swinters
Member
I did a repair today for a guy who pulled his winch right off his truck because he had used the wrong bolts to mount it. I've seen a lot of problems over the years where people are using Grade 8 bolts because they're supposed to be stronger and had the bolts break so thought it might save some headaches to explain when Grade 8 bolts are actually weaker than lower grades, such as Grade 5's. Most know the SAE marking system and that you count the number of lines and add 2, so a bolt with 3 lines is a Grade 5, 6 Lines is a Grade 8 and so on. What most people don't know is that you need to use the right bolt for the application. Simplified a bit, bolts normally see one or both of two types of stresses: Tension, and Shear. When the load on the bolt tries to stretch the length of the bolt, the bolt is in tension. When the load tries to cut the bolt in two, like where two plates are trying to slide against each other, the bolt is in shear. Grade 5's are flexible, so are stronger in tension than an 8. The head will pop off, or the bolt will break, on a Grade 8 long before a Grade 5 when in tension but you can cut a 5 in two a lot easier than an 8, so the bolt should be selected based on application. Use a 5 in tension and an 8 in shear.