How to Size and Install 5/16-18 Carriage Bolts in Wood

By Mutual Screw & Supply | Published: 2026-11-02

Category: How-to Guides

Learn how to choose the right hole size and length for 5/16-18 carriage bolts, then install A307 plain steel bolts in wood with a clean, tight fit.

Quick answer

For a 5/16-18 carriage bolt in wood, drill a 5/16-inch (8 mm) pilot hole for the shank and a shallow square hole sized to the bolt’s square neck so the head pulls in flush. Choose a bolt length that leaves at least one full thread exposed beyond the nut after the wood pieces are clamped.

Why hole size and neck seating decide the joint

A carriage bolt holds because the square neck under the domed head bites into the wood and stops the bolt from spinning while you tighten the nut. If the hole is too large, the neck never gets a grip; if it is too small, the neck splits the wood or refuses to seat. The 5/16-18 size refers to a 5/16-inch diameter with 18 coarse threads per inch, so the shank needs a matching 5/16-inch clearance through the wood.

Plain steel A307 carriage bolts are uncoated, so they can leave iron stains on damp or tannin-rich woods such as oak. That is a cosmetic trade-off, not a strength problem, but it matters for visible outdoor work. For hidden structural joints or painted assemblies, the lack of coating is usually irrelevant.

Length, thread, and clamping choices that affect the fit

Measure the total thickness of the wood pieces plus any washers, then add the nut height and at least two exposed threads. Because the square neck sits below the head, the smooth shank must pass through the joint before the threads reach the nut; if the neck bottoms out in the hole, the joint will not tighten. A washer under the nut spreads the load and keeps the nut from digging into soft wood.

For wood-to-wood connections, clamp the pieces before drilling so the holes line up and the neck seats squarely. For wood-to-metal connections, drill the metal clearance hole slightly larger than 5/16 inch so the bolt does not bind, and let the square neck seat in the wood side only.

Step-by-step installation

1. Measure the joint and pick the bolt length: Clamp the wood pieces together and measure the total thickness. Add the nut height plus at least two exposed threads; for a 5/16-18 nut, that is roughly 1/4 inch of nut plus 1/8 inch of thread. Check that the smooth shank, not the threaded section, spans the joint. If the threads sit inside the wood joint, the square neck cannot pull in; choose a longer bolt.
2. Drill the shank hole: Use a 5/16-inch (8 mm) bit for a snug clearance hole through both pieces. Drill straight and at the same angle on both sides; a wobbling bit enlarges the hole and reduces the neck’s grip. Test the bolt in the hole: it should slide through with light hand pressure, not rattle.
3. Cut the square neck recess: Mark the square neck’s width on the wood, then use a chisel or a router bit to cut a shallow square recess just deep enough for the neck to sit below the surface. Keep the recess centered on the hole. A recess that is too deep lets the head sink below the wood and weakens the clamp; stop when the neck is flush.
4. Insert the bolt and seat the head: Push the bolt through from the head side and tap the domed head with a mallet or hammer until the square neck pulls into the recess and the head sits flush. Do not drive it so hard that you crush the wood fibers around the neck. If the head will not seat, enlarge the recess slightly rather than forcing it.
5. Add the washer and nut, then tighten: Place a flat washer over the threaded end, thread on the nut, and tighten with a wrench while holding the domed head if it tries to turn. Stop when the washer just bites into the wood; over-tightening crushes fibers and can split the piece. Check that at least two full threads show beyond the nut after final tightening.
6. Inspect and protect: Confirm the head is flush, the neck is engaged, and the joint is tight. Because these are plain steel bolts, apply a coat of paint or a corrosion inhibitor if the joint will be exposed to weather or damp wood. Recheck the nut after the wood has had a day to settle, especially in softwood.

Product facts

5/16-18 X 7" Carriage Bolts | A307 Grade A Plain Steel (200 / Pkg.): 5/16-18 X 7-inch carriage bolts in A307 Grade A plain steel, sold in a 200-piece package. The 7-inch length suits thick built-up wood joints, and the bulk package keeps a large project supplied without repeated orders.
5/16-18 X 7" Carriage Bolts | A307 Grade A Plain Steel (200 / Pkg.)