In this article, we take a detailed look at the key considerations when comparing two fundamentally different types of bolts — namely, 12.9 grade alloy steel fasteners and stainless steel fasteners. If you've ever wondered whether stainless steel bolts can be used in place of high-tensile ones, this article is for you.

We'll explore not only their similarities and differences but also the practical implications of substituting one for the other in real-world applications. It's a question we field regularly — from tradies and engineers in Maroochydore and Warana through to DIY builders in Montville and out around Glass House Mountains.

Metric vs Imperial – A Quick Refresher

Before comparing materials, it's worth noting that different regions and industries use different measurement systems to describe fasteners. The two dominant systems are metric and imperial, each with its own terminology, testing methods, and mechanical rating systems. Understanding which system you're working within is critical before making any comparison or substitution.

Measured In: Metric: Megapascals (MPa) Imperial: Pounds per Square Inch (PSI)

Grade Names: Metric: Property classes (e.g. 4.6, 8.8, 10.9, 12.9) — defined under ISO 898-1 Imperial: SAE Grades (e.g. Grade 2, 5, 8) or ASTM designations (e.g. A325, A490)

Strength Ranges: Metric: From property class 4.6 (low-carbon steel) to 12.9 (high-tensile alloy steel) Imperial: From SAE Grade 2 (low-carbon) to Grade 8 (high-tensile) — note that "Grade 10" is not a standard SAE designation and should not be relied upon without manufacturer verification

Thread Pitches: Imperial: Unified National Coarse (UNC) and Unified National Fine (UNF) Metric: Metric Coarse (M) and Metric Fine (MF)

Thread pitch affects how a fastener engages with its mating part. Fine threads offer better load-carrying capacity and vibration resistance, while coarse threads are more forgiving of minor damage and easier to assemble in softer materials. Choosing the wrong pitch is one of the more common causes of thread stripping — and it's an entirely avoidable problem with the right knowledge upfront. For a deeper dive into the distinctions between the two systems, this article covers metric vs imperial fasteners in full.

Substituting a Fastener – Why It's Not Always Straightforward

At first glance, fasteners that appear similar in size and shape might seem interchangeable. They rarely are. The mechanical properties, material composition, heat treatment, and manufacturing standard all determine how a fastener performs under load — and even small variations can produce dramatically different outcomes in service.

When considering substitution, the first step is always to identify the standard the original fastener was specified to. A metric bolt and an imperial bolt of similar diameter may look nearly identical, yet differ in thread pitch, proof load, and surface treatment requirements. Without that foundational knowledge, substitution is guesswork — and guesswork has no place in load-bearing or safety-critical assemblies.

For a structured overview of how grades are classified and what they mean in practice, our article on understanding bolt grades is a good starting point. That brings us to the central question: the difference between stainless steel fasteners and high-tensile steel fasteners, particularly those manufactured to Grade 12.9 standards.

Can You Substitute a Stainless Steel Fastener for a High-Tensile Fastener?

In most cases — no. Standard stainless steel fasteners, such as A2-70 (304) or A4-70/A4-80 (316), are designed primarily for corrosion resistance. Their property class designations (70 or 80) indicate minimum tensile strengths of 700 MPa and 800 MPa respectively. This sits well below the performance of a Grade 12.9 high-tensile fastener, which carries a minimum tensile strength of 1,220 MPa and a proof load stress of 1,100 MPa. To put that plainly: a standard A4-70 stainless bolt has roughly 57% of the tensile strength of a Grade 12.9 bolt of the same diameter.

That's not a rounding error — that's a meaningful structural difference, and substituting one for the other in a high-load application can lead to deformation, thread stripping, shear failure, or worse. For more on how 12.9 grade steel is made and why its properties are achieved through alloy selection and heat treatment, that article explains the process in detail. There are exceptions — some manufacturers produce high-strength stainless fasteners using precipitation-hardened alloys such as 17-4 PH (AISI 630) or cold-worked A4-80 variants.

These can approach or, in limited cases, match the strength of conventional high-tensile grades while retaining corrosion resistance. However, they come with real-world trade-offs: higher cost, limited availability in standard sizes, differing friction coefficients that affect torque calibration, and the need for careful application-specific assessment. These options exist well outside the general-purpose fastener landscape. If you're not certain what you need, contact us directly — we've helped customers from Noosaville to Coolum Beach and all the way down to Caloundra sort this out without the guesswork.

Are Stainless Steel Bolts Stronger than High-Tensile Bolts?

In terms of tensile strength, no — not under standard grades. High-tensile alloy steel bolts are engineered for maximum load-bearing capacity, shear resistance, and durability under cyclic and dynamic stress. These properties make them the correct choice for automotive suspensions, engine mounts, heavy machinery, and structural steel connections. Our high-tensile fastener range covers the grades and sizes most commonly used across these industries. Stainless steel fasteners are not optimised for tensile strength — they're optimised for chemical stability.

They resist oxidation, rust, and staining, making them the go-to choice in marine, food-processing, architectural, and chemical processing environments where corrosion is the primary threat. For anyone working near the coast — whether in Mudjimba, Twin Waters, Peregian Beach, or out on the water — our article on using steel in a marine environment covers what you actually need to know. The comparison isn't simply about which material is "stronger." In a corrosive environment, a high-tensile carbon steel bolt that seizes, corrodes, or loses preload due to surface degradation may functionally underperform a stainless bolt of lower rated strength.

Context — loading type, environment, maintenance access, and service life — determines the right answer. Our steel types comparison article covers the broader landscape if you want to dig into the trade-offs across materials.

Why Would Someone Want to Substitute Stainless Steel for High-Tensile?

The appeal of stainless is real. The finish is clean, the corrosion resistance is excellent, and in many industries — marine, architectural, food production — stainless hardware is the standard. That's entirely appropriate where the application calls for it. The problem arises when stainless is chosen for structural or high-load applications where strength is the primary requirement.

Substituting a lower-rated stainless bolt into a joint designed for Grade 12.9 can reduce clamping force, compromise fatigue life, and in some jurisdictions void compliance with engineering standards or certifications. It's a mistake that tends to be invisible right up until it isn't. Legitimate reasons to consider stainless over coated high-tensile steel do exist: low-load, high-visibility components where aesthetics and corrosion resistance are the priority; environments with aggressive chemical exposure that would rapidly degrade carbon or alloy steel coatings; and applications that require regular disassembly, where stainless threads are far less prone to seizing than carbon steel in corrosive conditions.

If you're restoring an older vehicle, it's worth reading our piece on why fitting modern fasteners to a vintage chassis isn't always the right call — a different kind of substitution problem, but the same principle applies. And for anyone starting from first principles, what do I need to know about fasteners is worth a read before making any substitution decision. It's a question that comes up equally in coastal workshops in Marcoola and hinterland sheds out past Eumundi — and the answer is always the same: specification first, material second.

The Problem with Using Inadequate Fasteners

The risks extend well beyond mechanical failure in isolation. Avoiding common problems with bolts starts with specification — and substituting a high-tensile fastener with an underrated stainless one introduces several specific failure modes. Loss of preload is a significant concern: stainless alloys generally have a lower elastic modulus and yield strength than high-tensile alloy steel, meaning they stretch more under the same clamping load.

Over time — particularly under vibration — this leads to joint relaxation and loosening. There's more on how tight is tight enough in a dedicated article. Galling and cold welding are also genuine risks. Stainless-on-stainless thread engagement is prone to galling — a form of adhesive wear where threads cold-weld under load, making disassembly destructive.

A quality anti-galling lubricant — such as those available through Molytec or Inox — is essential when working with stainless fasteners. For more on drilling, tapping, and torquing stainless steel correctly, that article covers the practical steps in detail. Under impact or dynamic loading, an underspecified bolt can also fail suddenly and without meaningful warning deformation. In automotive, civil, or industrial contexts, failure of a fastener in a critical joint can have consequences well beyond the bolt itself.

Grade 12.9 fasteners are specified for suspension components, engine mounts, and drivetrain assemblies for good reason. Whether you're working in a workshop in Woombye, Landsborough, or Eudlo, using the wrong fastener in these applications is a risk that simply isn't worth taking. For a practical overview of the most common fastener mistakes and how to avoid them, DIY without the drama covers the basics clearly.

Additional Considerations: Coatings and Corrosion Protection

A common reason for reaching for stainless is to sidestep corrosion issues associated with carbon steel. It's a reasonable instinct, but modern high-tensile fasteners are available with a range of protective coatings that provide meaningful corrosion resistance without the strength trade-off. Options include zinc electroplating, hot-dip galvanising, black oxide, Geomet, and silver-zinc coatings — each suited to different exposure levels and environments.

For outdoor and exposed applications, our article on how to stop your outdoor fasteners corroding is a practical guide to choosing the right protection. It's also worth comparing stainless steel versus galvanised steel when working through outdoor or coastal applications — particularly relevant for builders and renovators in exposed areas like Golden Beach, Bokarina, or Buddina.

For a broad selection of coated high-tensile options, our general hardware range and trusted brands including Bremick, Brighton Best International, and Macsim Fasteners cover a wide range of coated and treated fasteners suited to demanding environments. You can also explore our full range of brands if you're looking for something specific. If you're uncertain which coating suits your application, get in touch — it's exactly the kind of question we enjoy answering.

In Conclusion

When selecting or replacing a fastener, physical dimensions alone are not sufficient. The mechanical properties, material grade, heat treatment, and application environment must all align with the original specification — or be verified by a qualified engineer before any substitution is made. Grade 12.9 high-tensile fasteners are purpose-built for demanding, high-load scenarios.

Replacing them with standard stainless steel bolts of equivalent dimensions can significantly reduce clamping force, fatigue life, and structural integrity — with consequences that may not be immediately apparent but can be serious over time. Where corrosion resistance is the priority, the better path is to explore coated high-tensile options or, where strength requirements genuinely allow for it, properly rated stainless grades — rather than making an uninformed like-for-like swap.

Always check the specification. When in doubt, ask someone who knows. You can browse our full fastener range or reach out to us directly — we're based in Maroochydore and ship Australia-wide. Both materials have a rightful place in the fastener toolkit: high-tensile steel delivers unmatched strength and fatigue resistance for mechanical and structural applications, while stainless steel provides superior corrosion resistance and long service life in harsh or aesthetic-critical environments. Use each for what it was designed for — and your assemblies will thank you for it.