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Certifications available: EN 10204 3.1 / 3.2 MTRs, NACE MR0175 compliance, Third-Party Inspection witness, ASME / ASTM / DIN / ISO / BS standards, and full export documentation packages.
Carriage Bolts
A world-class technical reference for EPC contractors, structural and mechanical engineers, procurement heads, TPI inspection agencies, and global project buyers specifying carriage bolts in timber connections, structural steel, heavy machinery, infrastructure, and industrial fabrication applications.
Geometry & Load
Characteristics
Carriage bolts — also designated as coach bolts or round-head square-neck bolts — are specialised fasteners characterised by a domed, smooth circular head and a square anti-rotation neck directly beneath the head. The square neck embeds into the material being fastened, preventing bolt rotation during nut tightening. They are specified in timber-to-timber, timber-to-steel, and steel-to-steel structural connections across construction, infrastructure, heavy machinery, and industrial fabrication projects.
1.1 — Technical Definition and Functional Role
A carriage bolt is a threaded fastener with a low-profile, dome-shaped (round) head, a square shank section (neck) immediately below the head, and a fully or partially threaded cylindrical shank terminating with a nut at the free end. The defining engineering feature is the square neck — dimensioned to fit tightly into a square hole or to embed into timber grain under the clamping force of the nut, creating a mechanical anti-rotation lock that eliminates the need for a wrench on the bolt head during assembly.
In EPC and heavy industrial contexts, carriage bolts appear in temporary and permanent structural connections: scaffolding systems, platform grating and checker plate attachments, timber framework in modular buildings and cooling towers, conveyor structure and materials handling equipment, marine and offshore deck timber installations, and bridge timber decking on infrastructure projects. Their primary engineering advantage is the flush, tamper-resistant head profile and the self-locking anti-rotation geometry.
RR Hydraulic manufactures and supplies carriage bolts to global EPC and infrastructure projects under all applicable international standards with full material traceability and EN 10204 3.1 / 3.2 documentation.
1.2 — Carriage Bolt Classification and Type Descriptions
Standard Carriage Bolt (Round Head Square Neck)
Dome head with full square neck per ASME B18.5, DIN 603, ISO 8677. Square neck dimensions exactly match hole size in timber or steel to prevent rotation. Thread: UNC (inch) or metric coarse. The most widely used type in timber and light structural applications.
Fin-Neck (Ribbed-Neck) Carriage Bolt
Round dome head with longitudinal ribs or fins on the neck instead of a square section. Fins embed into timber grain under load — preferred where pre-drilled holes are circular (not square). Provides superior pull-through resistance in soft-wood timber connections. Conforms to BS 4933 fin-neck variant.
Mushroom Head (Countersunk Carriage) Bolt
Low-profile mushroom head with square neck; head sits flush or near-flush with the surface. Used where a fully flush fastener head is required for safety, aesthetics, or material flow — conveyor belts, floor grating, sliding surfaces. Not suitable where a bearing face under the head is needed for load distribution.
Heavy Structural Carriage Bolt
Larger diameter (M20–M36 or ¾”–1½”), heavier gauge dome head, and deeper square neck for high-load timber beam connections, bridge decking, crane rail fastenings, and offshore deck timber. Typically supplied in ASTM A307 Grade A or high-strength A325 / A449 for structural applications.
Metric Mushroom Head Bolt (DIN 603)
The DIN 603 standard defines the mushroom head carriage bolt in metric series M5–M20. Widely used in European and Asian EPC projects, machinery fabrication, and modular structural steel. Dimensional system is ISO metric coarse thread; head profile and neck dimensions per DIN 603 table.
Stainless / Duplex Carriage Bolt
Carriage bolt in SS 304, SS 316, or Duplex 2205 for marine, coastal, chemical, and food-grade environments. Head profile and neck geometry identical to carbon steel variants; material selection governs corrosion performance. Used in offshore deck timber, marine pontoons, water treatment structures, and coastal infrastructure.
1.3 — Carriage Bolt vs Alternative Fastener Types: Engineering Selection
| Parameter | Carriage Bolt | Hex Bolt | Lag Screw / Coach Screw | Through-Bolt (Plain) | Engineering Implication |
|---|---|---|---|---|---|
| Head geometry | Dome (round), flush profile | Hexagonal — wrench required | Hex head, self-tapping thread | Any head; plain shank | Carriage: no head wrench access needed |
| Anti-rotation | Square neck — mechanical lock | Wrench on head during tightening | Thread engagement in timber | None — rotation possible | Carriage bolt: single-sided assembly possible |
| Timber application | Primary standard | Acceptable with washer | No pre-drill (or pilot hole) | Requires washer both sides | Carriage bolt preferred for timber connections |
| Steel-to-steel | Square hole required | Standard — round hole | Not applicable | Round hole — standard | Hex bolt preferred for steel-to-steel without timber |
| Tamper resistance | High — smooth dome head | Low — wrench access both sides | Moderate | Low | Carriage bolt preferred in public access areas |
| Load capacity | Moderate (shear + tension) | High (tension primary) | Moderate (withdrawal only) | High (shear + tension) | Heavy structural: hex bolt or high-strength carriage |
| Standard | ASME B18.5, DIN 603, ISO 8677 | ASME B18.2.1, DIN 931/933 | DIN 571, ASME B18.2.1 | ASME B18.8 | Verify standard against project specification |
1.4 — Structural Load Characteristics of Carriage Bolt Joints
Carriage bolts in structural timber and steel connections are subject to the following load conditions, which must be assessed in the connection design per applicable structural codes (AISC, Eurocode 5, AS 1720):
Shear (Lateral) Load
The primary load mode in timber connection design. Bolt shear capacity per NDS (National Design Specification) or Eurocode 5 depends on bolt diameter, timber species and density, embedment depth, and number of shear planes. Single-shear vs double-shear geometry determines the governing failure mode.
Withdrawal (Tension) Load
Axial tensile load pulling the bolt through the timber or pulling the nut through the washer face. Governed by washer size and bearing capacity of the timber species perpendicular to grain. Oversized washers are always required under the nut to distribute withdrawal load and prevent pull-through.
Combined Shear + Tension
Combined loading per Eurocode 5 interaction formula: (V/V_Rd)² + (N/N_Rd)² ≤ 1.0. Common in rafter-to-ridge timber connections, diagonal bracing, and purlin cleat bolting in industrial building frames and cooling towers.
Bearing Stress (Localised Crushing)
Timber bearing stress under the bolt shank at the hole face governs low-strength timber species. Engineering check: f_h = F / (d × t) ≤ f_h,k (characteristic embedding strength per timber grade). Bolt diameter and timber member thickness are the critical design variables.
Group Effect (Multiple Bolts in Row)
When multiple carriage bolts are arranged in a line parallel to load, group action reduces the effective capacity of each bolt. Eurocode 5 and NDS both apply a group reduction factor n_ef < n (number of bolts). Minimum bolt spacing and edge distances are mandatory per code.
Fatigue and Cyclic Loading
In platform grating, conveyor structures, and dynamic equipment bases, carriage bolts are subject to vibration-induced cyclic loading. Square neck embedment depth progressively reduces under cyclic shear — periodic inspection and retightening of nuts is required. Spring washers or prevailing-torque nuts (Nyloc) are specified to prevent vibration loosening.
1.5 — Square Neck Geometry: Anti-Rotation Mechanics
The square neck is the defining functional feature of the carriage bolt. Its geometry must be correctly matched to the receiving hole to ensure effective anti-rotation performance. Engineering requirements:
- Square neck diagonal: Must equal the bolt nominal diameter (d) such that when inserted into a pre-drilled circular hole of diameter d, the corners of the square neck dig into the hole wall as the nut is torqued
- Neck depth: Minimum neck depth (parallel to bolt axis) must be sufficient to prevent the neck from pulling through the timber under maximum nut torque — typically 0.6×d to 0.8×d per ASME B18.5 and DIN 603
- Hole tolerance in timber: Pilot hole diameter = d + 1 mm maximum overage; excess hole diameter reduces anti-rotation effectiveness and allows bolt rotation under nut torque
- Hole tolerance in steel: Pre-punched square holes (for steel-to-timber connections on steel gussets) must match square neck section within ±0.5 mm to maintain anti-rotation
- Head bearing: Dome head bears against the timber or washer face; head must not be recessed into a countersink for standard carriage bolt types — reduces pull-through resistance
k_mod = Modification factor for load duration and service class (0.6–1.1)
f_h,0,k = Characteristic embedding strength = 0.082 × ρ_k × d⁻⁰·³ (MPa) [Eurocode 5 Eq. 8.31]
ρ_k = Characteristic timber density (kg/m³) — e.g., 350 kg/m³ for C24 spruce
d = Bolt diameter (mm)
γ_M = Partial material factor = 1.3 (connections, Eurocode 5)
f_h,0,k = 0.082 × 350 × 16⁻⁰·³ = 0.082 × 350 × 0.4266 = 12.24 MPa
k_mod = 0.8 (medium-term load, SC2); γ_M = 1.3
f_h,0,d = 0.8 × 12.24 / 1.3 = 7.53 MPa
Single-shear load capacity (Johansen yield theory) governed by this embedding strength and member thicknesses.
1.6 — Design Safety Factors by Application Category
| Application Category | Safety Factor | Governing Code | Key Design Requirement |
|---|---|---|---|
| Timber structural connections | γ_M = 1.3 (EC5) | Eurocode 5 / NDS | Embedding strength + group effect check |
| Scaffolding and formwork | 3:1 to 4:1 (WLL to UTS) | EN 12811 / BS 5975 | Dynamic load factor; inspection every use cycle |
| Conveyor / platform grating | 3:1 | Project specification | Anti-vibration nut required; periodic retighten |
| Marine / offshore deck timber | 4:1 | DNV / project spec | Stainless or duplex grade; anti-galling lubricant |
| Bridge timber decking | 4:1 to 5:1 | AASHTO / BS 5400 | High-strength A325 or equivalent; HDG coating |
| Cooling tower timber frame | 3:1 | CTI (Cooling Technology Institute) | FRP or SS grade in chemical atmosphere zones |
| General structural steel | 3:1 to 4:1 | AISC / EN 1993 | Square hole preparation required in steel members |
| Outdoor public infrastructure | 4:1 | Local building code | HDG or SS grade; tamper-resistant dome head |
Submit your BOM, material specification, dimensional standard, and quantity for a fully documented RFQ within 24 hours.
Thread Systems
& Standards Compliance
Carriage bolt dimensions are governed by ASME B18.5 (inch series), DIN 603 (metric), ISO 8677 (metric mushroom head), and BS 4933 (inch / metric). Head diameter, dome height, square neck dimensions, shank diameter, thread length, and overall length are all standard-specific. All applicable standards are supported at RR Hydraulic with full certification capability.
Submit grade, standard, size, coating, and quantity to sales@rrhydraulics.com for a fully certified commercial offer.
2.1 — ASME B18.5 Carriage Bolt Dimensional Table (Inch Series)
The following table provides key dimensional data for round head square neck carriage bolts per ASME B18.5 — the governing standard for inch-series carriage bolts used in North American EPC, construction, and infrastructure projects. All dimensions are in inches.
| Nom. Dia. (in) | TPI (UNC) | Head Dia. Max (in) | Head Height Max (in) | Square Neck Width (in) | Square Neck Depth (in) | Thread Length Min (in) | Standard Length Range (in) |
|---|---|---|---|---|---|---|---|
| 3/16″ | 24 | 0.438 | 0.148 | 0.195 | 0.125 | 0.750 | ¾ – 3 |
| ¼” | 20 | 0.594 | 0.199 | 0.260 | 0.156 | 0.875 | ¾ – 6 |
| 5/16″ | 18 | 0.719 | 0.230 | 0.324 | 0.187 | 1.000 | ¾ – 8 |
| ⅜” | 16 | 0.844 | 0.279 | 0.387 | 0.219 | 1.125 | 1 – 10 |
| 7/16″ | 14 | 0.969 | 0.309 | 0.450 | 0.250 | 1.250 | 1¼ – 10 |
| ½” | 13 | 1.094 | 0.339 | 0.513 | 0.281 | 1.375 | 1¼ – 12 |
| ⅝” | 11 | 1.344 | 0.409 | 0.641 | 0.344 | 1.625 | 1½ – 16 |
| ¾” | 10 | 1.594 | 0.479 | 0.769 | 0.406 | 1.875 | 1½ – 20 |
| ⅞” | 9 | 1.844 | 0.545 | 0.895 | 0.469 | 2.000 | 2 – 20 |
| 1″ | 8 | 2.094 | 0.611 | 1.022 | 0.531 | 2.250 | 2 – 24 |
| 1¼” | 7 | 2.438 | 0.674 | 1.275 | 0.594 | 2.750 | 3 – 24 |
| 1½” | 6 | 2.906 | 0.802 | 1.525 | 0.656 | 3.250 | 3 – 24 |
2.2 — DIN 603 / ISO 8677 Metric Carriage Bolt Dimensional Table
The following table provides dimensional data for metric mushroom head square neck carriage bolts per DIN 603 (the metric carriage bolt standard most widely used in European and Asian EPC, construction, and machinery projects). All dimensions are in millimetres.
| Thread | Pitch (mm) | Head Dia. dk (mm) | Head Height k (mm) | Square Neck a (mm) | Square Neck c (mm) | Thread Length b (mm) | Standard Length Range (mm) |
|---|---|---|---|---|---|---|---|
| M5 | 0.80 | 14.0 | 2.9 | 5.5 | 3.5 | 16 | 16 – 80 |
| M6 | 1.00 | 16.0 | 3.3 | 6.6 | 4.0 | 18 | 16 – 120 |
| M8 | 1.25 | 21.0 | 4.3 | 8.7 | 5.0 | 22 | 20 – 160 |
| M10 | 1.50 | 26.0 | 5.2 | 10.8 | 6.0 | 26 | 25 – 200 |
| M12 | 1.75 | 30.0 | 6.0 | 13.0 | 7.0 | 30 | 30 – 260 |
| M14 | 2.00 | 35.0 | 7.0 | 15.1 | 8.0 | 34 | 40 – 260 |
| M16 | 2.00 | 40.0 | 8.0 | 17.2 | 9.0 | 38 | 40 – 300 |
| M20 | 2.50 | 50.0 | 10.0 | 21.3 | 11.0 | 46 | 50 – 300 |
| M24 | 3.00 | 60.0 | 11.5 | 25.5 | 13.0 | 54 | 60 – 360 |
| M30 | 3.50 | 72.0 | 14.0 | 31.6 | 16.0 | 66 | 80 – 400 |
| M36 | 4.00 | 84.0 | 16.5 | 37.8 | 19.0 | 78 | 100 – 400 |
2.3 — Applicable Standards and Compliance Framework
ASME B18.5
Round head bolts — inch series. The primary dimensional standard for carriage bolts in North American construction, EPC, and infrastructure projects. Defines head diameter, dome height, square neck dimensions, thread class (2A, UNC), and length tolerances. Companion nut standard: ASME B18.2.2.
DIN 603
Mushroom head square neck bolts — metric series M5–M36. The governing metric standard for carriage bolts in European and Asian EPC projects, machinery, and structural applications. Thread tolerance: 6g (bolt) / 6H (nut) per ISO 965. Widely cross-referenced with ISO 8677.
ISO 8677
Carriage bolts — metric series. International standard aligned with DIN 603 for metric mushroom head square neck bolts. Property class 4.6 and 8.8 per ISO 898-1. Used in international EPC projects where ISO-referenced project specifications are applied.
BS 4933
ISO metric black cup and countersunk head bolts for timber connections. The British Standard covering metric carriage (cup head) and mushroom head bolts with both square and ribbed (fin) neck variants. Used in UK construction, North Sea platform timber, and Commonwealth EPC projects.
ASTM A307
Low-carbon steel bolts and studs for general use. Grade A (structural) is the standard material specification for carbon steel carriage bolts in timber structural connections per NDS and IBC. Tensile strength: min 414 MPa (60 ksi). Suitable for general construction and non-critical structural connections.
ASTM A325 / A449
High-strength structural bolts. A325: medium-carbon steel, 830 MPa min UTS (for dia. ≤1″). A449: quenched and tempered bolts for general use — the equivalent for carriage bolt applications requiring higher shear and tensile capacity in heavy timber and bridge decking connections.
ISO 898-1
Mechanical properties of fasteners — bolts, screws, studs; metric series. Property classes 4.6 (low-carbon steel), 5.8 (medium-carbon), and 8.8 (alloy steel, Q&T) cover the full range of carriage bolt grades for metric supply. The metric equivalent to ASTM A307 / A325 material classifications.
EN 14399
High-strength structural bolting assemblies for preloading — applicable where carriage bolt equivalents in preloaded structural connections are specified. Covers HR (torque method) and HV (direct tension) assembly types. Referenced in Eurocode 3 (steel structures) and EN 1995 (timber structures) supplementary specifications.
NACE MR0175 / ISO 15156
Materials for H₂S sour environments. For carriage bolts in industrial chemical or oil field infrastructure where H₂S exposure is possible — hardness limit ≤22 HRC for carbon and alloy steel. Stainless steel, duplex, or controlled-hardness grades required. Rare application for carriage bolts but mandatory where specified.
2.4 — Thread System and Compatibility Reference
| Thread System | Standard | Size Range | Pitch / TPI | Tolerance Class | Governing Bolt Standard | Regional Application |
|---|---|---|---|---|---|---|
| UNC Coarse | ASME B1.1 | 3/16″ – 1½” | 6–24 TPI | 2A / 2B | ASME B18.5 | USA, Canada, global ASME |
| UNF Fine | ASME B1.1 | ¼” – ¾” | 12–28 TPI | 2A / 2B | Special order only | Precision machinery; rare in carriage bolts |
| Metric Coarse (ISO) | ISO 261 / DIN 13 | M5 – M36 | 0.80–4.0 mm | 6g / 6H | DIN 603 / ISO 8677 / BS 4933 | Europe, Asia, global metric EPC |
| BSW (Whitworth) | BS 84 | ⅛” – 1½” | 4–28 TPI | Class 1–3 | Legacy BS carriage bolts | UK legacy plant; Commonwealth projects |
| BA (British Association) | BS 57 | 0BA–16BA | Fine | BS 57 | Rare; instrumentation only | Legacy only; not used in structural carriage bolts |
2.5 — Torque Chart: Carriage Bolt Installation
Torque values below apply to carbon steel (Grade 4.6 / A307) and medium-high strength (Grade 8.8 / A449) carriage bolts with standard hex nut. Anti-rotation of the carriage bolt is provided by the square neck; torque is applied to the nut only. A washer must always be used under the nut to distribute bearing stress and prevent pull-through.
| Size | Thread | Grade / Class | Proof Load (kN) | Torque Dry (Nm) K=0.20 | Torque Oiled (Nm) K=0.15 | Torque Galv. (Nm) K=0.22 | Washer Min OD (mm) |
|---|---|---|---|---|---|---|---|
| M8 | M8×1.25 | 4.6 | 11.2 | 18 | 14 | 20 | 20 |
| M8 | M8×1.25 | 8.8 | 22.4 | 36 | 27 | 40 | 20 |
| M10 | M10×1.5 | 4.6 | 17.2 | 34 | 26 | 38 | 24 |
| M10 | M10×1.5 | 8.8 | 34.4 | 69 | 52 | 76 | 24 |
| M12 | M12×1.75 | 4.6 | 24.5 | 59 | 44 | 65 | 30 |
| M12 | M12×1.75 | 8.8 | 49.0 | 118 | 88 | 130 | 30 |
| M16 | M16×2.0 | 4.6 | 46.0 | 147 | 110 | 162 | 38 |
| M16 | M16×2.0 | 8.8 | 92.0 | 294 | 221 | 323 | 38 |
| M20 | M20×2.5 | 4.6 | 70.4 | 281 | 211 | 309 | 44 |
| M20 | M20×2.5 | 8.8 | 140.8 | 562 | 422 | 618 | 44 |
| M24 | M24×3.0 | 4.6 | 100.6 | 483 | 362 | 531 | 52 |
| M24 | M24×3.0 | 8.8 | 201.2 | 966 | 724 | 1063 | 52 |
| ½” UNC | ½”–13 | A307 Gr.A | 28.9 | 73 | 55 | 80 | 32 |
| ½” UNC | ½”–13 | A449 | 57.8 | 146 | 110 | 161 | 32 |
| ¾” UNC | ¾”–10 | A307 Gr.A | 65.8 | 249 | 187 | 274 | 44 |
| ¾” UNC | ¾”–10 | A449 | 131.6 | 498 | 374 | 547 | 44 |
| 1″ UNC | 1″–8 | A307 Gr.A | 116.0 | 587 | 440 | 646 | 54 |
| 1″ UNC | 1″–8 | A449 | 232.0 | 1174 | 880 | 1291 | 54 |
2.6 — Preload Calculation Guide
T = Applied nut torque (Nm)
K = Nut factor: 0.20 (dry/uncoated), 0.15 (oiled), 0.22 (HDG), 0.12 (PTFE/MoS₂)
d = Nominal bolt diameter (m)
Note for timber connections: Target preload for carriage bolts in timber should not exceed the timber bearing capacity — over-torquing crushes timber fibres and reduces effective shear area. In timber, specified torque is typically limited to 50–60% of the bolt proof load to protect the timber substrate.
d = 0.016 m, K = 0.15, Target preload @ 50% proof = 92.0 × 0.5 = 46.0 kN
Required torque T = 46,000 × 0.15 × 0.016 = 110.4 Nm
Table 2.D cross-check: M16 class 8.8 oiled = 221 Nm (at 100% proof load)
50% target → 221 × 0.50 = 110.5 Nm ✓ Consistent
Heat Treatment
& Manufacturing Process
Carriage bolt material selection is governed by structural load requirement, service environment (corrosion exposure, temperature), and project specification. RR Hydraulic manufactures carriage bolts in all standard and high-performance grades — from low-carbon A307 to stainless, duplex, and FRP — with full heat treatment documentation and EN 10204 material traceability.
3.1 — Material Grade Overview and Mechanical Properties
| Grade / Class | Spec | Base Material | UTS (MPa) | Yield / Proof (MPa) | Elong. (%) | Temp Range (°C) | Primary Use Case |
|---|---|---|---|---|---|---|---|
| Class 4.6 | ISO 898-1 / DIN 603 | Low-carbon steel | 400 | 240 | 22 | −20 to +300 | General timber connections, light structural |
| Class 5.8 | ISO 898-1 | Medium-carbon steel | 500 | 400 | 10 | −20 to +300 | Medium-load timber and steel connections |
| Class 8.8 | ISO 898-1 | Medium-carbon alloy Q&T | 800 | 640 | 12 | −20 to +300 | Structural connections, bridge decking, heavy machinery |
| A307 Gr.A | ASTM A307 | Low-carbon steel | 414–724 | — | 18 | −29 to +260 | General construction, NDS timber design |
| A449 (≤1″ dia) | ASTM A449 | Medium-carbon steel Q&T | 896 | 634 | 14 | −29 to +300 | Heavy timber, bridge decking, heavy structural |
| SS A2-70 (304) | ISO 3506 / DIN 267-11 | SS 304 / A2 | 700 | 450 | 20 | −196 to +300 | Light marine, food industry, mild chemical |
| SS A4-70 (316) | ISO 3506 / DIN 267-11 | SS 316 / A4 | 700 | 450 | 20 | −196 to +300 | Marine, offshore deck timber, coastal infrastructure |
| SS A4-80 (316) | ISO 3506 | SS 316 strain-hardened | 800 | 640 | 8 | −196 to +300 | High-load offshore deck, subsea timber, chemical plant |
| Duplex 2205 | UNS S31803 | Duplex SS | 620 | 450 | 25 | −50 to +315 | Offshore, seawater, high Cl⁻ environment |
| Hot Dip Galvanised (HDG) | ASTM A153 / ISO 1461 | CS base + Zn coat | Per base grade | Per base grade | Per base | −20 to +200 | Outdoor, construction, bridge, infrastructure |
3.2 — Yield Strength and Mechanical Properties by Heat Treatment
| Grade / Spec | Heat Treatment | UTS (MPa) | 0.2% Yield / Proof (MPa) | Elongation (%) | Hardness (HB) | Impact (J) @ Temp | Application |
|---|---|---|---|---|---|---|---|
| ISO 4.6 / A307 | As-rolled / normalised | 400–724 | 240 (4.6) | 18–22 | Max 250 | Not req. | General timber and light structural |
| ISO 5.8 | Cold-headed, no heat treat | 500 | 400 | 10 | Max 280 | Not req. | Medium construction; small sizes |
| ISO 8.8 (d ≤16mm) | Q&T or cold-work | 800 | 640 | 12 | 245–320 | Not req. | High-load timber and steel connections |
| ISO 8.8 (d >16mm) | Q&T — 840°C/600°C | 800 | 660 | 12 | 245–320 | Not req. | Heavy structural carriage bolts |
| ASTM A449 (d ≤1″) | Q&T | 896 | 634 | 14 | 255–321 | Not req. | Bridge decking, heavy timber frame |
| A4-70 (SS 316) | Solution annealed 1040°C | 700 | 450 | 20 | Max 220 | N/A (austenitic) | Marine, offshore, coastal deck timber |
| A4-80 (SS 316 SH) | Solution Ann. + strain hardened | 800 | 640 | 8 | 250–300 | N/A | Heavy offshore structural; subsea timber |
| Duplex 2205 | Solution annealed 1020–1100°C | 620 | 450 | 25 | Max 293 | N/A | Offshore, seawater, sour service |
3.3 — Corrosion Resistance by Material vs Service Media
| Material | Marine / Seawater | Treated Timber (CCA/ACQ) | Industrial Atmosphere | Chloride / Chemical | Freshwater / Potable | Outdoor / UV | Recommended Coating |
|---|---|---|---|---|---|---|---|
| Carbon Steel (bare) | Poor | Poor (CCA attacks CS) | Poor | Poor | Fair | Poor | HDG or epoxy coat mandatory |
| Carbon Steel (HDG) | Fair | Fair (Zn sacrificial) | Good | Fair | Good | Good | Standard for outdoor/construction |
| Carbon Steel (Geomet) | Good | Good | Very Good | Good | Good | Very Good | Preferred for treated timber, no thread issue |
| SS A2-70 (304) | Fair (pitting risk) | Very Good | Very Good | Poor (SCC risk) | Excellent | Excellent | Passivation only |
| SS A4-70 (316) | Very Good | Excellent | Excellent | Good | Excellent | Excellent | Passivation; standard marine grade |
| Duplex 2205 | Excellent | Excellent | Excellent | Excellent | Excellent | Excellent | Passivation; offshore structural |
| FRP (Fibreglass) | Excellent | Excellent | Excellent | Excellent | Excellent | Excellent | No coating req.; cooling tower preferred |
3.4 — Manufacturing Process
3.4.1 — Cold Heading (Cold Forming) of Dome Head
Carriage bolt dome heads are manufactured by cold heading (cold forging) from bar or wire stock. The cold heading process forms the dome head and square neck simultaneously in a multi-stage die set, producing a continuous grain flow from shank through neck and into the head — providing superior tensile and fatigue properties compared to machined heads. Square neck accuracy is critical: the neck width-across-flats must match the nominal bolt diameter within the tight tolerance of the applicable standard to ensure effective anti-rotation performance.
3.4.2 — Thread Rolling vs Thread Cutting
- Thread rolling: Cold-forming process; produces compressive residual stress at thread root; grain flow continuous — superior fatigue life. Standard for ISO 8.8 and high-strength carriage bolts. Thread root Ra ≤ 1.6 µm achievable.
- Thread cutting: Machining process; interrupts grain flow; tensile residual stress at root possible. Used for larger diameters (M24 and above) and exotic material grades (duplex, stainless) where rolling is impractical.
- Thread class tolerance: 6g (metric, ISO 965) and 2A (UNC, ASME B1.1) are standard. Tighter 5g/3A on special order for precision-loaded joints.
3.4.3 — CNC Machining Tolerances
- Head diameter: +0.0 / −0.8 mm for metric sizes per DIN 603; ±3% for inch sizes per ASME B18.5
- Square neck width: +0.0 / −0.3 mm (metric DIN 603); tight fit to pre-drilled pilot hole is mandatory
- Square neck depth (parallel to shank): ±0.3 mm per standard — shorter neck reduces anti-rotation effectiveness
- Dome head height: ±0.3 mm (metric); ±0.4 mm (inch ASME B18.5)
- Length tolerance: +2d / −0 for lengths up to 150 mm; +3 mm / −0 for lengths above 150 mm (DIN 603)
- Thread straightness (shank bow): max 1 mm per 100 mm of shank length
- Surface finish — shank: Ra ≤ 3.2 µm; thread form: Ra ≤ 1.6 µm; dome head bearing face: Ra ≤ 3.2 µm
3.5 — Surface Finish Options
| Finish | Specification | Coating Thickness (µm) | Salt Spray (h) | Treated Timber Compatible? | Max Service Temp (°C) | Nut Oversize Req.? | Typical Application |
|---|---|---|---|---|---|---|---|
| Plain / Black | As-manufactured | — | <24 | No | 300 | No | Interior, dry, immediate assembly |
| Black Oxide | MIL-DTL-13924 | 1–2 | 24–48 | No | 150 | No | Indoor machinery, short-term |
| Zinc Electroplate | ASTM B633 Type II | 5–25 | 96–200 | No — not for treated timber | 120 | No | Dry interior structural only |
| Hot Dip Galvanise (HDG) | ASTM A153 / ISO 1461 | 45–85 | 2000+ | Yes — minimum 45µm | 200 | Yes — oversize nut required | Outdoor construction, treated timber, bridge |
| Geomet / Dacromet | ISO 10683 | 8–12 | 1000–1500 | Yes | 300 | No | Treated timber, outdoor, no thread build-up |
| Epoxy Coat | Proprietary / ASTM D3359 | 60–150 | 2000+ | Yes | 120 | No (applied after threading) | Chemical environments, water treatment |
| PTFE / Teflon | Proprietary | 20–50 | 1000+ | Yes | 260 | No | Anti-galling for SS carriage bolts; easy assembly |
| Passivation (SS only) | ASTM A380 / A967 | Passive layer | 500–2000+ | Excellent | Per alloy grade | No | SS A2/A4, Duplex carriage bolts |
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from raw material heat to final packed shipment on all carriage bolt orders. Dimensional inspection reports, EN 10204 3.1 / 3.2 MTRs, NDT reports, surface finish certification, and complete EPC export documentation packages are provided as standard on project-grade supply.
4.1 — Inspection & NDT Protocol
4.2 — EN 10204 Material Test Certificate Requirements
| Certificate | Content | Signatory | Standard Requirement | When Mandatory |
|---|---|---|---|---|
| 2.1 | Conformity declaration only | Manufacturer | Non-critical general supply | General commercial procurement |
| 2.2 | Non-specific test results; not heat-traceable | Manufacturer | Low-criticality structural | Basic project supply; not accepted for EPC O&G |
| 3.1 | Heat-traceable mech. + chem. results; specific to production lot | Manufacturer’s authorised QC Inspector | Minimum for EPC and infrastructure projects | All structural, offshore, and bridge supply |
| 3.2 | 3.1 + countersigned by independent TPI (SGS / BV / DNV / Lloyds) | Manufacturer + TPI | Critical structural, offshore, NACE service | Offshore deck, bridge critical connections, sour service |
4.3 — Applications by Industry
Timber Frame Structures (EPC & Commercial)
Carriage bolts are the primary through-bolt fastener in glulam, LVL (Laminated Veneer Lumber), and solid timber beam connections in EPC module buildings, warehouses, and industrial structures. ASTM A307 Grade A or ISO Class 8.8; HDG or Geomet coating for treated timber. Connection design per NDS (USA) or Eurocode 5 (Europe).
Bridge and Infrastructure Timber Decking
Timber decking planks on pedestrian bridges, access platforms, and vehicle ramps are fastened with large-diameter (¾”–1½” or M20–M36) high-strength carriage bolts in A449 or ISO 8.8, HDG-coated. Dome head flush profile prevents trip hazard. Minimum 5:1 safety factor; periodic inspection and retorquing mandatory per bridge maintenance schedule.
Offshore & Marine Deck Timber
Timber grating on offshore platform decks, walkways, and helideck surrounds fastened with SS A4-70 or Duplex 2205 carriage bolts. Marine atmosphere and intermittent seawater wash require full SS specification — HDG is not acceptable in continuous seawater zone. Anti-galling PTFE lubricant mandatory for SS-to-SS installation.
Cooling Tower Timber Framework
Cooling tower structural timber frames use carriage bolts in CCA/ACQ-treated timber members. Minimum specification: HDG at 85µm or Geomet-coated. FRP (Fibreglass Reinforced Plastic) carriage bolts are specified in chemical dosing zones where even HDG is attacked by biocide or chlorine-based water treatment chemicals.
Conveyor & Materials Handling Equipment
Platform grating, walkway timber, and equipment base timber blocks on conveyor structures are fastened with carriage bolts in ISO Class 8.8, HDG or Geomet-coated. Vibration service requires spring washers (DIN 127) or Nyloc prevailing-torque nuts under the washer to prevent vibration loosening. Periodic retightening inspection specified in maintenance schedule.
Scaffolding & Temporary Works
Proprietary and system scaffolding timber decking boards are fastened with carriage bolts per EN 12811 / BS 5975. Dynamic load factor of 1.5 applied to design loads. Reuse criteria: visual inspection of dome head, neck, and thread for deformation or corrosion before each installation cycle. Bolts with deformed square necks or visible corrosion must be discarded.
4.4 — Export Packaging Specification
- Individual carriage bolts wrapped in VCI (Volatile Corrosion Inhibitor) poly film for plain and zinc-coated grades — prevents atmospheric corrosion during ocean freight and site storage up to 24 months
- Thread end protection: plastic thread protector caps on all bolt ends; dome head profile protection sleeve for HDG and coated grades where coating integrity during transit is critical
- Bundle identification: each bundle tagged with heat number, material grade / property class, standard (DIN 603 / ASME B18.5), nominal size, length, surface finish, quantity, and PO reference — full traceability from manufacturer to site installation
- Segregation: bolts of different dimensions, grades, and coating types in separate compartments / polybags within the same shipping box — prevents mix-up during goods receipt
- HDG carriage bolts: packed with companion oversize nuts and structural washers in the same bundle where specified — ensures matching coating and correct nut fit at site
- Outer crating: ISPM-15 heat-treated pine wooden crates for all international export — fumigation certificate available on request for specific destination countries
- Crate markings: Project PO, item tag, grade / finish, gross weight, net weight, dimensions (L×W×H mm), country of origin, handling symbols per ASTM D5276 / ISO 780
- Documentation: packing list enclosed inside crate lid + external waterproof pocket; cross-referenced to MTC certificate numbers and inspection release note
4.5 — Complete EPC Project Documentation Package
| # | Document | Standard / Format | Mandatory / Conditional | Notes |
|---|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | Mandatory — all structural supply | Heat-traceable; lot-specific |
| 02 | Chemical Composition Report | Heat / lot certified lab analysis | Mandatory (within MTC) | All alloying elements per grade spec |
| 03 | Mechanical Properties Report | UTS, proof load, elongation, hardness | Mandatory (within MTC) | Per ISO 898-1 / ASTM A307 / A449 |
| 04 | Hardness Test Report | ASTM E10 / E18 / E92 | Mandatory — Grade 8.8 / A449 / NACE | Brinell + Rockwell / Vickers per grade |
| 05 | Dimensional Inspection Report | Per DIN 603 / ASME B18.5 / ISO 8677 | Mandatory | Thread gauge + all head / neck geometry |
| 06 | Thread Gauge Report | Go / No-Go per ISO 965 / ASME B1.1 | Mandatory | 100% gauged; results per lot |
| 07 | Surface Finish / Coating Certificate | ASTM A153 / ISO 1461 / ISO 10683 | Mandatory — all coated supply | HDG thickness + salt spray results |
| 08 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new items / first batch | Released before batch production |
| 09 | MPI / LPI Report | ASTM E709 / E165; EN ISO 9934 / 3452 | Conditional — structural, offshore, NACE | Head-neck junction primary zone |
| 10 | PMI Report (XRF) | Per lot — SS and duplex grades | Mandatory — SS / duplex supply | 100% SS A2 vs A4 differentiation |
| 11 | Pull-Through Test Report | Destructive sample lot test | Conditional — bridge / offshore structural | Confirms head cold-heading quality |
| 12 | TPI Witness Certificate | SGS / BV / DNV / Lloyds countersigned | Mandatory — EN 10204 3.2 orders | Co-witness at manufacturer works |
| 13 | Treated Timber Compatibility Statement | Coating compliance declaration | Conditional — treated timber projects | Confirms HDG / Geomet / SS grade for CCA/ACQ |
| 14 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC / infrastructure | Current scope covers fastener manufacture |
| 15 | Country of Origin Certificate | Chamber of Commerce issued | Mandatory — all export | Required for customs and duty classification |
| 16 | Packing List | Item-level detail per shipment | Mandatory | Cross-references MTC serial numbers |
| 17 | Commercial Invoice | Per INCOTERMS 2020 | Mandatory | Includes HS code for customs clearance |
| 18 | Bill of Lading / Air Waybill | Per freight mode | Mandatory | Issued by freight forwarder |
| 19 | MSDS / Safety Data Sheet | For coated / treated surfaces | Conditional — HDG, Geomet, epoxy, PTFE | Required for hazardous material shipping |
4.6 — ISO and Quality System Compliance
ISO 9001:2015
Quality Management System covering the full manufacturing cycle: raw material supplier qualification, in-process dimensional and surface finish inspection, heat treatment verification, coating thickness QC, final inspection release, and full material traceability documentation. Mandatory for EPC and structural project procurement qualification.
ISO 10474
Steel and steel products — inspection documents. The source framework for EN 10204 certificate type definitions. Some legacy EPC project specifications reference ISO 10474 — maps to EN 10204 3.1 (type 3.1.B) and 3.2 (type 3.1.C). Current operative standard is EN 10204.
ISO 9606
Qualification testing of welders. Relevant where carriage bolts are part of a hybrid structural assembly involving welded connections in the same joint — weld quality and bolt mechanical fastening are co-designed. All welders engaged on such fabrication hold current ISO 9606-1 certification.
ISO 4413
Safety requirements for hydraulic fluid power systems. Indirectly relevant where carriage bolts are used in structural connections on hydraulic power unit (HPU) base frames, skids, and hydraulic equipment mounting structures. Structural bolt specification on HPU frames must meet project-specific engineering criteria consistent with ISO 4413 safety management framework.
Submit your BOM, material specification, standard, coating, and quantity to RR Hydraulic for a complete, certified commercial offer.