{"id":11595,"date":"2026-10-29T18:12:22","date_gmt":"2026-10-30T00:12:22","guid":{"rendered":"https:\/\/apisaf.com.mx\/en\/?p=11595"},"modified":"2026-10-29T18:12:22","modified_gmt":"2026-10-30T00:12:22","slug":"grade-5-vs-grade-8-vs-class-8-8-10-9-choosing-bolts","status":"publish","type":"post","link":"https:\/\/apisaf.com.mx\/en\/blog\/grade-5-vs-grade-8-vs-class-8-8-10-9-choosing-bolts\/","title":{"rendered":"Grade 5, Grade 8, Class 8.8 and 10.9: How to Choose the Right Bolt"},"content":{"rendered":"<p>It is one of the most common questions in purchasing and maintenance: Grade 5 or Grade 8? And how do they compare with metric classes 8.8 or 10.9? The wrong choice can mean a joint that loosens, a bolt that breaks or, the other way around, paying for strength you don&#8217;t need. Here is how to read each system and when to use each grade.<\/p>\n<h2>Two systems: SAE (inch) and ISO (metric)<\/h2>\n<ul>\n<li><strong>SAE J429<\/strong> defines grades for inch fasteners: Grade 2, Grade 5 and Grade 8 are the most common.<\/li>\n<li><strong>ISO 898-1<\/strong> defines property classes for metric carbon and alloy steel fasteners: 4.6, 8.8, 10.9 and 12.9, among others.<\/li>\n<\/ul>\n<p>Both systems measure the same things (tensile strength and yield strength), but with different numbers.<\/p>\n<h2>How to read a metric property class<\/h2>\n<p>In an ISO class, the first number times 100 gives the nominal tensile strength in megapascals (MPa), and the second number gives the yield strength as a fraction of that value:<\/p>\n<ul>\n<li><strong>8.8:<\/strong> 800 MPa nominal tensile; yield at 80%, i.e. 640 MPa.<\/li>\n<li><strong>10.9:<\/strong> 1,000 MPa nominal tensile; yield at 90%, i.e. 900 MPa.<\/li>\n<li><strong>12.9:<\/strong> 1,200 MPa nominal tensile; yield of 1,080 MPa.<\/li>\n<\/ul>\n<h2>Comparison table<\/h2>\n<table>\n<thead>\n<tr>\n<th>Grade \/ class<\/th>\n<th>Standard<\/th>\n<th>Approx. min. tensile<\/th>\n<th>Head marking<\/th>\n<th>Typical use<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Grade 2<\/td>\n<td>SAE J429<\/td>\n<td>74 ksi (\u2248510 MPa), up to 3\/4&#8243;<\/td>\n<td>No marks<\/td>\n<td>General purpose, light loads<\/td>\n<\/tr>\n<tr>\n<td>Grade 5<\/td>\n<td>SAE J429<\/td>\n<td>120 ksi (\u2248830 MPa), up to 1&#8243;<\/td>\n<td>3 radial lines<\/td>\n<td>Automotive, machinery, light structures<\/td>\n<\/tr>\n<tr>\n<td>Grade 8<\/td>\n<td>SAE J429<\/td>\n<td>150 ksi (\u22481,030 MPa)<\/td>\n<td>6 radial lines<\/td>\n<td>Suspension, heavy machinery, high vibration<\/td>\n<\/tr>\n<tr>\n<td>Class 8.8<\/td>\n<td>ISO 898-1<\/td>\n<td>800 MPa nominal<\/td>\n<td>&#8220;8.8&#8221;<\/td>\n<td>Close metric counterpart to Grade 5<\/td>\n<\/tr>\n<tr>\n<td>Class 10.9<\/td>\n<td>ISO 898-1<\/td>\n<td>1,000 MPa nominal<\/td>\n<td>&#8220;10.9&#8221;<\/td>\n<td>Close metric counterpart to Grade 8<\/td>\n<\/tr>\n<tr>\n<td>Class 12.9<\/td>\n<td>ISO 898-1<\/td>\n<td>1,200 MPa nominal<\/td>\n<td>&#8220;12.9&#8221;<\/td>\n<td>Tooling, high-strength socket head cap screws<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><em>Exact values vary with diameter; always check the current standard for your size.<\/em><\/p>\n<h2>What about the A325 structural bolt?<\/h2>\n<p>Structural steel connections use structural bolts such as the <a href=\"https:\/\/apisaf.com.mx\/en\/tornillo-estructural-a325\/\">A325<\/a> (now part of ASTM F3125). Its strength is comparable to Grade 5, but its heavy hex head, thread length and use with a hardened nut and washer are designed for bolted structural joints.<\/p>\n<h2>How to choose<\/h2>\n<ol>\n<li><strong>Start with the load and joint type.<\/strong> If the joint works in tension or shear with high loads or vibration, go up a grade. For light fastening, Grade 2 or 5 is usually enough.<\/li>\n<li><strong>Don&#8217;t mix systems.<\/strong> Don&#8217;t pair a metric bolt with an inch nut, or a Grade 8 bolt with a lower-grade nut: the nut must match the bolt&#8217;s strength.<\/li>\n<li><strong>Higher grade is not always better.<\/strong> Higher grades are harder and less ductile; in some applications with impact loads or acid-based coating processes, the risk of hydrogen embrittlement increases. That is why coating and baking (de-embrittlement) matter.<\/li>\n<li><strong>Check head markings on receipt.<\/strong> The 3 or 6 lines, or the stamped class, plus the manufacturer&#8217;s mark, are your first inspection.<\/li>\n<li><strong>Ask for material and test certificates.<\/strong> For critical applications, request tensile, hardness and chemical composition reports by lot.<\/li>\n<\/ol>\n<p>APISA Fasteners manufactures fasteners in SAE grades and metric classes, such as <a href=\"https:\/\/apisaf.com.mx\/en\/tornillo-g5-hexagonal-rosca-estandar\/\">Grade 5 hex bolts<\/a>, as well as <a href=\"https:\/\/apisaf.com.mx\/en\/custom-fasteners\/\">custom made-to-drawing parts<\/a>. If you&#8217;re unsure which grade your application needs, <a href=\"https:\/\/apisaf.com.mx\/en\/contact-us\/\">contact us<\/a>.<\/p>\n<h2>Frequently asked questions<\/h2>\n<h3>What is the difference between Grade 5 and Grade 8?<\/h3>\n<p>Grade 8 has higher tensile strength (about 150 ksi versus 120 ksi for Grade 5). It is identified by 6 radial lines on the head; Grade 5 has 3.<\/p>\n<h3>Is an 8.8 bolt the same as Grade 5?<\/h3>\n<p>They are close in strength but not identical or interchangeable: one is metric (ISO 898-1) and the other inch (SAE J429), with different threads.<\/p>\n<h3>What do the numbers in a metric class like 10.9 mean?<\/h3>\n<p>The 10 means 1,000 MPa nominal tensile strength, and the .9 means yield strength is 90% of that value, i.e. 900 MPa.<\/p>\n<h2>Sources<\/h2>\n<ul>\n<li><a href=\"https:\/\/www.sae.org\/standards\/content\/j429_201405\/\" rel=\"nofollow noopener\" target=\"_blank\">SAE J429: Mechanical and Material Requirements for Externally Threaded Fasteners<\/a><\/li>\n<li><a href=\"https:\/\/www.iso.org\/standard\/60266.html\" rel=\"nofollow noopener\" target=\"_blank\">ISO 898-1: Mechanical properties of steel fasteners<\/a><\/li>\n<li><a href=\"https:\/\/www.astm.org\/f3125_f3125m-23.html\" rel=\"nofollow noopener\" target=\"_blank\">ASTM F3125: High-strength structural bolts<\/a><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Differences between Grade 5 and Grade 8 bolts (SAE J429) and property classes 8.8, 10.9 and 12.9 (ISO 898-1): strength, head markings and when to use each.<\/p>\n","protected":false},"author":0,"featured_media":11253,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"apisa_traduccion":"https:\/\/apisaf.com.mx\/blog\/grado-5-grado-8-clase-8-8-10-9-como-elegir-tornillo\/","footnotes":""},"categories":[3],"tags":[],"class_list":["post-11595","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-standards-and-quality"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Grade 5 vs Grade 8 vs Class 8.8 and 10.9 Bolts | APISA<\/title>\n<meta name=\"description\" content=\"Differences between Grade 5 and Grade 8 bolts (SAE J429) and 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