How it connects
The bolt's square neck drops into the matching square hole, locking rotation; the threaded shank passes through and a nut (often with washer) on the far side clamps the joint. No seal.
Male and female halves
This is the female (receiving hole); the male is the carriage bolt's square neck. Mating is unthreaded; the thread engages a separate nut.
Usually connects to
- carriage bolt (square neck)
- round-head square-neck bolt
- plow/elevator bolts with square neck
Common sizes
- Square side matches bolt diameter, e.g. ~1/4 to 3/4 in (M6 to M20) bolts; a 3/8-16 bolt needs roughly a 3/8 in (~10 mm) square.
Where it is used
fence and gate hardwarepost clamps and bracketstrailers and agricultural equipmentplayground and wood constructionsteel framing
Variations
- square hole in metal plate/ear
- square washer hole (carriage washer)
- round hole in wood where the neck self-cuts its square
How to recognise it
A small square (not round) punched through-hole in a flat bracket, ear or strap, roughly the bolt diameter on each side; often paired coaxially on opposed clamp ears, and used with a domed-head bolt on the visible side.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| Fastener clearance hole (round) | Round clearance holes allow rotation; this hole is square to lock the bolt neck. |
| Square drive socket (e.g. 3/8 in drive) | A drive socket is a blind tool recess with a detent; a carriage-bolt hole is a through-hole in plate for a fastener. |
Also called
square punched holecarriage bolt square holesquare neck hole
Good to know
This square-profile clearance hole mates with a carriage-bolt neck to prevent rotation.
Design with real parts
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