The thin nut first, nearest the joint, with the full nut tightened against it - and most field installations get this wrong.

Once the pair is jammed, the outer nut carries the load and the inner one is held against it.

Putting the full nut first leaves the thin nut carrying the joint load, which it is not sized for.

The thin nut is run down and lightly tightened, then held with a spanner while the full nut is tightened against it.

That final action loads the two nuts' threads in opposite directions, which is what creates the lock.

Simply running both nuts down without holding the first achieves nothing.

Yes, when installed correctly - it is all-metal, has no temperature limit and consumes nothing.

The lock comes from the two nuts' threads being loaded in opposition, which does not relax with vibration.

There is no polymer to degrade and no coating to be consumed, so it works at temperatures that rule out nylon inserts.

Both nuts can be reused indefinitely provided the threads are undamaged.

The disadvantages are height, the extra part, and the fact that installation is easy to get wrong.

A distorted-thread or flexible-top lock nut gives the same all-metal locking in one part and less height.

It can be, and it downgrades the joint - its threads will strip before the bolt breaks.

Half the height means roughly half the thread engagement, so the nut cannot develop the bolt's full strength.

That moves the failure mode from a detectable stretched bolt to an undetectable stripped thread.

In a designed connection it should never be substituted for a standard nut.

In light non-structural work where load is well below capacity, it is used routinely and without incident.

The honest position is that it is a clearance solution in those cases, not a strength one.

Where a full-height nut will not fit - shallow recesses, crowded assemblies and linkages with moving parts.

Panel-mounted components frequently have short threaded bushings that a standard nut cannot engage fully.

Linkages and adjusters need the nut to clear a moving member.

In each case the nut is short because there is no room, not because anything is being locked.

The reduced strength has to be acceptable for the load, which in these applications it usually is.

Where it is not, redesigning for a longer thread is the correct answer.

The outer nut is tightened to the joint's torque while the inner one is held - the jamming is the relative action, not extra torque.

Holding the inner nut is what makes the outer nut's tightening load the two in opposition.

If both turn together, no opposition develops and there is no lock.

Additional torque beyond the joint's requirement does not improve the lock and risks the fastener.

Some references specify tightening the thin nut to a fraction of the final torque first.

The practical check is that the pair cannot be turned as a unit afterwards.

In common grades yes, though the range is narrower and the marking convention differs.

Because a thin nut cannot develop a high-strength bolt's full load, the very high classes are less relevant to it.

Standards define separate property requirements for thin nuts rather than applying the full-height ones.

That is a recognition of the geometry rather than a quality difference.

For locking use the grade of the thin nut matters less, since the full nut carries the load.

For load-carrying use in a designed joint, the reduced capacity is the governing issue whatever the grade.

A one-piece all-metal locking nut - distorted thread or flexible top - in less height than a jammed pair.

Both give prevailing-torque locking without a polymer element or a temperature limit.

Both install as a single part, which removes the installation-order error entirely.

Nylon-insert nuts are shorter still in some patterns but bring a temperature ceiling.

Thread-locking adhesive occupies no height at all and needs clean, dry threads and cure time.

Which is appropriate depends on temperature, reusability and whether the joint is assembled in the field.