Carbon Steel Carriage Bolts — Factory Direct Supplier | Huaxia-Steel

The carriage bolt — round domed head, square neck under the head, blunt threaded end — is one of the oldest and most widely used fasteners in steel-to-wood and steel-to-steel construction. The square neck locks into the hole in timber or into a punched square hole in steel, so the bolt cannot spin while the nut is torqued; the domed head leaves no protruding shank to snag people, livestock or cargo. As a factory-direct supplier we manufacture carriage bolts (also called coach bolts) in full size ranges from M6 to M24 and 1/4″ to 1″, in multiple grades and finishes, with export packaging and full documentation.

Product Overview

Item	Specification
Product name	Carbon steel carriage bolt / coach bolt (round head square neck)
Standards	ASME/ANSI B18.5 (inch); DIN 603 / ISO 8677 (metric mushroom head); GB/T 12
Diameter range	M6 – M24; 1/4″ – 1″
Length range	20 mm – 500 mm; 3/4″ – 20″
Thread	Coarse (UNC/ISO metric); partial thread per standard, full thread on request
Grades	Gr 2 / A307 Gr A (60 ksi), Gr 5, A325 type 1 option; class 4.6 / 8.8 metric
Finishes	Zinc plated (RoHS white/blue), hot-dip galvanized ASTM A153, black oxide, plain/self-color
Head height	Per B18.5 Table (e.g. 1/2″ bolt: head dia 0.812–0.875″, height 0.323–0.361″)
Matching nut	Hex nut per ASME B18.2.2 / DIN 934, same grade and finish

Dimensional Key Points (ASME B18.5, common sizes)

Nominal size	Head dia (in)	Head height (in)	Square neck (in)
1/4″	0.437–0.469	0.156–0.176	0.245–0.260
3/8″	0.594–0.625	0.218–0.242	0.364–0.386
1/2″	0.812–0.875	0.323–0.361	0.495–0.520
5/8″	0.969–1.031	0.405–0.442	0.622–0.648
3/4″	1.156–1.219	0.490–0.533	0.745–0.771
1″	1.531–1.625	0.660–0.707	0.990–1.019

Available Grades and Mechanical Properties

Grade / standard	Tensile strength	Yield (min)	Typical use
SAE J429 Gr 2 / class 4.6-4.8	74 ksi (510 MPa)	55 ksi (380 MPa)	General wood/steel construction
ASTM A307 Gr A	60 ksi (414 MPa)	—	Structural timbers, fence, trailer
SAE J429 Gr 5 / class 8.8	120 ksi (830 MPa)	92 ksi (635 MPa)	Higher-load connections, machinery
ASTM A325 Type 1	120 ksi (830 MPa)	85 ksi (586 MPa)	Heavy structural steel joints (carriage style specials)

Manufacturing Standards and Process

Carriage bolts are cold headed on bolt formers from wire rod (SAE 1018/1022 for Gr 2, 1038 or boron steel for Gr 5), the square neck and round head formed in the heading dies, threads rolled for fatigue strength, then heat treated where the grade requires (Gr 5 quenched and tempered) and finished by plating or hot-dip galvanizing. Our process includes: raw wire chemistry verification per heat, heading die maintenance checks for neck squareness, thread roll profile control, and for galvanized lots, plating thickness verification to ASTM A153 and re-tapping of threads ( HDG adds 4-10 μm buildup that affects nut fit). Metric production follows DIN 603 with the same controls.

Applications

• Timber construction: beams, posts, joist hangers, barn and pole buildings.
• Fencing, guardrail and livestock equipment (snag-free head).
• Truck trailers, flatbeds and container securing points.
• Solar mounting structures and agricultural machinery.
• Playground equipment and outdoor furniture (rounded head for safety).
• Steel grating and channel framing with punched square holes.

Quality Assurance

Every shipment includes an inspection report and material certificate covering: chemistry of the source wire, mechanical tests (tensile, wedge proof where grade applies, hardness), dimensional sampling per ASME B18.18 / ISO 3269 sampling plans, plating thickness (magnetic gauge) and salt-spray results on request (zinc clear 48-72 h; HDG per ASTM B117 expectations), and thread gauge fit verification. Third-party inspection (SGS/BV) can witness loading. Common failure modes we design out: under-height heads that pull through timber, undersized square necks that spin in the hole, and hydrogen embrittlement on plated high-strength bolts — controlled by post-plate baking for Gr 5.

Packaging and Shipping

Bolts are packed in 25 kg cartons or bulk bags, with nuts and washers optionally included in matched kits; cartons palletized and stretch-wrapped, each pallet labeled with grade, size, finish and quantity. Mixed-size orders (a full container of assorted carriage bolt SKUs) are a specialty — we pack per your picking list. Export terms FOB/CFR/CIF; standard lead time 3-5 weeks for production, faster for stock sizes.

Finish Selection Guide

Environment	Recommended finish	Expected outdoor life
Indoor dry	Zinc plated clear (RoHS)	Not applicable — indoor duty
Outdoor rural/urban	Hot-dip galvanized ASTM A153 (min 50 μm)	20–40 years
Coastal / industrial	HDG + paint duplex system	15–25 years with maintenance
Temporary / cost-driven	Zinc plated yellow or black oxide + oil	Months, protected storage

Installation Notes

Three installation details decide whether a carriage bolt joint performs to spec. First, hole fit: drill timber holes to nominal bolt diameter (a 1/2″ bolt wants a 1/2″ hole); oversize holes let the square neck spin and chew the wood. Second, neck seat: the square neck must fully bottom into the hole or material before torque is applied — otherwise the head spins while the neck hangs, rounding the hole. Third, torque discipline: for 1/2″ A307 bolts, hand-tight plus a quarter turn is usually sufficient for timber (crushing governs); in steel-to-steel joints follow the grade’s torque table — over-torquing Grade 2 bolts past yield is the most common site failure. For hot-dip galvanized assemblies, re-check nut tightness after the first week in service: HDG coatings embed slightly under load, relaxing initial tension.

Frequently Asked Questions

What is the difference between DIN 603 and ASME B18.5 carriage bolts?

Head geometry and neck size differ: DIN 603 (mushroom head) has a flatter, wider head and metric dimensions; B18.5 (round head) has a taller dome with inch dimensions and defined neck square. They are not interchangeable in stamped holes designed to one standard — confirm which drawing your project uses before ordering.

Can carriage bolts be used in structural steel connections?

Only where the design is non-critical — A307 carriage bolts are not permitted in primary structural steel connections governed by AISC; those require A325/F3125 hex-head structural bolts. Carriage bolts do serve in secondary steel framing, solar structures and equipment brackets.

Do you supply matching nuts and washers?

Yes — hex nuts and round/flat washers in matching grade and finish, either loose or pre-packed with bolts in job kits. Kits with one bolt, nut and washer per bag are available for retail and solar programs.

What is your minimum order for mixed-size programs?

Production minimum is one ton per size line, but mixed-size consolidation programs for distributors start at one 20 ft container with no per-SKU minimum — the container total is what matters. We pick to your SKU list, bag or box per your retail format if required, and palletize by destination store or DC, with a packing declaration and barcode labels generated from your spreadsheet format.

Why Buy From Us

• True factory-direct manufacturing: heading, threading, heat treatment and plating under one quality system.
• Full grade portfolio Gr 2/A307 to Gr 5/8.8 with matching nuts and washers.
• Both inch (B18.5) and metric (DIN 603) tooling in-house.
• Hot-dip galvanized lots with ASTM A153 thickness certificates.
• Mixed-SKU container programs for hardware distributors.

How Carriage Bolts Are Tested Before Shipment

Bolt quality fails in ways that are invisible in a carton, so our inspection plan targets the physics of the joint:

Test	Method / standard	Acceptance
Tensile & proof load	ASME B18.2.1 / ISO 898-1 wedge test	Per grade table above
Hardness	Rockwell on head/shank	Grade range (Gr 2: B80-100; Gr 5: 25-34 HRC)
Dimensional	Calipers, gauges per ASME B18.18 sampling	Head dia, height, neck square, thread fit
Plating thickness	Magnetic/X-ray gauge	ASTM A153 / ASTM B633 class
Hydrogen embrittlement control	Post-plate bake (Gr 5) + delayed fracture sample test	No cracking under sustained load
Thread fit	GO/NO-GO ring and plug gauges	2A/6g class fit

The wedge tensile test is the one that catches real-world problems: it pulls the bolt at an angle to expose head-to-shank weakness — exactly the stress path when a timber connection is loaded off-axis. A bolt that passes a straight tensile pull but fails the wedge test has a weak head forging, and it will fail on site in wind-loaded or vibration-loaded joints. For buyers purchasing by grade, requiring wedge test records per lot is the single highest-value line item in a bolt specification.

Raw Material and Grade Chemistry Reference

Carriage bolt performance starts at the wire rod. Our grade-to-raw-material mapping:

Bolt grade	Wire rod grade	Key chemistry (%C)	Heat treatment
Gr 2 / 4.6-4.8 / A307	SAE 1018–1022	0.15–0.22	None (cold drawn)
Gr 5 / 8.8	SAE 1038 / 10B21	0.35–0.40 (B: 0.0005–0.003)	Quench & temper, 425 °C min temper
A325 Type 1 specials	4140 / 10B33	0.33–0.40	Q&T per ASTM A325

Boron steel (10B21) deserves a note: it delivers Grade 5 strength with lower carbon, improving cold-heading die life and lowering cost — but boron effectiveness depends on precise titanium/nitrogen control, so we buy boron wire only from qualified mills with per-heat boron recovery data. For buyers in hot climates, we also flag tempering temperature: Grade 5 bolts tempered below 425 °C meet hardness but risk delayed brittle failure in service, so our standard is temper at 425-500 °C even where a customer’s spec is silent.

Metric DIN 603 Reference Sizes

For European-specification projects, our DIN 603 production covers the full standard size list. Common export sizes with key head dimensions:

Thread	Head dia max (mm)	Head height (mm)	Neck square (mm)
M6	13.5	3.4–4.0	5.06–5.68
M8	17.0	4.2–4.8	6.76–7.42
M10	21.0	5.0–5.8	8.76–9.42
M12	24.0	6.0–6.8	10.73–11.47
M16	30.0	7.8–8.8	14.73–15.47
M20	37.0	9.6–10.8	18.60–19.54

Lengths run from 16 mm to 500 mm with thread lengths per DIN 603 (roughly 2×d + 6 mm, longer threads available on request). Zinc flake finishes (geometry-protected, hydrogen-free) are available as an alternative to electroplate on high-strength metric sizes — worth specifying on automotive-adjacent programs where hydrogen embrittlement clauses apply.

Need carriage bolts in bulk, galvanized or plain? Send your size list, grade and quantity for a factory-direct quotation within 24 hours — samples and dimensional drawings available on request.