional standards, Grade 8.8 mechanical requirements, and ISO 1461 hot dip galvanizing specifications.
- ✅ Premium Strength + Durability Combination: Grade 8.8 mechanical performance combined with HDG long-term corrosion protection — the definitive specification for permanent outdoor structural fastening.
- ✅ Hydrogen Embrittlement Controlled: Controlled pickling parameters and process management per EN ISO 10684 to minimize hydrogen absorption during the HDG process for Grade 8.8 fasteners.
- ✅ Dual Material Options: Alloy Steel 10B21 or Carbon Steel 35K — both meeting Grade 8.8 requirements.
- ✅ Full Traceability: Material certificates (MTC), mechanical test reports, hardness records, and ISO 1461 coating thickness inspection records available per shipment.
- ✅ Wide Size Range: M5 through M20, lengths from 20 mm to 300 mm.
- ✅ Matched HDG Nuts & Hardened Washers Available: Complete HDG Grade 8.8 fastener sets (bolt + nut + washer) available for simplified procurement.
- ✅ Flexible Packaging: Bulk, box, and pallet options available to suit construction site and project requirements.
Applicable Standards & References
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DIN 603 — Round head square neck bolts (metric coarse thread)
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DIN EN ISO 898-1 — Mechanical properties of fasteners: bolts, screws and studs (property class 8.8)
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ISO 1461 — Hot dip galvanized coatings on fabricated iron and steel articles — specifications and test methods
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EN ISO 10684 — Fasteners — hot dip galvanized coatings
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ISO 9227 — Corrosion tests in artificial atmospheres — neutral salt spray (NSS) test
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GB/T 700 / GB/T 3077 — Carbon and alloy structural steels (35K and 10B21 material standards)
Ordering & Customization
FULLERKREG supplies DIN 603 Grade 8.8 Hot Dip Galvanized carriage bolts in standard and custom configurations. Please specify the following when placing an inquiry:
- Thread diameter and pitch (e.g., M10 × 1.5, M12 × 1.75)
- Nominal length (e.g., 50 mm, 100 mm, 200 mm)
- Fully threaded or partially threaded shank preference
- Material preference: Alloy Steel 10B21 or Carbon Steel 35K
- Whether matched HDG nuts and hardened washers are required
- Required quantity and packaging format (bulk / box / pallet)
- Certification requirements (ISO 1461 coating reports, material certificates, mechanical test reports, hardness records)
Contact our technical sales team for datasheets, samples, and volume pricing.
FULLERKREG — High-Strength Fasteners Built for the Long Term. Grade 8.8 Strength. Hot Dip Galvanized Durability.
