Because a nut gives no other clue to its pitch, and two pitches in the same nominal size look identical in the hand.

A bolt at least shows its thread along the shank, and a wrong match is often visible. A nut hides the thread inside it.

Coarse and fine threads exist in most sizes. Fine threads are used where vibration resistance or fine adjustment matters, and in thin-walled components; coarse is the general case.

The failure is quiet. A mismatched nut engages for a turn or two under fingers, then binds, and forcing it strips one or both threads.

Labelling that carries the full designation rather than just the diameter is what prevents this, and it is worth relabelling a kit that does not.

A thread pitch gauge resolves any doubt in seconds and belongs with the kit.

Both, weighted towards plain for assembly work and towards locking for anything that vibrates.

Plain hex nuts are the general case and the cheapest to hold in depth. They rely on the preload in the bolt to stay put, which is adequate for static joints.

Anything that vibrates loosens plain nuts eventually, and the fix is a locking nut rather than more torque.

Nylon-insert types are the common general-purpose locking nut - easy to identify, limited in temperature, and best treated as good for a small number of re-installations.

All-metal locking types cost more, tolerate heat, and reuse better, which suits machinery maintenance.

For a mixed workshop, holding both plain and nylon-insert in the two or three diameters used most covers the majority of work.

Yes, because each one removes a washer from the assembly - as long as the surface can take the flange.

The integral flange spreads the clamping load over a wider area, which is exactly what a washer does, in one part and one operation.

The serrated version bites into the joint face and resists loosening, which makes it useful on machinery and vehicle work.

That bite is also the limitation. Serrations mark the surface permanently, so they are wrong on finished, soft or coated faces.

Flange nuts need clearance around the fastener, so they do not suit counterbores or tight recesses.

In a general kit a small selection in the common sizes is usually enough; they are chosen deliberately rather than as a default.

Plain nuts usually yes; locking nuts only within limits, and never if they run down freely by hand.

A plain nut in good condition, with clean undamaged threads, is reusable indefinitely. Cleaning and inspecting it takes longer than replacing it, which is why most workshops replace anyway.

Nylon-insert nuts rely on the insert deforming against the thread. Each installation wears it, and manufacturers give a small number of re-uses.

The field test for any locking nut is whether it still requires effort to turn before it reaches the joint. If it spins freely down the thread, it has stopped locking.

Heat is the other disqualifier. A nylon insert that has been through a high temperature is finished even if it looks intact.

Any nut that has been stretched, galled or heated during removal should be discarded rather than judged by appearance.

Because they are deliberately tapped oversize to clear the thick zinc coating on a galvanised bolt.

Hot-dip galvanising deposits a coating far thicker than electroplating, and it builds up in the thread. If the nut were cut to standard size it would not run on at all.

So galvanised nuts are oversize and are supplied as a matched pair with galvanised bolts. On a plain or electroplated bolt they feel sloppy and have reduced thread engagement.

The reverse error is worse: a standard nut on a hot-dip galvanised bolt will bind and may seize before it is tight.

In a mixed kit this is a real risk, because the finishes are not always obvious once parts are in compartments.

Keeping galvanised fasteners in their own section, or their own kit, is the reliable answer.

Yes - they gall, and the usual precaution is an anti-seize compound and slower assembly.

Galling is a cold-welding of the two thread surfaces under pressure. Stainless is particularly prone to it, and once it starts the nut seizes solid and neither part can be saved.

Speed makes it worse. Running a stainless nut down with a power tool generates local heat at the thread and is the most common cause.

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

Mixing grades or pairing a stainless nut with a stainless bolt of the same grade raises the risk; a slightly different grade between nut and bolt reduces it.

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

Deeper than a screw kit, because nuts are consumed rather than selected.

A screw is chosen from a range by length; a nut is simply used, and any assembly taken apart tends to get new ones.

In practice two or three diameters account for most consumption, and those compartments will empty repeatedly while the extremes never move.

That makes a nut assortment the clearest case for refilling from bulk packs of the moving sizes rather than replacing the kit.

Breadth still has value for the unexpected size, which is the whole point of holding an assortment at all.

Recording which compartments are refilled is the cheapest way to decide what bulk stock should actually be held.