STAINLESS STEEL CARRIAGE BOLTS FULL THREAD

Carriage Bolts Full Thread — A carriage bolt is easy to identify by its smooth, domed head sitting above a short square-section neck: drop it into a matching square hole, or let the neck bed itself into timber, and the bolt resists turning while a nut is run up from the other side with a single spanner — no second tool holding the head is needed. This full-thread version runs the thread from the neck all the way to the tip, which suits variable grip thicknesses since there's always enough engaged thread whatever the stack-up. It's a familiar sight in timber and fencing construction, playground and outdoor furniture assembly (where the domed head avoids snagging or injury risk), agricultural fixings, and general joinery — anywhere a tamper-resistant, non-driveable head is wanted on the visible face of the work. Grade choice mirrors most outdoor fastener decisions: A2 is adequate for general external use away from coastal or heavily salted conditions, but A4 becomes the sensible specification for permanent structures near the coast, in agricultural environments with fertiliser or slurry exposure, or anywhere the bolt cannot easily be inspected or replaced once installed — the cost difference is minor set against the disruption of replacing a corroded structural fixing years later. There isn't a single carriage bolt standard as tightly codified as the hexagon head range; dimensions for the square neck, head diameter and thread follow established commercial and DIN-derived conventions that keep the neck sized to seat correctly in a matching drilled or punched square hole. Sizing should start with the hole or timber section the neck needs to seat into — undersized holes prevent proper seating and oversized ones let the bolt spin — followed by a length that clears the full clamped thickness with enough thread left proud for the nut. Where the joint will see meaningful shear load, the part-thread version is generally the safer choice, since its plain shank gives a fuller bearing section through the hole.

FAQs
What size square hole does a carriage bolt need?
The hole should be cut or punched to match the bolt's neck dimension so the square section seats snugly without a gap; a hole that's too loose lets the bolt spin during tightening, defeating the self-locking design.

Can a carriage bolt be installed without a pre-cut square hole?
Yes, in solid timber the square neck can be driven in so it beds itself into the wood fibres, which is how carriage bolts are traditionally used in fencing and framing; in metal or hard material, a matching square hole is needed.

Is a full thread carriage bolt suitable for shear-loaded joints?
It can carry light shear loads, but because the thread runs into the region that sits in the hole, a part-thread carriage bolt — with its plain shank across the shear plane — is the more robust choice where the joint is expected to see significant transverse load.

Do carriage bolts need a washer under the nut?
A washer is common and recommended, particularly in timber, to spread the nut's bearing load over a wider area of the material and reduce the risk of the nut crushing into softer wood fibres as it's tightened.