Carriage Bolt
Manufacturer
India · Mumbai
What Makes a Carriage Bolt — Mushroom Head + Square Neck Explained
A carriage bolt (also called a coach bolt in British and Australian engineering) is defined by two features that work together: a smooth, domed (mushroom) head with no drive socket or wrench flats, and a square or ribbed neck immediately below the head that bites into the substrate material as the nut is tightened. Together, these two features allow single-sided tightening — the nut is tightened with a wrench on the accessible side while the bolt head pulls flush to the substrate surface, with the square neck preventing rotation without any tool on the bolt head side.
This geometry makes carriage bolts the correct specification for connections where: (1) the bolt head side is inaccessible after assembly — timber framing, FRP grating, composite panels; (2) a smooth tamper-resistant head face is required — marine deck fixtures, public furniture, walkway grating; (3) the substrate material can deform around the square neck — wood, rubber, soft composite, PTFE-backed surfaces.
In exotic alloy service — SS 316L, Hastelloy C276, Monel K500 — carriage bolts are specified for FRP grating connections in chemical plant walkways and platforms, rubber-lined pipe support connections, PTFE-backed flange cover plates, composite panel connections in offshore platforms, and marine deck timber-to-steel composite connections where a smooth head and one-sided tightening are required.
Carriage Bolt Types — DIN 603 and Variants
Standard Mushroom Head Square Neck
The primary DIN carriage bolt form — smooth dome head with square neck that embeds into substrate on tightening. ISO 8677 is the ISO equivalent. Used in all wood, FRP grating, composite panel, and rubber-lined substrate connections requiring tamper-resistant smooth head and single-sided tightening. All materials from carbon steel through exotic alloy. Most widely stocked variant.
American Coach Bolt (ASME B18.5)
ASME B18.5 is the American dimensional standard for carriage bolts — inch UNC thread, slightly different head and neck proportions from DIN 603. Specified on US-origin engineering drawings, AISC structural timber connections, dock and marine construction, and American process industry applications. Available in carbon steel, SS 304/316L, and exotic alloys in inch UNC/UNF thread sizes.
Ribbed Neck Carriage Bolt
A variant with a ribbed or knurled neck instead of the square cross-section — the ribs grip soft substrates (rubber, PTFE, soft plastic) more effectively than a square neck that requires a specific square hole. Used in rubber-lined pipe support and equipment connections, PTFE-backed panel connections, and composite substrate bolting where the standard square neck profile is geometrically incompatible with the substrate pre-drilled hole.
Flat Head / Countersunk Variant
Countersunk (flat) head carriage bolt where the head must sit flush with or below the substrate surface — walkway grating where the head must not protrude above the grating surface level, FRP panel connections requiring a smooth flush surface on the top face, and marine deck connections where the carriage bolt head must be level with the deck surface. DIN 603 proportions with countersunk head form, custom-manufactured on drawing.
Long Shank / Extended Neck
Standard DIN 603 carriage bolt with extended unthreaded shank length — for thick substrate panels (FRP structural grating 40–80mm thick, composite wall panels, multi-layer rubber-lined equipment) where the standard DIN 603 grip length is insufficient and the square neck must extend through the full substrate thickness for anti-rotation. Custom drawn specification. Contact sales@akbarfasteners.in with substrate thickness.
Exotic Alloy Carriage Bolts
Carriage bolts in all exotic and engineering alloys — SS 316L, SS 304, Hastelloy C276, Monel K500, Inconel 625, Nickel 200, Duplex 2205 — for chemical plant FRP grating walkways and platforms, offshore composite deck connections, marine structure timber-composite connections, and all applications where corrosion resistance is required alongside the carriage bolt head form and anti-rotation square neck geometry.
Carriage Bolt Size Chart — DIN 603 MM
DIN 603 mushroom head square neck dimensions M6–M36. Head diameter (dk), head height (k), square neck size (a), and pitch. Lengths from 16mm to 300mm across the range. ASME B18.5 inch sizes on request.
| Size (M) | Pitch (mm) | Head Dia dk (mm) | Head Ht k (mm) | Sq Neck a (mm) | Neck Ht (mm) | Min Length (mm) | Max Length (mm) |
|---|---|---|---|---|---|---|---|
| DIN 603 MUSHROOM HEAD SQUARE NECK — CARBON STEEL / SS 316L / EXOTIC ALLOY — ISO 8677 | |||||||
| M6 | 1.0 | 14.0 | 4.0 | 6.0 | 4.0 | 16 | 100 |
| M8 | 1.25 | 18.0 | 5.0 | 8.0 | 5.0 | 20 | 150 |
| M10 | 1.5 | 22.0 | 6.0 | 10.0 | 6.0 | 25 | 200 |
| M12 | 1.75 | 26.0 | 7.0 | 12.0 | 7.0 | 30 | 200 |
| M16 | 2.0 | 34.0 | 9.0 | 16.0 | 9.0 | 40 | 250 |
| M20 | 2.5 | 42.0 | 11.0 | 20.0 | 11.0 | 50 | 300 |
| M24 | 3.0 | 50.0 | 13.0 | 24.0 | 13.0 | 60 | 300 |
| M30 | 3.5 | 62.0 | 16.0 | 30.0 | 16.0 | 80 | 300 |
| M36 | 4.0 | 74.0 | 19.0 | 36.0 | 19.0 | 100 | 300 |
ASME B18.5 inch sizes ¼”–¾” · custom lengths · ribbed neck variants · countersunk head — all on request: sales@akbarfasteners.in
Carriage Bolts in All Materials — Carbon Steel to Exotic Alloy
Every carriage bolt material PMI-verified before machining. Exotic alloy carriage bolts machined from same ASTM-certified rod stock as stud bolts and hex bolts. Full MTC and COC dispatched with every order.
Carbon & Alloy Steel
DIN 603 Standard →SS 304 / 316L
View SS Range →Hastelloy C276
View C276 →Hastelloy B3 / B2
View B3 Range →Monel K500
View K500 →Monel 400
View M400 →Inconel 625
View Inconel →Duplex 2205 / 2507
View SS Range →Carriage Bolt Application Areas
Single-sided tightening with anti-rotation square neck — the correct fastener wherever the bolt head side is inaccessible, a smooth flush head is required, or the substrate grips the square neck for anti-rotation.
Chemical Plant FRP Grating
walkway & platform grating
Marine / Offshore Deck
composite deck connections
Rubber-Lined Equipment
rubber panel connections
Timber & Wood Construction
structural timber connections
Offshore FPSO
FPSO deck panel fasteners
Food & Pharmaceutical
hygienic panel connections
Walkway & Stairways
tamper-resistant smooth head
Composite & FRP Panels
composite structural connections
Carriage Bolt Standards Coverage
Same ISO 9001:2015 quality system as stud bolts and hex bolts. PMI XRF before machining on all exotic alloy orders.
Complete Documentation Package
Heat & Lot Traceable
Conformance
Exotic Alloys
Tensile · Hardness
Inspection Report
Related Fastener Types
Carriage bolts for smooth-head anti-rotation connections. Hex bolts, stud bolts, and eye bolts for other connection types from the same exotic alloy range.
Hex Bolts
Where the bolt head is accessible for wrenching — standard hex bolts provide the connection strength of carriage bolts with full wrench engagement. DIN 931 (partial thread) and DIN 933 (full thread) in all materials M6–M100.
View Bolt Range →Stud Bolts
For ASME B16.5 pressure flange connections — stud bolts provide fully threaded engagement with matching heavy hex nuts. Available in all alloys from carbon steel B7 through Hastelloy C276, Monel K500, and Inconel 625 for exotic alloy process flanges.
View Stud Bolts →Hex Nuts
Matching hex nuts for DIN 603 carriage bolts — standard hex DIN 934 and heavy hex ASME B18.2.2 in all materials from carbon steel through exotic alloy. Same-alloy matched sets available with single MTC.
View Nuts Range →Washers
Matched washers for DIN 603 carriage bolt assemblies — DIN 125 standard flat or DIN 9021 large OD for soft substrate connections (FRP, rubber, composite) where bearing area under the nut must be distributed. Same alloy as carriage bolt available.
View Washers →Need Carriage Bolts in
Any Material?
Standard (DIN 603 / B18.5) · material · size · length · finish · quantity. Akbar Fasteners Pvt. Ltd. responds within 24 hours.
Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in