STAINLESS STEEL CARRIAGE BOLTS PART THREAD

Carriage Bolts Part Thread — The part-thread carriage bolt shares the same self-locking format as its full-thread sibling — a domed head above a square neck that beds into timber or seats in a matching square hole so the bolt can be tightened from a single side — but stops the thread short of the neck, leaving a plain shank to bear through the clearance hole. That plain section is the whole point of choosing this version over the full-thread one: it presents a full, unbroken diameter across the shear plane, which matters in thicker timber sections, framing joints, gate and fencing structures under load, and any assembly where the bolt shank needs to bear cleanly rather than have a thread crest doing the work. It's a natural pairing with structural or semi-structural timber connections where the joint will be loaded transversely as well as in tension. On grade, the same reasoning applies as elsewhere in the outdoor fastener range: A2 covers general external use comfortably, while A4 is the better investment for coastal, agricultural, or permanently damp installations, or for any fixing that would be expensive or disruptive to access and replace later — the extra material cost is trivial compared with a callback to replace a corroded structural bolt. There's no single tightly harmonised international standard for carriage bolts in the way there is for hexagon bolts; the square neck, head profile and thread proportions follow long-established commercial conventions that keep the neck matched to standard square-hole punching and drilling practice. Choosing the right one means sizing the neck to the hole it will seat into, then picking an overall length that spans the full clamped thickness — including any washers — while leaving the plain shank positioned across the load-bearing section of the joint and enough thread proud for the nut to seat and develop clamp load properly. Where grip thickness varies significantly across a project, the full-thread version offers more flexibility at the cost of some shear performance.

FAQs
How much plain shank does a part thread carriage bolt have?
The proportion of plain shank to thread varies with the bolt's overall length, following standard commercial sizing, so longer bolts carry proportionally more plain shank than short ones — check the specific size against the supplier's dimension table when shear-plane position matters.

Is a part thread carriage bolt reusable?
Yes; it's a structural fastener rather than a locking or deformable device, so it can be undone and refitted through normal maintenance cycles without loss of performance, unlike a split pin or self-locking nut designed to resist removal.

When should I choose part thread over full thread?
Choose part thread whenever the joint is expected to carry meaningful shear load or where the clearance hole should be filled by a full-diameter shank rather than a thread crest; full thread suits situations where grip thickness varies and flexibility in engagement matters more than shear performance.

Does the square neck need to be a tight fit in the hole?
Yes — a snug fit is what stops the bolt spinning as the nut is tightened from the other side; a loose or oversized hole undermines the single-sided tightening feature that defines this bolt type.