An externally threaded fastener with a head, designed to pass through clearance holes in the parts and be tightened by turning a nut. The difference from a screw is about intended use, not looks: the same hex part becomes a cap screw when it is driven into a tapped hole by its head. A bolt works as a stretched spring, so its job is to clamp the joint through preload rather than to act as a pin in shear unless the design says so.
Those are bolts, not cap screws, so put a nut and a hardened washer on every one and turn the nut, not the head.
Purists turn the nut, not the head, because most torque specs assume the friction happens under the nut face.
Also heard as
- through bolt
- hex bolt
Do not mix it up with
- Screw: A screw is meant to be turned by its head into a tapped or self-formed thread; a bolt is meant to be tightened with a nut.
Related terms
Screw
An externally threaded fastener tightened by turning its head, into a tapped hole or a thread it makes itself.
Cap screw
A precision-headed screw, hex or socket, made to close tolerances and usually driven into a tapped hole.
Nut
An internally threaded part that mates with a bolt or stud, graded so the bolt breaks before the nut strips.
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.