A block of metal, usually hex, with an internal thread that mates with an external thread to complete a bolted joint. A correctly matched nut is designed so the bolt fails in tension before the nut threads strip, which is why nuts carry their own grades: property class 8 or 10 nuts for class 8.8 or 10.9 metric bolts, Grade 5 or Grade 8 nuts for inch hardware. Common styles include finished hex, heavy hex, flange, jam, castle and prevailing-torque nuts.
Put a Grade 8 nut on that Grade 8 cap screw, a hardware-store nut will strip before the bolt ever gets to preload.
Also heard as
- hex nut
Related terms
Bolt
A headed, externally threaded fastener meant to be tightened by turning a mating nut rather than its own head.
Jam nut
A thin nut, roughly half to two-thirds standard height, run up against another nut or part to stop it turning.
Prevailing-torque nut
A nut built to resist turning on the thread before it seats, using a nylon insert or deliberately distorted threads.
Thread stripping
Failure where the threads of a nut, tapped hole or bolt shear off instead of the bolt breaking.
Property class
The metric strength grade of a bolt or nut, stamped on the head as a number such as 8.8, 10.9 or 12.9.