Enough to step through the common clamping thicknesses - in practice six or more, and the ratio is more informative than any headline figure.

Diameter is decided by the hole and rarely changes within a job. Length changes constantly, because it depends on the total thickness being clamped plus enough thread engagement beyond it.

A kit with three lengths per diameter will cover the middle of that range and miss both ends, which is exactly where a job stops.

The step size matters too. Even increments are more useful than a list with a gap in the middle.

Where a kit publishes only totals, the per-size list is worth requesting before buying; without it the two numbers that matter are invisible.

Too long is usually recoverable with a washer or by cutting; too short is not.

The ones already in the toolkit - and for most current work that means internal hex and star drives rather than slotted.

Drive type determines how much torque can be applied before the driver slips and damages the head. Slotted heads cam out early and are largely legacy; cross-head drives are the general case; internal hex and star take the most torque in the smallest head.

Mixing drives in one kit is normal and harmless as long as the bits exist. A kit whose drives do not match the bits on hand is a nuisance every time it is opened.

Head shape is a separate choice. Countersunk heads sit flush and need a matching recess; pan and button heads bear on the surface; hex heads take a spanner where access is poor.

Security drives appear in some kits and are worth having only where the work involves them, since each needs its own bit.

Where one drive type dominates the work, a single-drive kit is easier to use than a broader one.

No - assume they are not, unless the kit states a grade and the fasteners carry the markings.

General assortment fasteners are unmarked commercial-quality steel. They are perfectly sound for brackets, covers, panels, enclosures and repairs, which is most of what they are used for.

A designed connection needs a stated property class or grade, and needs it demonstrable. Head markings are the evidence, and assortment stock generally has none.

Traceability is the second problem. Structural work often requires knowing the batch and origin, which a mixed kit cannot provide.

The failure mode is not usually snapping. It is stretching under load, which loosens the joint quietly and shows up as a fastener that has to be retightened repeatedly.

Keeping graded structural fasteners physically separate from assortment stock is the practical safeguard.

A machine screw needs a thread to go into; a self-tapper makes its own.

Machine screws have a uniform thread along a plain shank and require either a tapped hole or a nut. Used in an untapped hole they will spin without holding.

Self-tapping and self-drilling screws have a sharper, coarser thread designed to cut or form a mating thread in sheet metal or plastic, and some carry a drill point that makes the hole as well.

Wood screws are a third family, with a coarse thread and often a partly unthreaded shank so the head can pull the two pieces together.

Using the wrong family is the most common misuse of a kit: a machine screw in timber holds poorly, and a self-tapper in a tapped hole destroys the thread.

Kits that separate these into labelled sections rather than mixing them by size prevent most of it.

Match the equipment being worked on, and keep the two systems in separate cases if both are needed.

Several metric and imperial threads are close enough to start and then bind, which damages both parts and leaves a fastener that cannot be removed cleanly.

Machinery and structures built in different regions use different systems, and older equipment often uses a system its own market has abandoned.

The error is made under time pressure, so physical separation works far better than labels on a shared box.

A thread gauge settles an unknown fastener in seconds and is worth keeping with the kits.

Where the work is genuinely mixed, buying two smaller kits costs little more than one large mixed one and removes the problem.

Zinc-plated for indoor work, and a separate stainless or hot-dip galvanised kit for anything exposed.

The default plating on assortment screws is thin and intended for dry interiors. Outdoors it rusts within a season, and the rust stains whatever it is fixed to.

Stainless is the usual answer for exposure, but it is softer than plated steel and the heads are easier to damage, so drive type matters more when using it.

Hot-dip galvanising gives the thickest coating and the coarsest thread fit, which is why galvanised screws and nuts are usually sold as matched pairs.

Treated timber is a specific case: some preservative chemistries attack ordinary plated steel quickly, and the screw manufacturer's compatibility statement is the thing to check.

Mixing plated steel with stainless or aluminium at the same joint starts galvanic corrosion, so a kit used on aluminium should be a stainless one.

For a general repair kit yes, but they run out at different rates and are better refilled separately.

Combined kits are convenient because a bolt is rarely useful alone, and having the matching nut and washer in the same case saves a second box.

The drawback is proportion. Most kits include far fewer nuts than bolts and far fewer washers than either, so the nut compartments empty first and the kit becomes unbalanced.

Where washers are used heavily - which is most assembly work - a separate washer assortment is the more economical answer.

Combined kits also tend to be shallow in lengths, because the case space is shared across three product types.

For occasional general repair the combined kit wins; for a workshop doing the same work daily, separate deeper kits per family are better value.