Nuts & Bolts

A page about everything nuts and bolts. I’ve tried to list them A to Z, but sorry if I’ve missed something.

Bolt – a cylindrical threaded fastener with a head that is formed to allow torque to be applied.

Nut – A type of hardware fastener with a threaded hole

With regards to different thread forms – see our page on threads and subsequent links to further information.

Across Corners (AC)

The distance on a nut from one corner to the opposite corner. The purpose of the AC dimension is to know how large a hole might be needed to insert a recessed nut.

Across Flats (AF)

The distance on a nut from one flat surface to the opposite flat surface. This is the size of the wrench needed to install or remove the nut.

Anchor Bolt

A special type of bolt used to attach objects or structures to concrete.

Bihexagon Nut

A bolt or screw whose cross section of its head is in the shape of a 12 pointed star.

Breakaway Torque

The torque necessary to put into reverse rotation a bolt that has not been tightened.

Breakloose Torque

The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.

Cap Nut

A nut that is closed at the threaded end often with a dome.

Captive Nut

A nut which fits into a cage and is welded in place. This is done where the nut is not easily accessible.

Cleveloc Nut

The collar of the nut is elliptical in cross section and it is this that provides the flexible locking element. The nut is pre-lubricated to reduce the tightening torque. Cleveloc is a registered trade name of Forest Fasteners.

Coach or Carriage Bolt

A small rounded head with a square section under the head designed to stop the bolt rotating.

CSK Bolt

Common abbreviation for Countersunk Bolt. This means the bolt is driven using: Allen Key, Flathead Screwdriver, Pozi-drive Screwdriver or Torx.

The countersunk allen key bolts in the image are used to mount the chuck to this lathe backplate as that then allows the backplate to mount flush to the lathe spindle.

Flanged Bolt

Flanged Bolt

A bolt with a built in washer on the head.

Flange Nut

Flanged Nut

A nut with a built in washer to spread the load.

Hexagonal Nut

A nut with a plain internal thread and hexagonal in shape.

Lock Nut

This can be either a second nut that is tightened against the first nut or a nut that has some form of system that prevents the nut from coming loose with vibration, eg Cleveloc or Nyloc. Sometimes called a Jam Nut.

Hexagonal Nut

This is the nut we’re all familiar with – 6 sides.

Hot Bolting

This term is used for the completion of maintenance work on a bolted joint when the joint is under load.

Machine Bolt

Bolt with a plain hexagonal head.

Nyloc Nut

A nut with an elastic ring that grips the bolt and stops he tendancy for the nut to vibrate loose.


The process of pulling parts of a joint together, most of the input turn during this process is absorbed in the joint with little tension being given to the bolt.

Snug Torque

The torque required to pull plates together so that direct contact occurs. Often used in angle control tightening.

Socket or Caphead Bolt

Fastened with a hexagonal Allen key.

Square Nut

As per the title, a plain thread square nut.

T Bolt

A bolt with a fixed square head that fits into the T-slot of a bed on a milling machine.

U Bolt

A U-shaped, bent iron bar that has bolts and threads at both ends.


Used to distribute load under a nut or bolt locking surface.

Wing Nut

A nut with wings that allow it to be tightened and released using fingers.

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