Part of its thread is deformed out of true, and forcing it back into shape on the bolt makes the steel grip.

The deformation is produced during manufacture - an oval section, an inward crimp or a deformed cone.

As the nut runs on, the bolt pushes the distorted thread towards its correct form and the elastic reaction clamps the thread.

That grip is prevailing torque and exists whether or not the joint is clamped.

Nothing is added to the nut, so nothing can be lost to heat or chemistry.

The mechanism is the nut's own metal, which is the whole point of choosing it.

Effectively the steel's own limit - there is no polymer element to soften.

That is the main reason to choose one over a nylon insert nut.

Engine bays, exhaust systems, ovens, furnaces and process heat are all within reach.

Material and plating still set a practical ceiling, and stainless and alloy versions extend it further.

Very high temperature also relaxes bolt preload over time, which is a separate problem the nut does not solve.

For temperature-critical joints the whole assembly is specified rather than just the nut.

Prevailing torque applies along the whole thread engagement, not just at the end.

The distorted section resists from the moment it engages, so every turn is loaded.

That is slow by hand and puts real load on power tools, particularly on long bolts.

It also generates heat in the threads, which contributes to galling in stainless assemblies.

Running the nut down quickly with a power tool and finishing by hand is common practice.

The resistance is the locking feature working, so it cannot be designed out.

Prevailing torque plus the torque that produces the required clamping load.

Using a plain-nut figure under-tightens the joint, because part of the applied torque never reaches the clamp.

Manufacturers publish prevailing torque values for each size and class.

Prevailing torque falls with each reuse, so the correct total changes over the nut's life.

Where preload matters, angle-controlled tightening or tension indication avoids the arithmetic entirely.

Minimum prevailing torque is also the criterion for whether a used nut may be refitted.

More than a nylon insert nut, within a stated limit - and the criterion is the remaining prevailing torque.

Manufacturers publish both a permitted number of cycles and a minimum prevailing torque that must remain.

A nut that runs down freely has lost its lock and is scrap regardless of appearance.

Each cycle also wears the bolt thread, so the bolt's condition matters as much as the nut's.

Galling during removal disqualifies both parts.

In documented maintenance regimes, single use is often specified in preference to checking.

It works against it, and repeated cycles wear both parts - which is the trade for an all-metal lock.

A nylon insert deforms itself and leaves the bolt largely untouched; an all-metal nut cannot.

On a bolt that will be reused many times, that wear accumulates and eventually reduces engagement.

Stainless assemblies are the worst case, since metal-on-metal friction at high pressure invites galling.

An anti-seize compound helps but changes the torque-tension relationship and the prevailing torque.

Inspecting the bolt thread at each disassembly is worthwhile in high-cycle applications.

Very high vibration, and any joint that requires a positive rather than frictional lock.

Prevailing torque resists loosening well but does not make it impossible.

Wedge washer pairs work by geometry rather than friction and outperform prevailing-torque nuts in transverse vibration.

Slotted and castle nuts with a cotter pin cannot back off at all, which is why aerospace and lifting applications use them.

Multi-piece lock nuts are the specialist answer for the most severe conditions.

The design requirement usually names the method rather than leaving it to the assembler.