Fasteners 101: How To Choose the Right Material for Any Project (Without Being an Engineer)

At Reliable Bolt in Edenvale, we’ve spent over 40 years learning that selecting a fastener is about much more than just finding a bolt that matches a hole. It’s about moving past assumptions or simply "making a plan" and instead choosing the right material to handle the torque—how much you can tighten the fastener, and the environment where it will do its work.

With an inventory of nearly 72,000 items and a team that understands the unique engineering challenges of the South African industry, we make sure you walk away with the exact fasteners you need. To help you navigate your next project, we’ve broken down the different grades we stock and why they matter for your project.

1.   Mild-Steel (Grades: 4.6 & 4.8):
Mild-Steel fasteners are the standard for everyday projects that don’t carry massive structural weight. In the Local Market you’ll usually see these marked as Grade 4.6 or Grade 4.8.

• Grade 4.6: A standard commercial grade fastener that is commonly used for lower-load connections, such as brackets and general furniture assembly. It is often the most cost-effective choice for non-critical parts.

• Grade 4.8: The most common mild steel grade for general hardware. While it has the same breaking point as 4.6, it is tougher and more resistant to bending or deforming permanently when tightened.  

The Advantage: Mild Steel is valued for its ductility. This means the metal is flexible enough to bend or give slightly under pressure rather than snapping.

Where to use it: This is the best choice for woodworking projects and timber trusses. For example, standard roofing regulations typically call for Grade 4.8 fasteners because they provide a reliable hold without the unnecessary hardness that might split the timber.

Pro-Tip: If you need to weld something on-site, mild steel is your best friend. It handles the heat of welding much better than heat-treated high-tensile fasteners, which can lose their strength if exposed to extreme heat (such as welding)

2.   High-Tensile Steel (Grade 8.8, 10.9 & 12.9):
When you are dealing with heavy machinery, car engines, or steel fabrication, you need the ‘Heavy Lifters’ of the fastening world. These fasteners undergo a heat-treatment process (quenching and tempering) to make them incredibly hard and strong.

• Grade 8.8: The most common High-Tensile Fastener. These are known as ‘Structural Grade’ fasteners and are the standard for Steel Sheds, Trailers and General Machinery.

• Grade 10.9: These fasteners are 25% stronger than the Grade 8.8 equivalent and are commonly used in the automotive industry and with mechanical equipment. It’s important to distinguish standard Grade 10.9 Bolts from ‘Stretch Bolts’.
Stretch Bolts are designed to stretch permanently and normally cannot be reused after removal.

• Grade 12.9: The strongest metric grade we have available, reserved for high-stress mining machinery and heavy-duty industrial equipment.

The Advantage: These High-Tensile Grades offer exceptional strength-to-weight ratio and superior resistance to fatigue. They are engineered to maintain their clamping force even in environments with heavy vibration or high mechanical loads.

Where to Use It: Use Grade 8.8 for structural framework and standard mechanical joints, Grade 10.9 is commonly used for high-performance automotive and industrial parts, and Grade 12.9 for critical mining gear and high-pressure drives where failure isn’t an option.

Pro-Tip: High-Tensile fasteners require precision. Roughly 90% of the effort you apply when tightening is used just to overcome friction in the threads and under the head, while only 10% goes into the clamping force. To ensure you aren’t just fighting friction, we always recommend using a calibrated torque wrench for heavy-duty high-tensile assembly.

Caution: Don’t fall for the myth that ‘Stronger is always better’. As steel gets harder, it can also become more brittle. In a joint subject to heavy vibration or sudden impact, common in mining, a Grade 12.9 bolt might snap, whereas a Grade 8.8 bolt would have the toughness and elasticity to survive the shock.

03.   The ‘In-Betweeners’: EN8 & Grade 5.6
Sometimes mild steel is a bit too weak, but full high-tensile fasteners are complete overkill. That’s where EN8 & Grade 5.6 fasteners come in.

• Grade 5.6: Often called the "bridge" grade, it sits right in the middle between standard mild steel and structural Grade 8.8. It offers a reliable middle ground for projects that need more backbone than basic hardware.

• EN8: An Engineering Steel that is significantly tougher than mild steel. We frequently supply this for parts that need better durability without the high cost of a specialised structural alloy like Grade 8.8.

The Advantage: These materials provide a specific balance of strength and ‘workability’. Grade 5.6, for example, is less hard than Grade 8.8, which allows it to be slightly manipulated during installation without the risk of cracking or snapping.

Where to Use It: Grade 5.6 is a favourite for agricultural machinery and interior vehicle parts. EN8 is a preferred choice for custom-engineered parts like axles, shafts, gears, and bespoke studs where standard mild steel would simply not work.

Pro-Tip: Grade 5.6 offers gear ‘nudge-ability’, if the holes on the workpiece don’t align with the studs, Grade 5.6 can often be manipulated more easily than a more brittle high-tensile bolt. EN8 is easy to machine while still being strong enough to support moving parts.

04.   Stainless Steel (A2 vs. A4):
If you are busy with a project outside, near water or in a food-handling area, you need stainless steel. This material creates its own natural ‘shield’ against rust and corrosion.

• Grade A2/304: Known as the industrial "workhorse," this grade contains approximately 18% chromium and 8% nickel. It’s perfect for urban areas, food preparation equipment, and general home DIY projects where you want a clean finish to the piece.

• Grade A4/316: Referred to as "Marine Grade," it includes a critical addition of 2–3% molybdenum. This chemical composition significantly boosts resistance to chlorides (salt air) and pitting corrosion. This is your go-to material for coastal and humid areas.

The Advantage: Stainless steel is self-passivating, meaning it builds its own protective oxide layer that can heal itself if scratched. It offers long-term corrosion protection without needing extra coatings or platings.   

Where to use it: Use A2 for projects in non-coastal urban environments or kitchens. Use A4 (Marine Grade) for any installation near the ocean or in chemical processing sites where corrosion is severe.   

Pro-Tip: To prevent "Galling"/Cold Welding, where stainless threads cold weld and lock together permanently during tightening, always apply a specialised lubricant like a dedicated anti-seize compound to the threads before assembly.   

05.   Brass & Nylon
Sometimes your project would not require steel at all. Stainless steel is too expensive, doesn’t conduct a current. And high-tensile steel or mild steel isn’t functional or looks ugly.

• Brass: Brass fasteners are the ‘Gold-Standard’ for electrical components. It conducts power efficiently. Brass is also non-sparking, which is essential if you are working near explosive or flammable substances such as fuel or gas.

• Nylon Fasteners: Nylon, on the other hand, is about 80% lighter than the steel alternatives, but does not conduct electricity. Plastic fasteners are widely popular on solar panels, mounted circuit boards and lightweight components such as drones or robotics. Nylon fasteners handle vibration very well, often staying tight where metal or steel alternatives would work loose.

The Advantage: Brass offers superior electrical conductivity and safe usage in hazardous zones. Nylon provides complete electrical insulation, extreme weight reduction, and superior resistance to vibration induced loosening.   

Where to use it: Use brass for electrical terminals, grounding connections, and decorative hardware. Use nylon for solar arrays, sensitive electronics, and high vibration components where weight saving is a priority.   

Pro-Tip: Brass is significantly softer than steel; tighten with care to avoid stripping the threads or stretching the bolt shank. When using Nylon, store your fasteners in a sealed bag until needed. Nylon is hygroscopic and can absorb moisture from the air, which may cause it to swell or change its mechanical properties.

06.   Understanding the South African ‘Rust Map’
Where you live in South Africa often determines the materials that suit projects best. Most people know the coast as a ‘Rust Zone’, but people forget about the Industrial Areas. Some industrial areas in South Africa have corrosion and rust rates as high as the seaside. This is because industrial pollution and chemicals in the air can be just as aggressive as saltwater. 

• Dry Inland & Rural Areas: Standard Electroplating (Zinc Plating) is usually sufficient. It provides a basic shiny protective layer but will rust if exposed to rain or dampness for long periods.

• Standard Urban Areas: Areas with moderate moisture require more robust protection. Hot Dip Galvanising (HDG) is the rugged choice for outdoor structures because it has a self-healing property where the zinc protects exposed steel if the surface is scratched.

• Industrial Sites & Coastal Regions: The air is aggressive here. You need heavy-duty protection like Stainless Steel (Grade A2) for moderate urban sites, or specialised Zinc Flake and Zinc-Nickel coatings which offer extreme corrosion resistance without making high-strength bolts brittle.

• Shoreline & Chemical Plants: In these extreme environments, salt spray and chemicals will destroy standard steel in months. Marine Grade Stainless Steel (A4), Brass, or Nylon are mandatory as they do not rust and provide the necessary chemical or electrical stability required for these niches. 

07.   Offer The Exceptional…
In the industrial world, even a minor error, such as using the incorrect fasteners in a given environment, can lead to massive failure, hours of fault-finding, and an expensive repair bill.

Don’t leave your projects to that chance. Our sales team at Reliable Bolt Industrial Supplies is dedicated to ensuring you have the right tools for the job. We combine over 40 years of industry experience and local knowledge. And the highest standards to help you find the right fit for your needs.