This huge torque wrench has...
This huge torque wrench has no place on a small bolt like this one; we just thought it looked cool. Torque values vary depending on the application and dimension of the bolt, as we will learn.
For many racers, nuts and bolts are some of the most misunderstood parts on the car. Many of us do not really know a great deal about these fasteners in general. We might misuse them or make poor selections.
Engine hardware aside, we tend to not really consider the role of a nut or a bolt until it breaks and causes another serious issue. For example, a bolt that holds a weight onto the chassis fails during a race, and the weight falls off the car as a result. You win the race, not necessarily because a 5-pound weight fell off. The tech inspectors find that you were light across the scales after the race, and you are disqualified. A 10-cent bolt causes you to lose a hard-earned victory.
This is an issue that can cause some real grief. It's a very simplistic illustration, but the issues could be much greater (e.g., a suspension system failure). These situations are completely preventable if we learn how to select the right hardware for the given application.
Back in high school, we were introduced to simple machines in science class: the lever, inclined plane, wedge, wheel, axle, and screw. There are really only five simple machines, as the screw is just an inclined plane wrapped around a rod. The thread itself is a variation of the inclined plane.
From a racer's perspective, bolts and nuts are the primary method for holding the car together. We start with a frame of welded tubes and sheetmetal, but the remainder of the ancillary parts are held in place primarily with nuts and bolts, or possibly some rivets.
The components of a bolt are...
The components of a bolt are not that complex. Once you know the nomenclature, it is no big deal. If you want a 51/416x18x111/42-inch bolt, 51/416 is the diameter in inches, 18 is the number of threads per inch pitch, and 111/42 is the length in inches.
Let's spend some time examining how bolts really work. What happens when you tighten a bolt or run a nut down on a stud and torque it into place? A bolt or a stud turns into a spring that exerts a load on the parts you are trying to hold together when it is tightened. It is not that difficult to imagine.
The bolt stretches as it is tightened. As long as you do not tighten it past the point where the metal the bolt is made from yields or breaks, the bolt will exert force as the result of it trying to return to its free state. A collection of springs trying to get back to a free state holds your race car together. This force can be significant; in fact, if you use multiple bolts or studs, the amount of load or clamping force they can place on the structure is quite high.
From both a physical and material perspective, bolts are very well-defined parts. From a performance perspective, there are specifications for multiple characteristics of the bolt: load, grip, shear forces, clamping force, and tension force. The characteristics of the bolt from a manufacturing perspective are also very well defined and highly controlled. Bolts are not as simple as they seem. A good bit of engineering has gone into developing threaded fasteners.
Once you have established the need for a bolt and determined the forces the bolt will endure, you can derive which bolts and nuts you need for a specific application. Points to consider include the following: hex size (the wrench size used to drive the bolt, or this can also be an internal hex drive), length, thread length, thread pitch, and nominal diameter.
Grade 5 bolts have three lines...
Grade 5 bolts have three lines stamped in the head of the bolt. This level is the minimum that should be used on the race car. Typical uses include holding on body parts, lightweight attachments such as tachometers and ignition switches, and so on. Never use this grade to bolt structural components together.
Bolts are also classified by grade (i.e., the level of material used to make the bolt and the amount of heat treatment it has been exposed to). We select a bolt for a given application according to its grade. The most common grades range 2 through 8, with Grade 8 being the hardest and the strongest.
Unfortunately, many racers just use what they have on hand and are not too concerned about grades or specifications. While this is the accepted norm, the proper selection can save you more than money. It could save you from a failure that may have more dire consequences.
Bolts are graded against a scale developed by the SAE (Society of Automotive Engineers). The rating scale starts at Grade 2. Grade 2 bolts are made of low-carbon steel and are usually zinc plated (silver in color). There are no markings on the head of a Grade 2 bolt (more on this later). This grade is usually found in hardware stores and intended for use in low-strength applications, such as gates and fences.
This type of bolt has no place on a race car. Not to say that this grade may not be used to attach sheetmetal panels to each other or be used for some other low-load requirement. The main reason this grade shouldn't be used on a race car is that it may find its way into an application that requires a fastener with a higher grade.
In an emergency situation on the track, use of this grade bolt could be downright dangerous. The best way to prevent this is to make sure that Grade 2 bolts are never on your race car.