When repairing or replacing any component on your car, the possibility exists that one or more of the original bolts, washers or nuts will be lost, stripped or broken during the process. It is probably one of Murphy’s Laws, but it is usually that last piece need to complete the installation is the one that gives you the most trouble. We all have coffee cans or mayonnaise jars full of miscellaneous hardware in the garage that were extras or leftover from a previous project, but finding the one item that is the correct size or thread pitch is not necessarily the end of the story – you should also make sure that the hardware has the same strength limitations as the other pieces you are using.
Hardware can be made from many different materials and each of these materials have different mechanical properties for their minimum yield strength and their minimum tensile strength. An industry standard grading of automotive and industrial hardware helps identify the hardware that has been removed and is the same grading system used to aid you in finding the correct replacement hardware of equal or greater strength to meet the factory recommended original requirements.
Bolts are the easiest to identify because each bolt has been marked with a series of radial lines that tell you how much stress it can handle. Tensile strength is the ability of the component metal to be stressed before it will fail. Because even metals have an amount of elasticity, they can often be stressed but still return to form. The yield strength is a point where the component materials deform without having the ability to revert back to their original shape. Rated hardware will have a lower yield strength than their breaking tensile strength, however, once the metal has been stretched, it is recommended it be replaced, as this stretching reduces its tensile strength under subsequent stresses.
Bolts with no head markings at all are referred to as Grade 2. They are manufactured from low or medium carbon steel and used extensively where strength is not an issue, usually in household applications, but hardly ever on an automobile except for carriage bolts. Grade 2 bolts have a yield strength of 57,000 PSI and a tensile strength of 74,000 PSI, provided they are under 3/4-inch in diameter. Larger sizes up to 1-1/2 inches have a yield strength of 36,000 PSI and a tensile strength of 60,000. Most automotive hardware is Grade 5 rated and is identified by three radial lines on the head of the bolt. They are made from a medium carbon steel that is quenched and tempered. Quenching and tempering are rapid heating and cooling processes performed on the hardware after shaping that provide additional strength to the steel. Grade 5 hardware up to an inch in diameter is rated for 92,000 PSI yield strength and 120,000 PSI tensile strength. Bolts that are one inch and larger have weaker ratings, yield strength goes down to 81,000 PSI and tensile strength goes down to 105,000 PSI. Grade 8 bolts are made of a medium carbon alloy steel that is also quenched and tempered and are the most durable bolts you can purchase for automotive use. They can be identified by the five radial lines on the head of the bolt and are usually (but not necessarily) gold in color. Yield strength on Grade 8 bolts jumps to 130,000 PSI and tensile strength is 150,000 PSI. Grade 8 bolts are usually found in critical fastening applications, such as suspension parts, cylinder head bolts and flywheel bolts.
Metric bolts use their own numbering system as well, unmarked bolts are often used for light-duty automotive applications such as plastic retention or used with body retaining clips. Metric bolts have their own rating system based on Mega Pascals (MPas). A Mega Pascal is equivalents to about 145 PSI. Grade 8.8 bolts are manufactured from medium carbon steel that is quenched and tempered. Bolts (below 16 millimeters) have a yield strength of 640 MPa (92,800 PSI) and a tensile strength of 800 MPa (116,000 PSI). Heavier duty Grade 10.9 bolts are made of quenched and tempered alloy steel and have a yield strength of 940 MPA or 136,000 PSI and a tensile strength of 1040 MPa or 151,000 PSI. The premium metric bolts used in the most critical stress areas are Grade 12.9. They are also made of alloy steel that is quenched and tempered and have a yield strength of 1100 MPa or 160,000 PSI and a tensile strength of 1220 MPa or 177,000 PSI. Metrics will sometimes be made more identifiable by a red coloring, but finding the head mark on the bolt is the only way to be sure it is strong enough for your application.
Stainless steel bolts are usually made with a steel alloy that contains up to 19 percent chromium and 13 percent nickel. Standard thread stainless bolts are rated as 18-8 and have roughly the same yield and tensile strength as a Grade 5 bolt. Metric stainless bolts are rated as A-2 and are rated slightly under the standard metric 8.8 ratings.
When replacing nuts, washers or lock washers, it is essential that you use hardware of the same grade or higher to ensure the bolts can still meet the specified strengths for each grade. A Grade 8 bolt can be compromised when held in place with a grade 5 or nylon-insert style nut. When the original nut or bolt is missing, assume it is at least a Grade 5 or metric 8.8 when selecting replacement hardware.