THREADED FASTENER GRADES
by Mike Mavrigian
photos by author
Grade 5 steel bolt. Three radial line marks. Theoretically, this bolt will feature a 120,000 psi minimum tensile strength.
The grade label specifies the minimum strength properties the fastener is intended to meet. Industrial fasteners are also marked with a label that identifies the maker of the fastener. Grade markings on domestic fasteners meet ASTM and SAE specs, while metric fasteners meet ISO and SAE specs.
Bolts whose heads are unmarked can be assumed to carry a grade of either 1, 2 or 4. These bolts will be made from low or medium carbon steel. At best, they may be cold-drawn, but certainly have not been tempered.
Contrary to popular opinion, there are more than only three grades of bolts (the most popular being grades 3, 5 and 8). There are about 17 grades (other grades may exist in aerospace or other niche industries).
A threaded fastener’s grade indicates its tensile strength and hardness. The higher the clamping load required, the higher the grade you’ll need. In automotive applications, although it’s difficult to make broad generalizations, you’ll need at least Grade 8 (or 12.9 in metric) for high-clamping load applications, such as connecting rods, cylinder heads and main caps, flywheel bolts, clutch cover bolts, etc. Never use any threaded fastener below a Grade 5 (or 8.8 metric) for any automotive application, regardless of the area of use. Always follow the vehicle or aftermarket component maker’s recommendation for fastener grade.
Grade 5 fasteners are made from medium-carbon steel that has been quenched and tempered. Minimum tensile strength is 120,000 psi for bolts up to one-inch diameter (tensile strength drops for diameters larger than 1 inch). Grade 5 bolt heads are marked with three radial lines. Rockwell hardness for bolts up to one-inch diameter is usually C25-34.
Grade 8 fasteners generally feature a minimum tensile strength of 150,000 psi and a Rockwell hardness of C38-39. Conventionally, a head marking of six radial lines indicate Grade 8.
Grade 9 fasteners (also called by trade names L9, PFC9, Tru-Torq and Bow Malloy) feature taller heads and 180,000 psi tensile strength and Rockwell hardness C38-42.
INCH NUT GRADES
As far as nuts are concerned, there are basically three grades: 2, 5 and 8. In general terms, always use a nut of the same or higher grade as the bolt in use. Grade 2 nuts don’t have to be marked. All Grade 5 and Grade 8 nuts (in the one-quarter-inch to one-and-a-half-inch range) do require markings.
The identification marks may be handled in one of three ways: grade 5 nuts may be marked with a single dot on the face of the nut and a radial or circumferential line at 120-degrees counterclockwise from the dot; or a dot at one corner and a radial line at 120-degrees clockwise from the nut; or one notch at each of the six corners of the nut. Grade 8 nuts may be marked with a dot on the nut face, with a radial or circumferential line at 60-degrees counterclockwise from the dot; or a dot at one corner and a radial line at 60-degrees clockwise; or two notches at each of the nut’s six corners. Why can’t this be simpler, with one type of mark for each grade? It’s one of life’s great mysteries. It sounds as though there were too many cooks in the kitchen when this whole scenario was devised.
As with most things metric, the numbers actually mean something. When a metric bolt head is marked for grade identification, you’ll see two or three digits with a decimal point. The first number refers to 1/10 of the minimum tensile strength (in kgf/mm2, it’s not important for you to understand the increments of measure used here; simply know that the numbers actually represent the bolt’s strength properties). The second figure, following the decimal point, represents 1/10 of the ratio between the minimum yield stress and the minimum tensile strength, expressed as a percentage. As an example, let’s consider a bolt head marked with a identification 8.8 label. The first number represents 1/10 the bolt’s minimum tensile strength (in this case, 80 kgf/mm2). The second number represents 1/10 of the ratio between tensile and yield. The higher the second number, the longer it takes to bring the bolt to yield.
Wanna know more? The first one or two numbers (the number before the decimal point) indicates minimum tensile strength in Mpa. (Note: Mpa is the symbol for megapascals, which is a metric unit of pressure or stress equal to one Newton per square millimeter, which equals about 145.038 pounds per square inch). The final number (after the decimal point) indicates 1/10 of the ratio between minimum yield stress and minimum tensile stress. For example: if the bolt head is marked 8.8, this indicates a minimum tensile strength of 800 Mpa (about 116,030 psi) and a yield stress of 0.8 x 800, or 640 Mpa (about 92,824 psi). Metric screws are manufactured in Grades 3.8 to 12.9, but for automotive applications, common grades include 8.8, 10.9 or 12.9 (tensile strength increases as the numbers grow).
If we want to approximately compare metric grades to U.S. grades, a metric 8.8 is roughly equivalent to a Grade 5. Grade 10.9 is roughly equal to a Grade 8; and 12.9 is roughly equal to a Grade 9. Metric nuts are marked with a single or double numerical symbol (8, 10 or 12). Always match bolts and nuts of comparable grades (use a Grade 8 nut with a Grade 8 bolt; use a metric grade 10 nut with a 10.9 grade bolt, etc.). When dealing with metric fasteners, the 8.8 bolts are similar to a Grade 5. If you need higher tensile strength in metric, stick to 10.9 or 12.9.
The higher the first number, the stronger the bolt in terms of tensile strength. The higher the second number, the longer it will take to enter the yield point.
Note: Metric steel bolts are only required to show a grade mark if they are 6mm and larger and/or are Grade 8.8 or higher. Nuts must be grade labeled if they are Grade 8 or higher.