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.
A few times each year we receive calls from fastener suppliers who are in conflict with their customer over the quality of stainless steel bolts and nuts. The customer’s complaint is that during installation the bolts are twisting off and/or the bolt’s threads are seizing to the nut’s thread. The frustration of the supplier is that all required inspections of the fasteners indicate they are acceptable, but the fact remains that they are not working. This problem is called “thread galling.” According to the Industrial Fastener Institute’s 6th Edition Standards Book (page B-28), Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminum, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing – the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out. Carpenter Technologies, the fastener industry’s largest supplier of stainless steel raw material, refers to this type of galling in their technical guide as “cold welding.” Anyone who has seen a bolt and nut with this problem understands the graphic nature of this description.