Carriage bolts (round head, square neck) · Inch or metric standard, property class or grade, material and finish, and how length is measured
A carriage bolt has a smooth domed head with a square neck underneath; the neck pulls into the hole so the bolt does not turn while the nut is tightened from the other side. Fix three things before asking for a price: the dimensional standard (inch ASME B18.5 or metric DIN 603 / ISO 8677) and size, the strength grade or property class with its material, and the finish for the environment.
Round head square neck carriage bolts (coach bolts) in carbon and stainless steel, inch and metric. Hex bolts, flange bolts, lag screws, elevator bolts, and square-neck bolts with other head forms are outside this page; hex and flange bolts have their own pages.
CNFX editorial research · Sources checked October 11, 2026 · 10 sources
ASME B18.5 covers data for the various types of inch series bolts classified as round head bolts. [2] For metric bolts, DIN 603 is the standard for mushroom head square neck bolts, listed with ISO 8677, CSN 021319, PN 82406 and UNI 5731 as equivalent norms. [3]
Linked makers list carriage bolts to ANSI/ASME B18.5 and DIN 603 [4], DIN 603 bolts from M5 to M16 [6], carriage bolts from M5 to M20 [5], and production to DIN 603, ISO 8677 and Chinese GB/T 12. [7] One also lists square neck bolts to DIN 608. [9]
Shuifu Hanyang notes that carriage bolt length is measured from under the head, including the square neck, not from under the neck. [10]
Editorial note: ask the supplier to quote to one named standard and to send the head diameter, head height and neck dimensions for your size so the square hole can be cut to match.
Give the supplier the standard (ASME B18.5, DIN 603, ISO 8677 or other), diameter and thread pitch, length measured from under the head, thread length, grade or property class, material, finish and coating thickness, matching nut and washer, packaging and quantity.
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Set grade, material and finish
ASTM A307 covers carbon steel bolts, studs and threaded rod; its Grade A has a minimum tensile strength of 60 ksi for general applications, and Grade B a tensile strength of 60 to 100 ksi for flanged joints in piping systems with cast iron flanges. When tested, specimens must meet the specified chemistry, hardness, tensile strength, yield point, elongation and dimensions. [1]
Gangtong Zheli lists DIN 603 carriage bolts in property class 4.8 (Q235 carbon steel, zinc plated or black), 8.8 (medium carbon steel, black or hot-dip galvanized) and A2 (304 stainless steel, natural finish). [6] Haiyan lists A307 Grade A and A2-304 and A4-316 stainless grades, with plain, zinc plated, black oxide and hot-dip galvanized finishes. [4]
Fastenwell lists Grade 5 and Grade 8 carriage bolts with black, zinc plated and hot-dip galvanized finishes and gives 120,000 psi tensile strength for a Grade 5 carbon steel bolt. [5] Sinsun lists Grade 2 and heat-treated Grade 5 with electro-galvanized, hot-dip galvanized, black oxide and plain finishes. [7] Other listings include Dacromet-coated and blue-white zinc plated DIN 603 bolts. [8][9]
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Compare what suppliers offer
Suppliers linked to this page list carriage bolts; each card links to the page reviewed.
The listings reviewed cover inch and metric carriage bolts in carbon and stainless steel, from class 4.8 to 8.8 and Grade 2 to Grade 8, with zinc, black, hot-dip galvanized and Dacromet finishes. Ask each for the standard, property class marking, test certificate and coating thickness. [4][5][6][7][8][9][10]
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Trace what each feature does
This is a functional reference built from the sources cited; it is not one maker's specification.
Fill the specification fields with values for your joint
A carriage bolt quotation needs values for your joint. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
Field
What it means and published values
Ask the supplier for
Applicable Standard
ASME B18.5; DIN 603 / ISO 8677; GB/T 12. [2][3][7]
Standard and edition.
Nominal Size / Range
M5–M20 listed; length from under the head. [5][10]
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
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Compiled and source-reviewed by the CNFX editorial team.
Published product pages show what a company offers, not tested performance. Confirm the legal seller, maker and certificates for your order. How we source and review
· Page last updated 13 October 2026
Buyer questions
What standard covers metric carriage bolts?
DIN 603, with ISO 8677 listed as an equivalent. [3]
What standard covers inch carriage bolts?
ASME B18.5 covers inch series round head bolts. [2]
How strong is an A307 carriage bolt?
ASTM A307 Grade A has a minimum tensile strength of 60 ksi. [1]
How is carriage bolt length measured?
From under the head, including the square neck. [10]
Sources and what still needs a direct check
Sources checked 2026-10-11. ASTM A307 and ASME B18.5 summary pages and the DIN 603 directory entry set the standards; the rest are linked suppliers.
