Fasteners look simple until a project requires the right combination of thread size, length, head style, drive, material and finish. A screw that is perfect for an electronics enclosure may be a poor choice for machinery, while a nut that works in a static assembly may loosen in a vibrating joint. This guide explains the main families of screws, nuts and bolts and shows how to narrow the choice without relying on guesswork.
When shopping for metric fasteners, start with the joint rather than the product name. Identify what is being joined, whether the fastener must sit flush or proud, how often the joint will be serviced, the available tool access, environmental exposure and the amount of load or vibration. Those decisions point toward the right family before you choose the exact M-size and length.
Quick answer: Fasteners look simple until a project requires the right combination of thread size, length, head style, drive, material and finish.
What to know about types of screws nuts and bolts
Machine screws
Machine screws use a defined machine thread and are commonly paired with tapped holes or nuts. Pan head, cheese head, countersunk and socket head versions change how the fastener sits on the surface and how torque is applied.
Hex bolts
Hex bolts are driven from an external hex head with a spanner or socket. They are common in general mechanical assemblies because they are easy to hold, tighten and inspect, especially where there is enough clearance around the head.
Nuts
Hex nuts provide a reusable threaded connection. Nyloc nuts add prevailing resistance to rotation, while dome or acorn nuts cover the exposed thread end and are useful where appearance or edge protection matters.
Washers and inserts
Plain washers spread bearing load and protect the joint surface. Threaded inserts create a durable female thread in softer materials such as plastics, allowing repeated assembly without repeatedly cutting the parent material.
How to choose the right fastener
- Thread size: Match the nominal metric thread, such as M3, M4 or M6, across the screw, nut and tapped hole.
- Length: Measure from the correct reference point. Countersunk screws are typically measured overall, while many non-countersunk screws are measured from under the head.
- Head and drive: Choose a head profile that fits the surface and a drive that suits available tools and torque needs.
- Material and finish: Use stainless steel where corrosion resistance is important, and coated steel where strength, appearance or cost is the priority.
- Joint behavior: For vibration, repeated servicing or soft materials, consider locking nuts, washers, inserts or other joint-control methods rather than simply tightening harder.
Quick comparison
| Fastener family | Typical purpose | Common advantage |
|---|---|---|
| Countersunk screw | Flush surface assembly | Head sits level with a prepared countersink |
| Socket head cap screw | Machine and equipment assembly | Compact head with internal hex drive |
| Hex bolt | General structural/mechanical fastening | Easy external-tool access |
| Nyloc nut | Vibration-prone threaded joint | Prevailing resistance to rotation |
| Plain washer | Under bolt head or nut | Spreads load and protects surfaces |
| Threaded insert | Thread in plastic/soft material | Reusable internal thread |
A practical ordering workflow
Before placing an order for types of screws nuts and bolts, turn the application into a short specification. Start with the mating thread and physical envelope, then confirm the head or drive, material and finish. If the fastener is part of a loaded, vibrating, outdoor or frequently serviced joint, add the relevant mechanical, corrosion and maintenance requirements before choosing a SKU.
For repeat purchases, record the exact approved description in your bill of materials or purchasing system instead of relying on a photograph or an old sample. That record should include the dimensions that control fit, the material or property class, the finish, and any standard or certificate requirement. This makes replacement orders easier to compare and helps prevent accidental substitutions.
When a dimension, torque value, strength requirement or corrosion condition is safety-critical, use the applicable engineering drawing, current standard and manufacturer documentation as the final authority. Catalogue guidance is useful for narrowing choices, but it should not override the requirements of the actual joint.
How to verify the specification before ordering
Make the first verification on the actual assembly or approved drawing. For thread size, match the nominal metric thread, such as M3, M4 or M6, across the screw, nut and tapped hole. For length, measure from the correct reference point. Countersunk screws are typically measured overall, while many non-countersunk screws are measured from under the head. Do not assume that an older fastener removed from the machine is still the approved specification; previous repairs may have introduced substitutions.
If the part is being introduced into production, buy a small validation quantity before committing to a large lot. Check thread engagement by hand, seating of the head or nut, tool clearance, surface marking, interference with nearby parts and the condition of the mating thread after assembly. Where traceability or mechanical properties matter, compare received markings and documents with the purchase specification. This small verification step is especially useful when changing suppliers, materials, finishes or dimensional standards.
Common mistakes to avoid
- Choosing only by diameter and ignoring thread pitch or mating thread.
- Using the wrong length so the screw bottoms out before clamping the parts.
- Mixing fastener materials without considering corrosion between dissimilar metals.
- Using a decorative or protective nut where the joint actually requires a load-rated locking solution.
- Assuming every fastener with the same M-size has the same head dimensions or strength.
Recommended products and categories
You can continue from this guide by viewing shop all Screws, Nuts & Bolts, Allen socket head screws and Nyloc nuts.
Technical references
The following independent technical references are useful for checking terminology, standards and engineering background. They are provided for reference and do not imply endorsement of any product sold on this site.
Frequently asked questions
What is the difference between a screw and a bolt?
The distinction depends on design and application, but in practical purchasing a bolt is commonly used with a nut or through-hole, while a machine screw is often driven into a tapped feature. Standards and industry usage can overlap.
Which fastener type is best for a flush surface?
A countersunk screw is normally chosen when the head needs to sit flush with the surrounding surface, provided the part has a suitable countersink.
When should I use a washer?
Use a washer when you need to spread bearing load, protect the surface, bridge an oversized hole or provide a controlled bearing face under a nut or bolt head.
Can I mix stainless-steel screws with zinc-plated nuts?
It may work in benign indoor conditions, but mixed metals can create galvanic-corrosion concerns in wet or aggressive environments. Match materials where practical and assess the service environment.
Related reading
Final buying checklist
- Confirm the nominal thread diameter and thread pitch.
- Confirm the required length and the correct head-to-tip measuring method.
- Check head style, drive style and installation-tool access.
- Match material and finish to the service environment and mating materials.
- Check strength, locking, documentation and application limits where they matter.
A reliable fastener choice comes from matching geometry, material and joint behavior—not from picking the closest-looking part. Once the fastener family is clear, the exact diameter, length, pitch and finish become much easier to specify.
Ready to select a part? Browse our Screws, Nuts & Bolts catalogue and filter by thread size, material, finish, head style and drive type.