Bolt Grade Guide: SAE & Metric Strength Ratings

Understand bolt grade markings for SAE (Grade 2, 5, 8) and metric (8.8, 10.9, 12.9) fasteners. Learn tensile strength, proof load, and when to use each grade.

<p>A bolt's <strong>grade</strong> (SAE) or <strong>class</strong> (metric) tells you how strong it is — its ultimate tensile strength, its yield point, and how much clamping force it can safely hold. Using the wrong grade is one of the most common and most dangerous fastener mistakes a DIYer can make: a bolt that looks identical can be less than half as strong, and a grade that's too low in a suspension, brake, or engine joint can shear under load. This guide explains every common marking so you can read a bolt head at a glance and match the manufacturer's spec with confidence.</p>

<div><span class="font-bold text-amber-800">Never downgrade a fastener.</span><span class="text-amber-700"> If the manufacturer calls for Grade 8 or class 10.9, a lower-grade bolt may fail even when torqued correctly. Grade also affects the target torque value — a stronger bolt is usually torqued higher. When in doubt, match the original exactly.</span></div>

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<h2 class="text-2xl font-bold text-slate-900 mb-2">What Do Bolt Grade Markings Mean?</h2>

<p class="text-slate-600 text-sm mb-4">SAE and metric fasteners use two different marking systems stamped right on the bolt head.</p>

<p>SAE bolts use raised <strong>radial lines</strong> on the head to indicate strength: no lines means Grade 2, three lines means Grade 5, and six lines means Grade 8 — count the lines and add two. Metric bolts instead carry a stamped <strong>two-number class</strong> such as 8.8, 10.9, or 12.9. There are no lines to count; you read the printed number directly. Both systems are widely used, and many vehicles mix them, so it pays to recognize both.</p>

<h2 class="text-2xl font-bold text-slate-900 mb-2">SAE Bolt Grades Explained</h2>

<thead><tr style="background-color:#01696F;color:#fff"><th class="text-left px-4 py-3 font-semibold">Grade</th><th class="text-left px-4 py-3 font-semibold">Head Markings</th><th class="text-left px-4 py-3 font-semibold">Material</th><th class="text-left px-4 py-3 font-semibold">Tensile Strength</th><th class="text-left px-4 py-3 font-semibold">Typical Use</th></tr></thead>

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<tr class="bg-white"><td class="px-4 py-3 font-semibold text-slate-800">Grade 2</td><td class="px-4 py-3 text-slate-700">None</td><td class="px-4 py-3 text-slate-700">Low/medium carbon steel</td><td class="px-4 py-3 font-bold whitespace-nowrap" style="color:#01696F">74,000 psi</td><td class="px-4 py-3 text-slate-700">Non-critical hardware, brackets, trim</td></tr>

<tr class="bg-slate-50"><td class="px-4 py-3 font-semibold text-slate-800">Grade 5</td><td class="px-4 py-3 text-slate-700">3 radial lines</td><td class="px-4 py-3 text-slate-700">Medium carbon, quenched & tempered</td><td class="px-4 py-3 font-bold whitespace-nowrap" style="color:#01696F">120,000 psi</td><td class="px-4 py-3 text-slate-700">General automotive — e.g. the <a href="/vehicles/toyota/camry/2020/" class="font-medium hover:underline" style="color:#01696F">2020 Toyota Camry</a></td></tr>

<tr class="bg-white"><td class="px-4 py-3 font-semibold text-slate-800">Grade 8</td><td class="px-4 py-3 text-slate-700">6 radial lines</td><td class="px-4 py-3 text-slate-700">Medium carbon alloy, quenched & tempered</td><td class="px-4 py-3 font-bold whitespace-nowrap" style="color:#01696F">150,000 psi</td><td class="px-4 py-3 text-slate-700">High-stress: suspension, axles, the <a href="/vehicles/ford/f-150/2022/" class="font-medium hover:underline" style="color:#01696F">2022 Ford F-150</a></td></tr>

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<p>Grade 5 is the workhorse of most automotive applications, while Grade 8 is reserved for high-stress, safety-critical areas like suspension links, axle hardware, and engine mounts. Grade 2 is fine for light brackets and interior trim but should never appear in a load-bearing joint. Stainless fasteners (marked 18-8, A2, or A4) resist corrosion well but are generally <em>weaker</em> than Grade 5 — do not assume "stainless" means "stronger."</p>

<h2 class="text-2xl font-bold text-slate-900 mb-2">Metric Bolt Classes Explained</h2>

<thead><tr style="background-color:#01696F;color:#fff"><th class="text-left px-4 py-3 font-semibold">Class</th><th class="text-left px-4 py-3 font-semibold">Tensile (MPa)</th><th class="text-left px-4 py-3 font-semibold">Yield (MPa)</th><th class="text-left px-4 py-3 font-semibold">SAE Equivalent</th><th class="text-left px-4 py-3 font-semibold">Common Use</th></tr></thead>

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<tr class="bg-white"><td class="px-4 py-3 font-semibold text-slate-800">4.6</td><td class="px-4 py-3 font-bold" style="color:#01696F">400</td><td class="px-4 py-3 text-slate-700">240</td><td class="px-4 py-3 text-slate-700">~Grade 2</td><td class="px-4 py-3 text-slate-700">Low-strength general hardware</td></tr>

<tr class="bg-slate-50"><td class="px-4 py-3 font-semibold text-slate-800">8.8</td><td class="px-4 py-3 font-bold" style="color:#01696F">800</td><td class="px-4 py-3 text-slate-700">640</td><td class="px-4 py-3 text-slate-700">~Grade 5</td><td class="px-4 py-3 text-slate-700">General automotive</td></tr>

<tr class="bg-white"><td class="px-4 py-3 font-semibold text-slate-800">10.9</td><td class="px-4 py-3 font-bold" style="color:#01696F">1000</td><td class="px-4 py-3 text-slate-700">900</td><td class="px-4 py-3 text-slate-700">~Grade 8</td><td class="px-4 py-3 text-slate-700">High-strength: suspension, brakes</td></tr>

<tr class="bg-slate-50"><td class="px-4 py-3 font-semibold text-slate-800">12.9</td><td class="px-4 py-3 font-bold" style="color:#01696F">1200</td><td class="px-4 py-3 text-slate-700">1080</td><td class="px-4 py-3 text-slate-700">> Grade 8</td><td class="px-4 py-3 text-slate-700">Extreme stress: engine internals</td></tr>

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<div><span class="font-bold text-blue-800">The quick formula.</span><span class="text-blue-700"> For a metric class <em>a.b</em>: ultimate tensile strength ≈ <strong>a × 100 MPa</strong>, and yield strength ≈ <strong>a × b × 10 MPa</strong>. So a 10.9 bolt is about 1,000 MPa tensile and 900 MPa yield. A metric 8.8 is the rough equivalent of an SAE Grade 5.</span></div>

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<div class="text-xs font-semibold uppercase tracking-wide text-slate-500 mb-1">Recommended Tool <span class="ml-1.5 normal-case font-normal">· grade-correct torquing</span></div>

<h3 class="font-bold text-base sm:text-lg leading-snug text-slate-900">1/2" Drive Click Torque Wrench</h3>

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<p class="text-sm text-slate-600 mb-4 pl-8">A bolt's grade sets how hard you can pull it — guessing by feel either under-clamps a joint or snaps the head. A calibrated click wrench lets you hit the exact spec for the grade you're installing.</p>

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<h2 class="text-2xl font-bold text-slate-900 mb-2">Choosing the Right Grade for Your Job</h2>

<p>Always match the manufacturer's specification, especially in safety-critical locations. When you replace a bolt, check its head marking and buy the same grade — or higher only if the service manual allows it. Never substitute a lower grade (such as an 8.8) where a 10.9 or 12.9 is required. To find the exact OEM fastener and torque for your vehicle, use our free <a href="/vehicles" class="font-medium hover:underline" style="color:#01696F">Vehicle Fastener Specs</a> tool.</p>

<h2 class="text-2xl font-bold text-slate-900 mb-2">Frequently Asked Questions</h2>

<div><h3 class="font-bold text-slate-800 mb-1">Can I use a Grade 8 bolt where Grade 5 is specified?</h3><p class="text-slate-600 text-sm">Usually yes for strength, but not always. A higher-grade bolt is harder and less ductile, and some joints are designed to flex slightly or to have the bolt yield before a more expensive part. Follow the service manual rather than assuming stronger is better.</p></div>

<div><h3 class="font-bold text-slate-800 mb-1">What does "10.9" actually mean?</h3><p class="text-slate-600 text-sm">The first number times 100 is the approximate ultimate tensile strength in MPa (10 × 100 = 1,000 MPa), and the two numbers multiplied by 10 give the yield strength (10 × 9 × 10 = 900 MPa).</p></div>

<div><h3 class="font-bold text-slate-800 mb-1">Are stainless steel bolts stronger than graded steel?</h3><p class="text-slate-600 text-sm">No. Common 18-8 (A2) stainless is corrosion-resistant but typically weaker than a Grade 5 / class 8.8 bolt. Use stainless for rust resistance, not for high clamp load.</p></div>

<div><h3 class="font-bold text-slate-800 mb-1">My bolt has no markings — what grade is it?</h3><p class="text-slate-600 text-sm">No radial lines on an SAE bolt means Grade 2 (low strength). Treat unmarked hardware as non-structural and replace it with a marked, known grade in any load-bearing location.</p></div>

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<p>Next, learn how to confirm the right diameter and pitch in our <a href="/thread-pitch" class="font-medium hover:underline" style="color:#01696F">Thread Pitch Guide</a>, see the differences between systems in our <a href="/knowledge-hub/metric-vs-standard" class="font-medium hover:underline" style="color:#01696F">Metric vs Standard Bolts</a> guide, or browse the full <a href="/bolt-size-chart" class="font-medium hover:underline" style="color:#01696F">Bolt Size Chart</a>.</p>