Using the use the correct grade of fastener is vital. Most everyday automotive applications will use a Grade 5, For our racing clients we recommend Grade 8. Bolts are graded by tensile strength and are easily identified by the number of slash marks on the head of the bolt. The more marks the higher the quality. Beware of fasteners with no marking on top as these are usually soft, mild steel, grade 2 quality and should be avoided as they will easily shear.
Another thing to watch is torque specs. A bolt that has been over tightened can be just as lethal as one that hasn’t been tightened enough. A bolt that has been tightened beyond recommended torque specs can easily break in service. Keep in mind that torque specs will be less for bolts that have oil or lubricant on them than for clean, dry bolts. Use the following tables to determine what grade of bolt you are working with and how tight to torque it.
|I.D. Marks||No markings||3 lines||6 lines|
|Material||Low carbon||Medium-carbon, tempered||Medium-carbon, quenched & tempered|
|Tensile strength (Minimum)||74,000 psi||120,000 psi||150,000 psi|