Nut

A nut is a small metal fastener with a threaded hole, designed to screw onto a bolt or threaded stud. Its job is to provide the clamping force that holds a bolted joint together.

How it works

The nut’s internal threads engage with the external threads of a bolt. As the nut is turned, it travels along the bolt’s shank until it contacts the surface of the part being clamped. Further tightening stretches the bolt slightly and compresses the parts between the bolt head and the nut, creating the preload that holds the joint rigid.

Common types

• Hex nut: The standard six-sided nut, compatible with open-end and box-end wrenches.
• Lock nut (nyloc): Contains a nylon insert that deforms against the bolt threads, preventing the nut from loosening under vibration.
• Wing nut: Has two large “wings” for hand-tightening without tools — common on battery terminals and filter covers.
• Flange nut: Has a wide, integrated washer face that distributes clamping force over a larger area.
• Castle nut (castellated): Has slots cut into the top so a cotter pin can pass through and lock it in place.
• Cap nut (acorn): Has a domed top that covers the protruding bolt end, protecting against sharp edges.

Materials

Nuts are typically made from carbon steel, stainless steel, or brass, and are often zinc-plated or galvanised for corrosion resistance. The material and grade should match the bolt — using a soft nut on a high-strength bolt can cause the threads to strip under load.

How to identify a nut

Nuts are sized by width across flats — the distance between two opposite faces, which is what a wrench grips. A “19 mm nut” means a 19 mm spanner fits it, not that its threads are 19 mm. The thread size is separate: an M8 nut has 8 mm internal threads but takes a 13 mm wrench. This mismatch between thread size and wrench size trips up almost everyone the first time.

To match a loose nut to a bolt, test the thread engagement by hand first: it should spin on freely for several turns before firm resistance. If it binds immediately, the pitch is wrong — an M8 × 1.0 fine-pitch nut will not seat on an M8 × 1.25 standard bolt, even though both are “M8”. Forcing it cross-threads and ruins both parts.

Common confusion: nut vs. thread insert

A nut clamps from outside the parts; a threaded insert (like a Rivnut) is pressed into thin sheet metal and becomes the internal thread. Both accept a machine screw, but an insert is fixed in the workpiece while a nut turns. If the “nut” spins freely without clamping because it is spinning in place in the material, you are looking at a spinning insert, not a nut.