The worst mistake you can make with automotive fasteners is to assume that all are created equal. Even though two bolts might look identical with the same length, diameter and threads, they could have different strength grades, which suggest utterly different uses and locations. One of the most important things about deciding what set of fasteners to use is their strength. This is vital for OEMs during the production process and can be even more crucial for aftermarket users. Here is how you can determine the strength or grade class of automotive fasteners using bolts as an example.
Metric or Imperial
There are two main sets of standards that are commonly used in the industry – imperial and metric. In the world of automotive fasteners, mainly bolts, screws and washers, there are three main classes:
SAE Grade 2 (Metric 5.8): This is the most common grade made of low or medium-carbon steel used throughout the vehicle, primarily for interiors, body hardware, brackets and trim. You can quickly recognise it since the bolt’s head has no markings or has “5.8” stamped on it. The Grade 2 bolts have proof of load 385M Pa.
SAE Grade 5 (Metric 8.8): Made out of quenched and tempered medium carbon steel, Grade 5 bolts are significantly stronger than Grade 2 and have proof of load around 600M Pa. This means that they are used on more substantial components like key brackets, steering and suspension components. The 8.8 bolts can be recognised by three lines in the head of the bolt and/or the number 8.8 stamped on them.
SAE Grade 8 (Metric 10.9): Produced from medium alloy carbon steel, which is quenched tempered, the 10.9 Class bolts are much more robust with proof of load around 840M Pa. Due to their extreme strength, they are used in torqued engine components, main drivelines, and suspension parts. You can recognise them by six lines pressed in the head of the bolt.
Metric 12.9 (Exceeds Grade 8): This is the highest-grade fastener designed to endure extreme forces; it is not commonly used. Made of alloy steel (quenched and tempered), it has proof of over 1100M Pa load.
How To Determine Which Type To Use
The grades or classes are just the classifications regarding the materials or proof of load. However, they are not the only determining factor you should consider. Even more important are the metallurgy, tensile strengths, ductility and toughness. The tensile strength is the total strength of the material. Ductility is the amount of stretching the fastener can do before it is going to snap and the fatigue resistance and ability to endure stress. Those three factors will be more critical for your application.
For example, you might think that bolts holding the seat belt would be from Grade 8, at least. They are either Grade 2 or 5, which might sound strange, but there is perfectly valid logic behind that decision. As the bolt grades get higher, they are more durable but also more brittle and are known for snapping when they reach the maximum load. Therefore, the lower-grade bolts are known to be more ductile, which can stretch a little under the circumstances and will not snap instantly.
A similar approach can be used when deciding which bolts to use in the engine bay or suspension. Higher-grade bolts with minimal tensile strength should be used in structural components, brackets and mounts since they will resist vibrations and firmly keep the whole assembly. In aftermarket applications, especially with vehicles designed to withstand high forces or in racing cars, it is safe to assume that you should always go a grade higher than stock.
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