Choosing the Right Fastener
Walk into any hardware store and the fastener aisle can feel overwhelming, but almost every choice comes down to a handful of decisions: what type of fastener, how strong, what thread, what head, and what material. Getting these right is the difference between a joint that lasts for decades and one that strips, corrodes, or shakes loose. This guide walks through each decision in turn.
Bolt, screw, or stud?
The three basic forms are often confused. A bolt passes through clearance holes in the parts and is tightened with a nut on the far side, so the threads engage the nut rather than the parts. A screw threads directly into a tapped hole in one of the parts, so the part itself acts as the nut. A stud is a headless rod threaded at both ends — one end screws permanently into a component, the other takes a nut — which suits assemblies that are taken apart repeatedly, such as an engine cylinder head, because the vulnerable threads live in the replaceable nut rather than the casting.
Property classes: how strong?
Metric steel fasteners carry a two-part number stamped on the head, such as 8.8 or 10.9. This is the property class, and it encodes real mechanical properties:
- The first number, multiplied by 100, gives the tensile strength in MPa.
- The two numbers multiplied together give the yield (proof) strength in units of 10 MPa.
So a class 8.8 bolt has roughly 800 MPa tensile strength and 640 MPa yield. The common classes:
| Class | Tensile (MPa) | Yield (MPa) | Typical use |
|---|---|---|---|
| 4.6 | 400 | 240 | Light-duty, general |
| 8.8 | 800 | 640 | Structural, automotive |
| 10.9 | 1000 | 900 | High-strength machinery |
| 12.9 | 1200 | 1080 | Highest, precision |
Higher classes are stronger but also harder and more brittle, and more susceptible to hydrogen embrittlement, so bigger is not always better.
Coarse versus fine thread
For a given diameter you can usually choose a coarse or a fine thread pitch. Coarse threads are more tolerant of dirt and damage, assemble faster, and cut better into softer materials like aluminium or cast iron. Fine threads have a shallower helix, so they resist loosening from vibration slightly better, offer a little more tensile strength through a larger stress area, and allow finer adjustment. Fine threads also strip and cross-thread more easily, so coarse is the sensible default unless you have a specific reason to go fine.
Heads and drives
The head shape sets how the load spreads and how the fastener sits, while the drive is how you turn it. Common heads include hex (external, for a spanner or socket), socket cap (cylindrical with an internal hex), countersunk (flat, sits flush), and button or pan (rounded, low-profile). Drives range from the humble slot and Phillips to hex (Allen), Torx, and square. Torx and hex drives resist cam-out — the tendency of the tool to slip out under load — far better than a Phillips, which is why power-tool assembly has largely moved to them.
Washers earn their keep
- Plain washers spread the clamping load over a wider area, protecting soft surfaces and bridging oversized holes.
- Spring and star washers add a little axial tension to resist loosening, though on high-strength joints correct preload does this job better.
- Hardened washers stop a high-strength bolt head from digging into the part as it is torqued.
Thread engagement
A joint is only as strong as the threads holding it. A good rule of thumb is that the length of thread engagement should be at least one bolt diameter when threading into steel, and closer to two diameters in aluminium or other soft metals. Too little engagement and the threads strip before the bolt reaches its rated load; going much beyond two diameters gains almost nothing, because the first few engaged threads carry most of the load anyway.
Matching materials and corrosion
Fastener material should suit the environment. Plain steel rusts and needs zinc plating or another coating outdoors. Stainless steel resists corrosion but is generally weaker than an alloy-steel high-strength bolt, and it can gall (cold-weld) on tightening unless lubricated. The trap to avoid is galvanic corrosion: joining two dissimilar metals in a damp environment sets up a tiny battery that eats the less noble metal away. A plain steel bolt in an aluminium part near saltwater, for example, corrodes fast unless the metals are isolated or a compatible coating is chosen.
Picking the right size and thread gets easier with the numbers in front of you. Look them up with the Thread & Fastener Size Reference on MechKit.