Because that height gives enough thread engagement for the bolt to fail before the nut's threads strip.

Standards are written so the ductile failure happens first - the bolt stretches and breaks, which is detectable.

A stripped nut thread gives no warning and no visible sign beforehand.

Reducing the nut height, as a jam nut does, moves the failure mode towards stripping.

That is why a thin nut is not a general substitute for a standard one.

The same reasoning sets minimum thread engagement in tapped holes, where the depth depends on the material's strength.

As much as the bolt's - the joint is only as strong as the weaker of the two.

Nuts carry property class markings just as bolts do, and the classes are designed to be paired.

A high-strength bolt with a general-purpose nut is limited by the nut, so the bolt's cost is wasted.

The reverse pairing is safe but pointless.

Unmarked nuts from general assortment stock have no demonstrable class and are unsuitable for designed connections.

Manufacturers publish the correct pairings, and following them is simpler than reasoning it out per joint.

On the thread usually yes, on the spanner size often not - across-flats dimensions differ between standards.

Thread form and pitch are what determine whether a nut runs on a bolt.

The across-flats dimension is a separate specification and varies between regional standards for the same thread size.

That matters most in maintenance, where whoever comes to the joint brings a set of spanners rather than the right one.

Mixed-standard assemblies are a persistent nuisance on equipment sourced from different markets.

Where consistency matters, specifying the standard as well as the size prevents it.

To the joint's specified torque or preload - and torque is a proxy for tension, not the thing being controlled.

Friction under the nut face and inside the threads absorbs most of what is applied to the spanner.

Friction varies with finish, lubrication, surface condition and whether a washer is present, so the same torque gives different preload in different assemblies.

Torque figures are therefore specific to a stated condition - usually clean, dry threads unless otherwise stated.

Where preload really matters, tension-indicating methods or angle-controlled tightening are used instead.

Under-tightening causes far more joint failures than over-tightening does.

Plain hex nuts with clean, undamaged threads generally yes - which is a real difference from locking nuts.

Nothing in a plain nut is consumed by installation, so its capacity is unchanged after removal.

Threads that have galled, stretched or been cross-started should be discarded rather than judged by eye.

A nut that has been heated during removal has had its properties altered and should be replaced.

In practice nuts are cheap enough that replacement is often faster than inspection.

Documented maintenance regimes frequently mandate replacement regardless of condition.

They are tapped oversize to clear the thick zinc coating on a galvanised bolt.

Hot-dip galvanising deposits far more material than electroplating, and it builds up in the thread.

A standard-tapped nut would bind or seize on a galvanised bolt before it was tight.

So galvanised nuts and bolts are supplied as matched pairs, and the nut feels sloppy on an uncoated thread.

Thread engagement is reduced on a plain bolt, which reduces stripping capacity.

Keeping galvanised fasteners separate from plated ones prevents the mix-up.

Anti-seize compound and unhurried assembly - stainless galls readily against stainless.

Galling is a cold-welding of the thread surfaces under pressure, and once it starts neither part can be saved.

Running a nut down quickly with a power tool generates local heat at the thread and is the usual cause.

An anti-seize compound is the standard mitigation, and it changes the torque-tension relationship so torque figures need adjusting.

Pairing slightly different stainless grades between nut and bolt reduces the risk.

Clean threads help, since grit in the thread is a starting point for galling.