Screw Set Buying Guide: Sizes, Materials and Kit Choices

A screw set is only useful if its sizes match the work you actually do. The choices that decide that are the size system (metric or imperial), the head and drive type, the material and finish, and the assortment itself — how many sizes, how many pieces per size, and how they are separated. This guide answers the questions buyers ask most often, in the order they come up.

What Is a Screw Set — and What Should Be in One?

A screw set is a packaged assortment of fasteners covering several sizes, held together by a case, a card or a box. The value is not the screw; it is the coverage. One case replaces six part numbers, three trips to a hardware store and the drawer of loose screws nobody can identify.

Three formats dominate, and they solve different problems.

A well-built screw assortment kit has four properties. The sizes form a usable ladder rather than a random spread. Quantities per size reflect how fast each size is consumed. The case is labelled so a size can be found without counting. And the material is stated, because "steel" alone tells a buyer nothing about whether the screw will rust.

Metric or Imperial — Which Screw Set Do You Need?

Metric and imperial screws are not interchangeable, and no kit can substitute one for the other. This is the single most common source of returned kits, and it starts with how metric vs imperial screw sizes are defined.

Metric fasteners are designated by nominal diameter in millimetres, preceded by M: M3, M4, M5, M6. The number is the thread diameter, not the head size and not the wrench size. Imperial fasteners use gauge numbers for smaller screws (#4, #6, #8, #10) and fractions of an inch for larger ones (1/4", 5/16"), with threads per inch defining the pitch.

The practical consequences when choosing a screw set:

• Decide by the equipment, not by preference. A machine assembled in Europe or Asia is almost certainly metric. Equipment built in the United States may be either, and older equipment is more often imperial.
• Nearby sizes are not equivalent. An M4 screw does not fit a #8 hole. The diameters are close enough to look right and far enough apart to strip a thread or fall through.
• Do not mix systems in one kit. A kit holding both metric and imperial sizes in one compartment layout invites the wrong screw being picked under time pressure. If a workshop genuinely needs both, two cases are safer than one.
• Pitch matters as much as diameter. A metric screw stated only as M6 may be M6×1.0 coarse or M6×0.75 fine. Where the mating thread already exists, the pitch must match, not just the diameter.

A metric screw set covers the majority of global industrial and consumer equipment. If a buyer's market is unclear, the correct question is not "metric or imperial" but "what does the installed equipment use".

How Do You Read Screw Sizes in a Kit?

A screw size is written diameter × pitch × length, and each part of that string changes whether the screw works.

Diameter. For metric screws, M4 means a 4 mm nominal thread diameter. For imperial screws, the gauge number is a size code, not a measurement — a #8 screw is not 8 of anything.

Pitch. The distance between threads. Coarse pitch is the default and is left unwritten in most callouts (M6 implies M6×1.0 coarse). Fine pitch is written explicitly and is used where vibration or adjustment demands it.

Length. This is where most measurement errors happen. Length is measured from the bearing surface under the head to the tip — excluding the head — for most screw types. Countersunk (flat head) screws are measured overall, including the head, because the head sits flush and the cone occupies material. A 20 mm flat head screw and a 20 mm pan head screw do not reach the same depth.

Head type changes the effective length. Two screws with identical thread length behave differently if one has a wafer head and the other a countersunk head, because the head sits differently in the material.

When a kit's size chart lists only "M4" with no length, the kit is incomplete for anyone who needs to order a replacement. Screw kit sizes should be printed as diameter × length, with pitch added wherever a fine thread is included.

Which Head and Drive Types Belong in a Kit?

Head type decides how the load is carried and whether the fastener sits flush. Drive type decides whether the tool slips.

Cam-out is not a minor annoyance. It damages the head, damages the bit, and in automated assembly it stops the line. Where a kit is used on powered drivers, Torx or hex socket drives reduce that risk — which is why serious wood screw sets are built around star drives rather than Phillips.

Head style should follow the joint:

• Countersunk (flat) head — sits flush, centres the joint, used where the surface must stay clear.
• Pan and button head — general purpose, larger bearing area than countersunk.
• Wafer / washer head — an integrated washer spreads load and stops pull-through in soft material, which is why it appears on self tapping screw assortments for sheet and plastic.
• Hex head — high torque with an external wrench, common on structural and automotive work.

A kit that contains only one head style is a kit for one job. A kit with three head styles but no size discipline is a kit for none.

Stainless or Zinc Plated? Choosing Material for a Kit

Material decides service life more than any other specification, and it is the most frequently under-specified field on a purchase order.

Carbon steel with zinc plating is the default for indoor work: strong, economical, and adequate where humidity is controlled. Black zinc adds a darker appearance and modest additional corrosion resistance; yellow zinc is common on automotive and hardware items for the same reason.

304 stainless steel is the choice where moisture, outdoor exposure or cleaning chemicals are present. It resists rust without a coating, so a scratched surface does not become a rust site — which is exactly what happens when plated carbon steel is scratched. A 304 stainless steel screw assortment is the safer default for outdoor, marine-adjacent, food-adjacent and washdown environments.

Two rules prevent most corrosion complaints:

1. Match the fastener to the environment, not to the budget. A plated screw in a damp environment fails at the plating, and the failure looks like a quality problem when it is a specification problem.
2. Watch the mating material. Joining stainless to aluminium or to plain steel creates a galvanic pair. Where dissimilar metals must meet, isolating washers or a compatible finish avoid the slow corrosion that appears months later.

A kit that mixes both materials without labelling which is which is a kit that will be misused. If a set contains stainless and plated items, the compartments need to say so.

How Many Pieces and Which Sizes Should a Kit Contain?

The honest answer is that quantity follows consumption, not preference. A kit with twelve sizes and three pieces each looks comprehensive and runs out of the one size that is actually used.

A better construction method:

• Start from the top three sizes. In most maintenance work, a small number of sizes accounts for most usage. Build those in larger quantities — tens of pieces, not units.
• Add a size ladder around them. One size below and one above each core size covers adjacent joints without doubling the part count.
• Set the length range by the material thickness. Lengths should bracket the thicknesses actually being joined, not spread evenly from 6 mm to 50 mm.
• Keep the total count in proportion to the case. A compartment holding 200 pieces of one size dominates the kit and crowds out coverage.

This is why a 420-piece computer screw assortment kit works: the pieces are concentrated in the ten fastener types a PC actually uses, rather than spread thinly across forty sizes nobody needs. A 400-piece self tapping set follows the same logic — nine sizes across three lengths, all in the range that furniture, panels and plastic housings require.

Are all screws the same size across different kits? No, and this is a real source of frustration for anyone maintaining several kits. Each kit is built to its own specification: a 300-piece set screw kit covering M3–M8 is not interchangeable with a 195-piece thumb screw kit covering M4–M6. Two kits from different suppliers may even use different compartment orders. When replacing or combining kits, compare the printed size chart rather than assuming continuity.

Do Set Screws, E-Clips and Cotter Pins Belong in a Screw Set?

Strictly, no — but the practical answer is that maintenance buyers group them anyway, and a supplier should be able to build to that grouping.

The distinction is functional. Screws fasten by thread engagement. Set screws lock a component onto a shaft by point contact. E-clips retain by snapping into a groove. Cotter pins lock by deformation. Each solves a different retention problem, and each fails differently.

The practical rule:

• A screw set covers threaded fasteners: machine screws, self-tapping screws, set screws, thumb screws.
• A fastener assortment may extend to clips, pins, clamps and thread inserts, because they are consumed in the same maintenance event.
• A repair kit combines fasteners with the tools needed to install them.

This is where kit design earns its keep. A set screw kit with hex keys included removes the most common reason the kit sits unused; a thread insert kit that clearly states installation tooling is required prevents a returned order. Buyers should check what is included — inserts alone, or inserts with the tap and driver.

Loose Screws or a Kit — When Does a Kit Pay Off?

A kit costs more per piece than bulk and less per incident. The crossover is a handling question.

A kit pays off when: several sizes are used in one task, the work happens away from a stockroom, parts must be counted or issued to a technician, or the product is sold to an end user who needs coverage rather than volume.

Bulk pays off when: one size dominates consumption, the line has automated feeding, or the parts are consumed at a rate that empties a case before the next order.

The hidden cost of loose fasteners is not the fastener. It is the search, the count, the re-order and the rework when the wrong size is fitted. A screw kit with storage case and a size chart removes all four at once, which is why field service and maintenance operations standardise on kits even where bulk is cheaper per piece.

Can a Screw Set Be Customized for Retail or OEM?

Yes, and this is where a custom screw kit becomes a product rather than a purchase.

The customization dimensions that matter in practice:

For a screw kit for retail packaging, three details decide whether it sells: the size chart must be readable without opening the case, the compartment layout must match the chart's order, and the label must survive handling. A kit that requires the buyer to open it and count is a kit that gets opened, counted and put back.

For OEM buyers, the useful starting point is not a catalogue number but a usage list: which joints the kit must cover, how often each is serviced, and which market it ships to. The assortment follows from that.

A Checklist Before Ordering a Screw Set

This is the short version of how to choose a screw assortment kit: ten points, in the order they cause problems.

1. Size system confirmed — metric or imperial, matching the installed equipment.
2. Sizes written as diameter × pitch × length, not diameter alone.
3. Length measurement rule understood — overall for countersunk, under-head for others.
4. Head and drive types listed, with the drive matched to the tools in use.
5. Material stated per item, and labelled in the case if the kit mixes stainless and plated.
6. Quantities concentrated on high-use sizes, not spread evenly.
7. Case labelled with a size chart in the same order as the compartments.
8. Included accessories confirmed — bits, hex keys, taps — or explicitly excluded.
9. Packaging fit for purpose — bulk carton, retail box, or private label.
10. Replacement availability checked, so the kit can be topped up rather than replaced.

Build the Kit Around the Job

A screw set is a small product with a large specification surface: size system, pitch, length rule, head, drive, material, assortment, case and label. Getting those nine right is the difference between a kit that is used and a kit that is stored.

GANGGU manufactures screw kits and fastener assortments in 304 stainless steel and carbon steel with zinc, black zinc and yellow zinc finishes, supplied in compartment cases with printed size charts, from 55-piece specialty sets to 420-piece assortment kits. Assortment, compartment layout, label, barcode, box and language can all be built to the buyer's specification, with production in China and Vietnam under an ISO9001:2015 quality management system.

Browse screw assortment kits or hardware kits, or send a usage list to ggscrew.com/contact and the assortment will be proposed from it.

Frequently Asked Questions

1.What is a screw assortment kit?

A packaged set of fasteners covering several sizes, usually in a compartment case with a size chart. It replaces multiple individual part numbers with one item that covers the range a task or product needs.

2.Are metric and imperial screws interchangeable?

No. An M4 screw does not fit a #8 hole, and the two systems use different thread geometry. Choose the system that matches the installed equipment, and keep the two in separate kits if both are needed.

3.How do I measure screw length correctly?

For most screws, from the bearing surface under the head to the tip, excluding the head. Countersunk screws are measured overall including the head, because the head sits flush in the material.

4.What does M6×20 mean on a screw kit label?

A 6 mm nominal thread diameter, 20 mm length, with the default coarse pitch. If a fine thread is included, it must be written separately as M6×0.75, because coarse and fine are not interchangeable.

5.Stainless steel or zinc plated for outdoor use?

304 stainless steel. Plated carbon steel relies on a coating that fails at scratches and cut edges, while stainless resists rust through the material itself. Plated steel remains appropriate for dry indoor work.

6.How many pieces should a screw kit contain?

Enough that the high-use sizes do not run out before the low-use ones. Concentrate quantity on the two or three sizes used most and keep a single size step either side, rather than spreading a large total thinly across many sizes.

7.Why do the same size screws from two kits not match?

Each kit is built to its own specification, and compartment order, pitch and length ranges can differ between suppliers. Compare the printed size chart rather than assuming two kits covering "M4" are identical.

8.Do screw kits include the tools to install them?

Not always. Some set screw kits include hex keys; thread insert kits typically supply inserts only and require an STI tap and installation tools. Check the inclusion list before ordering.

9.Can I order a screw set with my own brand and packaging?

Yes. Assortment, compartment layout, lid card, label, barcode, box and language can be specified, which is what turns a standard assortment into a retail-ready product.

10.Is a kit more expensive than buying loose screws?

Per piece, usually yes; per incident, usually no. The kit removes search, counting, re-ordering and wrong-size fitting, and those costs are larger than the difference in unit price for most maintenance work.

About GANGGU — Dongguan Ganggu Hardware Product Co., Ltd. has manufactured hardware fasteners and precision CNC machined parts since 2011, supplying screw kits, fastener assortments and custom non-standard parts from production bases in China and Vietnam under an ISO9001:2015 quality management system.

Contact: angel@ganggus.com ｜ WhatsApp +86 18602085825