Carriage Bolt
Bolts · coach bolt · round head square neck bolt
Definition: A carriage bolt has a smooth domed head and a square shoulder under the head that bites into wood, letting it be tightened from the nut side only.
Anatomy
■ Head■ Shank■ Threads■ Point
Governing standards: ASME B18.5 · DIN 603
Common Sizes
| Inch | Metric | Head width | Head height |
|---|---|---|---|
| 1/4-20 | M6 x 1.0 | 9/16 | 5/32 |
| 3/8-16 | M10 x 1.5 | 13/16 | 7/32 |
| 1/2-13 | M12 x 1.75 | 1-1/16 | 9/32 |
Representative sizes. Full dimensional data lives in the standards listed below.
Grades & Strength
| Grade | Material | Tensile (psi) | Marking |
|---|---|---|---|
| Grade A | low carbon steel | 60,000 | No head marking |
| Grade 5 | medium carbon steel, Q&T | 120,000 | 3 radial lines |
Pilot Hole Sizes
| Screw size | Softwood pilot | Hardwood pilot |
|---|---|---|
| 1/4 | 3/16 | 13/64 |
| 3/8 | 5/16 | 11/32 |
| 1/2 | 13/32 | 7/16 |
Drill the pilot hole to roughly the screw root diameter. Always test on scrap.
Materials
- Zinc-plated steel
- Stainless steel
- Hot-dip galvanized steel
Pros & Cons
Pros
- One-wrench installation (head cannot turn once the neck bites)
- Smooth, snag-free domed head is safe for exposed surfaces
- Cheap and widely available
Cons
- Lower strength than a comparable hex bolt
- Cannot be used where the head must be countersunk
- Square neck can strip out of soft or oversized holes
Typical Uses
- Wood-to-wood and wood-to-metal connections
- Decks and fencing
- Furniture and playground equipment
- Timber framing
Alternatives & Related
- Hex Bolt — Higher strength and removable from either side
- Lag Bolt — Heavier duty, threads directly into wood
Where to Buy
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FAQ
What is a carriage bolt used for?
Wood connections where the head should stay flush and snag-free, and where you can only reach the nut side with a wrench — decks, fences, docks, and furniture.
Do I need to pre-drill for a carriage bolt?
Yes. Drill a hole sized to the bolt shank so the square neck seats into the wood and prevents the bolt from turning as you tighten the nut.