Carriage Bolts (Mushroom Head Square Neck Bolts)
When a joint is assembled from one accessible side—deck frames, guardrails, cable trays, HVAC hangers, or sheet-metal brackets—rotation control becomes the failure mode long before “bolt strength” does. A carriage bolt solves a very specific field problem: the bolt shank spinning while the installer tries to tighten the nut, especially after coating buildup, wet wood shrink/swell, or vibration cycles.
Its mushroom head spreads bearing pressure on wood and soft substrates, while the square neck locks into the mating material to resist rotation. In production, this reduces second-tool dependency and speeds single-side assembly. In maintenance, it prevents rework caused by rounded heads, damaged coatings, and “free-spinning” fasteners that cannot be re-torqued reliably.
- Lock rotation via square neck
- Distribute load with round head
- Enable one-side installation workflows
- Offer zinc / Geomet corrosion systems
- Supply A2 / A4 stainless options
- Support DIN 603 / ASME B18.5
Technical Specifications
Product Name
Carriage Bolts / Mushroom Head Square Neck Bolts
Standards
DIN 603 (metric mushroom head square neck), ASME/ANSI B18.5 (inch carriage bolts), custom drawings accepted
Material Grades
Carbon steel (low/medium carbon), Alloy steel (when higher clamp load is required), Stainless steel A2 (304) / A4 (316)
Thread
Partial thread (common for shear plane control) or full thread (on request)
Diameter Range
Metric: M6–M20 common; Inch: 1/4″–3/4″ common (other sizes on request)
Grades
Steel: typical 4.6 / 5.8 / 8.8 (per ISO 898-1 for metric property classes, as specified); Stainless: A2-70 / A4-70 / A4-80 (per ISO 3506, if applicable)
Surface Finish
Zinc plated (Cr3), Zinc-Nickel, Hot-dip galvanizing (program-dependent), Geomet/Dacromet (flake coating), Passivation (stainless)
Certifications
Material cert EN 10204 3.1, RoHS/REACH declarations on request, PPAP/traceability packages for OEM programs
1: Bolt spins during tightening (installation stall).
What happens in the field: In timber joints, the nut side may be accessible but the head side is hidden or recessed. As friction changes (wet wood, coating stack-up, re-used hardware), the shank starts rotating and the joint cannot reach target clamp load.
How carriage bolts solve it: The square neck is designed to bite into wood fibers or seat into a prepared hole in softer metals. Once seated, it provides anti-rotation so torque is applied at the nut—improving assembly repeatability and reducing tool juggling.
2: Bearing crush and embedment in wood (preload decay).
What happens in service: Wood compresses under the head; seasonal moisture cycling accelerates embedment, lowering preload and causing joint slip or squeaks.
Engineering response: The mushroom head provides a larger bearing interface than a small hex head contact edge. For soft woods or oversized holes, pairing with a fender washer further reduces contact stress and slows preload loss.
3: Corrosion at interfaces (coating damage + crevice).
What happens in outdoor builds: Under-head crevices trap moisture; damaged coatings become initiation points. Stainless on carbon steel structures can also create galvanic concerns depending on environment.
Solution approach: Select finish by exposure class (zinc-nickel or flake coating for higher corrosion performance; A4/316 for chloride environments), and specify controlled lubrication to avoid false torque and coating galling.
Why engineers search “carriage bolt dimensions / square neck size”
Because anti-rotation depends on geometry. If the square neck is undersized for the hole or the substrate is too hard, it won’t lock, and torque becomes non-transferable. A dimensional check against DIN 603 / ASME B18.5 is often the fastest way to prevent assembly failures.
This is a dimension example table to capture “dimensions” intent. For release drawings and PPAP, confirm values against the specified standard edition and your tolerance requirements.
| Thread d | Pitch P (coarse) | Head Ø dk (typ.) | Head height k (typ.) | Square neck (across flats) v (typ.) | Thread length b (example) |
|---|---|---|---|---|---|
| M6 | 1.0 | 16 | 3.3 | 6.5 | 18 (for L ≤ 40) |
| M8 | 1.25 | 20 | 4.4 | 8.5 | 22 (for L ≤ 45) |
| M10 | 1.5 | 24 | 5.5 | 10.5 | 26 (for L ≤ 60) |
| M12 | 1.75 | 30 | 6.6 | 12.5 | 30 (for L ≤ 65) |
| M16 | 2.0 | 38 | 8.8 | 16.5 | 38 (for L ≤ 90) |
Dimension notes that matter in production
dk / k influence bearing stress and countersink clearance.
Square neck size v is the real “anti-rotation feature”—match it to hole prep and substrate hardness.
b (thread length) affects whether the shear plane runs through threads (fatigue risk in dynamic joints).
Hole design and clearance (ISO 273 for round holes)
For steel plates using a round clearance hole, apply ISO 273 guidance (close/normal/large) to control fit and assembly variability.
If the square neck must lock in metal, consider square hole prep or anti-rotation washers/plates; relying on the square neck to bite into hard steel can be unreliable.
Seating the square neck (avoid false torque)
Ensure the head is fully seated before final tightening. If the square neck is not seated, torque increases from rubbing/spin rather than bolt elongation—preload will be lower than expected.
Torque, preload, and friction control
Torque is a proxy; friction dominates scatter. Use process control: define dry vs lubricated condition, and lock down finish/topcoat friction windows where possible.
Common shop-floor relationship: T = K × F × d
Typical K-factor ranges (indicative): dry 0.18–0.25, light oil 0.14–0.20, anti-seize 0.10–0.16.
For stainless carriage bolts, manage galling risk: use appropriate lubrication and compatible nuts.
Washer strategy (when flange area is insufficient)
On softwood, slotted holes, or thin sheet metal, use fender washers to reduce bearing stress and slow embedment-driven preload loss.
If vibration is present, choose locking method based on validation: prevailing torque nut, threadlocker, or mechanical locking washer.
Field failure prevention checklist
Square neck spins → hole too large / substrate too hard / neck not fully seated.
Preload decays → wood embedment; increase bearing area or re-torque plan.
Corrosion at head interface → choose coating system and protect installation surface.
Related Products
FAQ
What is a carriage bolt used for?
A carriage bolt is used to prevent bolt rotation during tightening in wood or one-side-access assemblies. The square neck locks the bolt while the nut is tightened, reducing spin-out and installation rework.
What standard covers metric carriage bolts?
Metric carriage bolts are commonly specified under DIN 603. For inch series, the typical reference is ASME/ANSI B18.5, and OEM drawings may add additional geometry or finish requirements.
Why does my carriage bolt keep spinning?
A carriage bolt spins when the square neck cannot lock into the substrate or the hole is oversized. Corrective actions include tighter hole control, using an anti-rotation plate/washer, or switching to a different head/drive style for metal joints.
Are carriage bolts structural fasteners?
Carriage bolts can be used in structural or semi-structural joints if the grade, preload requirement, and joint design are validated. Selection should be based on property class (e.g., 5.8/8.8), shear plane design, and service conditions (vibration, moisture, corrosion).
What material should I choose—zinc plated steel or stainless?
Zinc plated steel is often chosen for cost-effective indoor or mild outdoor use, while A4/316 stainless is preferred for chloride or coastal exposure. The correct choice depends on corrosion class, galvanic pairing, and whether coating damage during installation is acceptable.