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Fasteners are used to secure or hold different parts together or to mount a component. Many types and sizes of fasteners are used in automobiles. Each fastener is designed for a specific purpose and condition. The most commonly used is the threaded fastener. Threaded fasteners include bolts, nuts, screws, and similar items that allow for easy removal and installation of parts. The threads can be cut or rolled into the fastener. Rolled threads are 30% stronger than cut threads. They also offer better fatigue resistance because there are no sharp notches to create stress points. There are four classifications for the threads of Imperial fasteners: Unified National Coarse (UNC), Unified National Fine (UNF), Unified National Extrafine (UNEF), and Unified National Pipe Thread (UNPT or NPT). Metric fasteners are also available in fine and coarse threads. NPT is the standard thread design for joining pipes and fittings. There are two basic designs: tapered and straight cut threads. Straight cut pipe thread is used to join pipes but it does not provide a good seal at the joining point. Tapered pipe threads provide a good seal because the internal and external threads com press against each other as the joint is tightened. Most often a sealant is used on pipe threads to provide a better seal. Pipe threads are commonly used at the ends of hoses and lines that carry a liquid or gas. Coarse (UNC) threads are used for general- purpose work, especially where rapid assembly and disassembly are required. Fine threads (UNF) are used where greater holding force is necessary. They are also used where greater vibration resistance is desired. --- Various sizes of pipe fittings used with lines and hoses. --- Basic terminology for bolt identification. Head wrench size Shank Length Bolt diameter ---- STANDARD BOLT HEAD SIZES
Bolts have a head on one end and threads on the other. They are identified by their head size, shank (shoulder) diameter, thread pitch, length, and grade. The threads of a bolt travel from below the shank to the end of the bolt. The bolt head is used to loosen and tighten the bolt; a socket or wrench fits over the head and is used to screw the bolt in or out. The size of the bolt head varies with the bolt's diameter and is available in Imperial and metric wrench sizes. Many confuse the size of the head with the size of the bolt. The size of a bolt is the diameter of its shank. Bolt diameter is the measurement across the diameter of the threaded area or bolt shank. The length of a bolt is measured from the bottom surface of the head to the end of the threads. The thread pitch of a bolt in the Imperial system is the number of threads that are in 1 inch of the threaded length and is expressed in number of threads per inch. A UNF bolt with a 3/8-inch diameter is marked as a 3/8 = 24 bolt. It has 24 threads per inch. Likewise a 3/8-inch UNC bolt is called a 3/8 = 16 bolt. The distance, in millimeters, between two adjacent threads determines the thread pitch in the metric system. This distance will vary between 1.0 and 2.0 and depends on the diameter of the bolt. The lower the number, the closer the threads are placed and the finer the threads are. The bolt's tensile strength, or grade, is the amount of stress or stretch it’s able to withstand before it breaks. The material that the bolt is made of and the diameter of the bolt determine its grade. In the Imperial system, the tensile strength of a bolt is identified by the number of radial lines (grade marks) on the bolt's head. More lines mean higher tensile strength. Count the number of lines and add 2 to determine the grade of a bolt. --- Bolt grade markings. Grade 2 Grade 5 Grade 7 Grade 8 Customary (inch) bolts-identification marks correspond to bolt strength-increasing numbers represent increasing strength. Metric bolts-identification class numbers correspond to bolt strength-increasing numbers represent increasing strength. 4.6 4.8 5.8 8.8 9.8 --- Bolt fillet detail. Wrench pad; Shank; Threads; Fillet; Washer face ---- STANDARD NUT STRENGTH MARKINGS Inch System--Metric System Grade--Identification | Class--Identification Hex Nut Grade 5 3 Dots Hex Nut Property Class 9 Arabic 9 Hex Nut Grade 8 6 Dots Hex Nut Property Class 10 Arabic 10 Increasing dots represent increasing strength. Can also have blue finish or paint dab on hex flat. Increasing numbers represent increasing strength. -- Threads are not straight on line Threads are straight on line Stretched bolt, Unstretched bolt; A comparison of a stretched and an unstretched bolt. ---- On metric bolts, a property class number on the bolt head identifies its grade. The property class is expressed with two numbers. The first represents the tensile strength of the bolt. The higher the number, the greater the tensile strength. The second is a percentage rating of the bolt's yield strength. This denotes how much stress the bolt can take before it’s not able to return to its original shape. For example, a 10.9 bolt has a tensile strength of 1,000 MPa (145,000 psi) and a yield strength of 900 MPa (90% of 1,000). A 10.9 metric bolt is similar in strength to a grade 8 bolt. Nuts are graded to match their respective bolts; a grade 8 nut must be used with a grade 8 bolt. If a grade 5 nut is used, a grade 5 connection would result. GARAGE TIP -- Bolt heads can pop off because of fillet dam age. The fillet is the slightly curved area where the shank flows into the bolt head. Scratches in this area introduce stress to the bolt head, causing failure. Replace all bolts that are damaged. Tightening Bolts -- Any fastener is near worthless if it’s not as tight as it should be. When a bolt is properly tightened, it will be "spring loaded" against the part it’s holding. This spring effect is caused by the stretch of the bolt. Normally a properly tightened bolt is stretched to 70% of its elastic limit. The elastic limit of a bolt is that point of stretch from which the bolt won’t return to its original shape when it’s loosened. Not only will an over-tightened or stretched bolt not have sufficient clamping force, it also will have distorted threads. The stretched threads will make it more difficult to screw and unscrew the bolt or a nut on the bolt. Fatigue breaks are the most common causes of bolt failure. A bolt becomes fatigued when it’s able to move in its bore due to being under-tightened. Washers Many different types of washers are used with fasteners. Flat washers are used to spread out the load of tightening a nut or bolt. This stops the bolt head or nut from digging into the surface as it’s tightened. Also place flat washers with their rounded, punched side against the bolt head. Soft flat washers, some times called compression washers, are also used to spread the load of tightening and help seal one component to another. Copper washers are often used with oil pan bolts to help seal the pan to the engine block. Grade 8 and other critical applications require the use of fully hardened flat washers. These won’t dish out when tightened like soft washers. Lock washers are used to lock the head of a bolt or nut to the workpiece to keep them from coming loose and to prevent damage to softer metal parts. ---- Hexagonal nuts; Slotted hexagonal nut (castellated nut); Jam nut; Regular square nut; Jam nut; Locknut Free-running seating lock unit; Pre-locked position spring nut; Formed prongs; Arched base; Crown nut; Wing nut; Stamped nut; Locknut Initial tension --- Many different types of nuts are used on automobiles. Each type has a specific purpose. --- Other Common Fasteners The use of other fastener designs depends on the purpose of the fastener. Some of the more commonly used fasteners are described here. Nuts -- Nuts are used with other threaded fasteners. Many different designs of nuts are found on today's cars. The most common one is the hex nut, which is used with studs and bolts and tightened with a wrench. Locknuts are often used in places where vibration may tend to loosen a nut. Locking nuts are standard nuts with nylon inserted into a section of the threads. The nylon cushions the vibrations. Studs -- Studs are rods with threads on both ends. Most often, the threads on one end are coarse while the threads on the other end are fine. One end of the stud is screwed into a threaded bore. A bore in the part that will be mounted by the stud is fit over the stud and held in place with a nut that is screwed onto the stud. Studs are used when the clamping pressures of a fine thread are needed and a bolt won’t work. If the stud is being screwed into soft (such as aluminum) or granular (such as cast iron) material, that end of the stud will have coarse threads. The opposite end will have fine threads. As a result, a coarse thread is used to hold the stud in a component and a fine-threaded nut is used to clamp the other part to it. This provides for the clamping force of fine threads and the holding power of coarse threads. Cap Screws -- Cap screws are similar to bolts, but they don’t have a shoulder. The threads travel from the head to the end of the screw. Never use a cap screw in place of a bolt. Setscrews -- Setscrews are used to prevent rotary motion between two parts, such as a pulley and shaft. Setscrews have a square head and are moved with a wrench, or they are headless and require an Allen wrench or screwdriver to move them. Machine Screws--The length of machine screws is entirely threaded. These screws have a head on one end and a flat bottom on the other. Machine screws are used to mount one component to another that has a threaded bore. They are also used with a nut to hold parts together. Machine screws can have a round, flat, Torx, oval, or fillister head. Self-Tapping Screws--Self-tapping screws are used to fasten sheet metal parts or to join together light metal with wood or plastic parts. These screws form their own threads in the material into which they are screwed. Thread Lubricants and Sealants It’s often recommended that the threads of a bolt or stud be coated with a sealant or lubricant. The most commonly used lubricant is antiseize compound. Antiseize compound is used where a bolt might become difficult to remove after a period of time- For example, in an aluminum engine block. The amount of torque required to properly tighten a bolt treated with antiseize compound should be reduced. Thread lubricants may also cause hydrostatic lock; oil can be trapped in a blind hole. When the bolt contacts the oil, it cannot compress it; therefore, the bolt cannot be properly tightened or fully seated. Thread sealants are used on bolts that are tightened into an oil cavity or coolant passage. The sealant prevents the liquid from seeping past the threads. Teflon tape is often used as a sealant. Another commonly used thread chemical, called threadlocker, prevents a bolt from working loose as the engine or another part vibrates. Thread Pitch Gauge The use of a thread pitch gauge provides a quick and accurate way to check the thread pitch of a fastener. The leaves of the tool are marked with the various pitches. To check the pitch of threads, simply match the teeth of the gauge with the threads of the fastener. Then read the pitch from the leaf. Thread pitch gauges are available for the various threads used by the auto motive industry. --- A container of threadlocker. By Permatex, Inc. Taps and Dies The hand tap is a small tool used for hand cutting internal threads. An internal thread is cut on the inside of a part, such as a thread on the inside of a nut. Taps are also available that only clean and restore threads that were previously cut. Taps are selected by size and thread pitch. Sequence 1 goes through the correct procedure for repairing damaged threads with a tap. When tapping a bore, rotate the tap in a clockwise direction. Then, turn the tap counterclockwise about a quarter turn to break off any metal chips that may have accumulated in the threads. These small metal pieces can damage the threads as you continue to tap. These metal chips are gathered in the tap's flutes, which are recessed areas between the cutting teeth of the tap. After backing off the tap, continue rotating the tap clockwise. Remember to back off the tap periodically and make sure all of the existing threads in the bore have been re-cut by the tap. Hand-threading dies are the opposite of taps because they cut external (outside) threads on bolts, rods, and pipes rather than internal threads. Dies are made in various sizes and shapes, depending on the particular work for which they are intended. Dies may be solid (fixed size), split on one side to permit adjustment, or have two halves held together in a collet that provides for individual adjustments. Dies fit into holders called die stocks. Threaded Inserts When the threads in a bore are excessively damaged, it’s better to replace them than try to tap them. A thread insert can be used to restore the original threads. Inserts require drilling the bore to a larger diameter and tapping that bore to allow the insert to be screwed into it. The inner threaded diameter of the insert will provide fresh threads for the bolt. Spark Plug Thread Repair--Sometimes when spark plugs are removed from a cylinder head, the threads have traces of metal on them. This happens more often with aluminum heads. When this occurs, the spark plug bore must be corrected by installing thread inserts. When installing spark plugs, if the plugs cannot be installed easily by hand, the threads in the cylinder head may need to be cleaned with a thread-chasing tap. There are special taps for spark plug bores, simply called spark plug thread taps. Be especially careful not to cross-thread the plugs when working with aluminum heads. Always tighten the plugs with a torque wrench and the correct spark plug socket, following the vehicle manufacturer's specifications. Also, when changing spark plugs in aluminum heads, the temperature of the heads should be ambient temperature before attempting to remove the plugs. GARAGE TIP -- Never change spark plugs when the cylinder head is hot. The bores for the plugs can take on an oval shape as the cylinder head cools without spark plugs in the bores. ---A tap and die set. ---Metal chips are gathered into the flutes of a tap. Flute; Chips --- Using a threaded insert (heli-coil) to repair damaged threads. Emhart Fastening Teknologies. Drill hole to proper size; Tap hole to proper size; Install insert on mandrel; Install insert into threaded hole; Driving tang Insert === Repairing Damaged Threads with a Tap: ___1 Using a thread pitch gauge, determine the thread size of the fastener that should fit into the damaged internal ___2 Select the correct size and type of tap for the threads and bore to be repaired. ___3 Install the tap into a tap wrench. ___4 Start the tap squarely in the threaded hole using a machinist square as a guide. ___5 Rotate the tap clockwise into the bore until the tap has run through the entire length of the threads. While doing this, periodically turn the tap backward to clean the threads. This prevents breaking the tap. ___6 Drive the tap back out of the hole by turning it counterclockwise. ___7 Clean the metal chips left by the tap out of the hole. ___8 Inspect the threads left by the tap to be sure they are acceptable. ___9 Test the threads by threading the correct fastener into the threaded hole. Next: Automotive Measuring Instruments/Tools Prev.: LEARNING GOALS; Measuring Systems Home Article Index top of page |
