Many times we are asked “What grade of bolt should I use ?” Unfortunately, that is a hard question to answer from our side of the counter. We are not architects nor engineers.
However, we can shed light on the basic classifications ….
In the American world (a.k.a. SAE) the main classifications are Grades 2, 5, 8, Allen heads, and Stainless.
Grade 2 bolts ( A307) are a standard hardware grade steel. This is the least expensive and most common. They have a tensile strength of 60,000 PSI.
Grade 5 bolts are a heat treated carbon bolt. Hardened to increase its strength, it’s most commonly found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines … resembling an airplane propeller. They have a tensile strength of 120,000 PSI
Grade 8 bolts are a heat treated alloy steel that are hardened more than the Grade 5. Thus, they are stronger and can be used in more demanding situations. Grade 8 bolts have 6 evenly spaced radial lines. Grade 8 bolts have a tensile strength of 150,000 PSI
Allen head bolts can easily be identified as they require the use of an allen wrench. Industry standard allen heads have a tensile strength of 160,000 PSI
Stainless Hex Bolts ….. there are different grades of stainless but the most common are bolts made in the 303 and 304 series of stainless. Many people think stainless bolts are very hard. This is not true. 300 series stainless is just a tad under a Grade 5 in strength. Usually a tensile strength in the range of 112,000 to 116,000 PSI.
In the metric world (a.k.a. ISO) the most common bolts are marked with an 8.8, 10.9, or 12.9.
If there are no head markings at all, they are most likely comparable to the SAE grade 2.
The 8.8 is comparable to the SAE Grade 5.
The 10.9 is comparable to the SAE Grade 8.
The 12.9 is comparable to the SAE allen head.