Fasteners Many types and sizes of fasteners are used in the automotive industry. Each fastener is designed for a specific purpose and condition. One of the most commonly used type of fastener is the threaded fastener. Threaded fasteners include bolts (also called cap screws), nuts, screws and similar items that allow a part to be easily installed or removed. Over the years, a number of terms have been used to identify the various types of fastener threads. The Unified National Series consists of four basic classifications. Unified National Coarse (UNC or NC) (USS) Unified National Fine (UNF or NF) (SAE) Unified National Extrafine (UNEF or NEF) Unified National Pipe Thread (UNPT or NPT) Two thread styles are most commonly used on inch fasteners: UNC-USS coarse thread and UNF-SAE fine thread. Fine thread fasteners provide about 10% more clamping force than coarse thread fasteners of the same size and grade. Clamping force is the amount of tension force that holds two or more pieces together when a fastener is tightened. The two common metric threads are coarse and fine and can be identified by the letters SI (Systeme International d Unites or International System of Units) or ISO (International Standards Organization). Bolts A bolt has several parts (Figure 5). The bolt head is used to tighten the bolt. A socket or wrench fits over the head, which enables the bolt to be tightened. Bolt diameter is the measurement across the major diameter of the threaded area or across the bolt shank. The thread pitch of a bolt in the English system is determined by the number of threads that are in one inch of threaded bolt length and is expressed in number of threads per inch. The thread pitch in the metric system is determined by the distance in millimeters between two adjacent threads. Bolt length is the distance measured from the bottom of the head to the tip of the bolt. The bolt s tensile strength, or grade, is the amount of stress or stretch it is able to withstand. Figure 1. Typical Measurements for a Bolt or Hex Head Cap Screw NAPA FastTrack Counter Sales Training Fasteners Page 1
The type of bolt material and the diameter of the bolt determine its tensile strength. In the English system, the tensile strength of a bolt is identified by the number of radial lines (grade marks) on the bolt head (Figure 6). More lines mean higher tensile strength. In the metric system, tensile strength of a bolt or stud can be identified by a property class number on the bolt head. The higher the number, the greater the tensile strength. Figure 2. Grade Markings for U.S. Bolts and Hex Head Cap Screws Although there are many different grades of fasteners, some are more commonly used than others. Grade 3 fasteners should be used for nonmechanical applications that do not require high strength. Grade 5 fasteners, identified by three radial lines on their head, should be used where moderate strength is required. Grade 8 fasteners, identified by six equally spaced radial lines on their head, are required for critical applications such as flywheels, universal joints, transmission mounts and cases, motor supports, suspensions, and other high-load applications. The most common metric property classes are 8.8, 10.9, and 12.9. A bolt with a lower tensile strength should never be used to replace a bolt with a higher strength. Remind your customers that all fasteners need to be properly tightened. They should refer to and follow the tightening instructions and specifications given by the vehicle s manufacturer. Over tightening and under tightening a bolt can cause it to fail. When a bolt is tightened, it stretches. This stretching provides clamping pressure. If the bolt is under tightened, the pieces that should be held together will be able to move. This can cause a bolt to break, become looser, or cause damage to the pieces. If a bolt is over tightened, the bolt will be stretched beyond its elastic limit. The elastic limit of a fastener is the maximum stress that can be applied to a fastener that will allow it to regain its original dimensions after it is removed. Once a bolt is over tightened, it is longer than it should be and must not be reused. Over tightening can also cause a bolt to break or damage the threads. NAPA FastTrack Counter Sales Training Fasteners Page 2
Nuts Nuts are internally threaded fasteners used with bolts or other externally threaded fasteners. The grade is marked on one of the flat sides of a nut. If no markings are present, the nut is a Grade 2. Nuts and bolts should be matched to not only the same grade, but also the same thread and diameter dimensions. The common types of nuts used on automobiles are the finished hex nut, prevailing torque lock nut, flanged lock nut, nylon insert lock nut, heavy hex nut, hex jam nut, acorn nut, and castellated nut (Figure 7). Figure 3. Common Types of Nuts Used on Automobiles Screws Screws (Figure 8) are headed, threaded fasteners that are either designed to turn into threaded holes or are designed to form their own threads as they are turned. This type of screw includes self-drilling screws, sheet metal screws, wood screws, and thread cutting screws. Screws used in threaded holes or nuts include cap screws, carriage screws, and machine screws. NAPA FastTrack Counter Sales Training Fasteners Page 3
Figure 4. Different Types of Screws Washers Washers are mainly used as a seat for bolts, nuts, and screws. They prevent damage to the head fillet as the bolt is being tightened. They also distribute the tightening load over a greater area than would be provided by the fastener s head. Washers also provide springlike tension between the fastener and the piece receiving the fastener. This helps to keep the bolt from loosening. Some washers, called lock washers, are specifically designed to prevent bolts, nuts, and screws from loosening. Washers can also be used to take up space or seal a bolt or nut. Figure 9 shows some of the common washers used in an automobile. Figure 5. Common Automotive Washers NAPA FastTrack Counter Sales Training Fasteners Page 4
Other Common Fasteners An automobile is held together by many other fasteners. You should be familiar with all of them. Figure 10 shows just a few of these. To learn more about these, refer to your Rockford catalog. Figure 6. Other Common Types of Automotive Fasteners NAPA FastTrack Counter Sales Training Fasteners Page 5
Thread Damage If a fastener has failed or broken, its mating piece must also be replaced or repaired. There are many ways to repair threads and your store sells the tools and equipment to do this. Extractors, tap and die sets, and Heli Coil-thread repair kits are the most commonly used. An extractor is used to remove a broken bolt from its hole. A tap is used to cut new internal threads, while a die cuts external threads. The Heli Coil-thread inserts new threads into a damaged hole. All of these tools can be found in the NAPA Tools and Equipment Catalog. Key Terms Bolt an externally threaded fastener designed to hold components together. Also called a cap screw. Bolt diameter the measurement across the major diameter of the threaded area. Bolt head used to tighten the bolt with a socket or wrench. Bolt shank the diameter of the threaded area of a bolt. Cap screw an externally threaded fastener designed to hold components together. Also called a bolt. Clamping force the amount of tension force that holds two or more pieces together when a fastener is tightened. Grade marks markings on the head of a bolt that indicate its tensile strength. Nuts internally threaded fasteners used with bolts or other externally threaded fasteners. Screws headed, threaded fasteners that are either designed to turn into threaded holes or are designed to make their own threads. Thread pitch in the English system is determined by the number of threads that are in one inch of threaded bolt length and is expressed in number of threads per inch. The thread pitch in the metric system is determined by the distance in millimeters between two adjacent threads. Washers inserts that are mainly used as a seat for nuts, bolts, and screws. NAPA FastTrack Counter Sales Training Fasteners Page 6