Trim and hardware on the second floor. Bolts in aisle five, clips in aisle seven…The ever-growing array of fasteners, and tools to work with them, is one more challenge facing the collision repair technician today. But fastening systems have more in common than not. Basic mechanical principles define their form and function. Engineers making decisions about which type of fastener to use face many choices: screws, bolts, clips, one-time or reusable, rivets, hook and loop, and adhesives. Estimators and technicians need to be familiar with all of these and how to use them.
Fasteners: Simple Machines
Simple machines are recognized historically as those basic tools with which industrial society was built. The lever, inclined plane and screw are such simple machines. They are mechanisms that transform energy into a more useful form. For example, turning a screw translates into a clamping force applied to a door trim panel.
Screws are a very common fastener, available as sheet metal and machine screws. Like wood screws, sheet metal screws have widely spaced threads. However, unlike wood screws, only the very point is tapered. They are inserted into a hole smaller than the shank diameter and form a single revolution thread as they are turned. How much force a screw can apply depends on the depth of the thread and the thickness of the sheet metal. Apply too much torque to a sheet metal screw and the sheet metal will distort, which may strip the thread. Sheet metal screws are typically used to hold parts that do not require high forces, such as trim pieces and speaker mounts. Machine screws differ in that the threads are more finely pitched and used with female threads or nuts.
Screws of all types also vary with the drive head. The history of slotted, cross-point, or Phillips, square drive, hex, torx and other types of drivers is long and varied as inventors tried to make, and market, a better system. It is important to understand that the correct driver must be used or the head may become distorted and unusable. This is caused by point loads on small surfaces of the screw head if the tool does not fit tightly. Slotted screws should always be driven with the largest screwdriver blade that will fit in the slot. Every other design of screwdriver and head has been an attempt to improve the ability to apply torque to the screw head.
Bolts are like machine screws with regular male threads matched to a female thread in the part or nut. They are driven on the outside of the head. This makes the head of the bolt stronger than that of a machine screw, making it capable of applying more force to hold a part in place. Though the most common, hex head bolts are often replaced with more complex shapes to allow much more surface area to drive the head. In conventional hex head bolts, only a small portion of the metal is actually driven by the wrench or socket.
Many designs address this concern, starting with precisely made wrenches and sockets, which are more costly but work much better than cheaper tools. Some patented designs have modified inside corners on the tool to allow the forces to work more on the flat part of the bolt head. Heads with complex shapes, such as 12-point heads and inverted torx, are also used.
Thread types are divided into metric, measured as the distance between threads, and SAE, which is measured as the number of threads in an inch. The threads, of course, must match exactly to work properly.
The strong forces the bolts encounter dictate that they be very sturdy. Bolts are rated for strength, or grade. In the Society of Automotive Engineers (SAE) standard, there are slash marks on the head. The number of slashes—plus two—indicates the grade. A grade five bolt has three slashes; a grade eight would have five. In the metric standard, the hardness is indicated by a number on the head. Unmarked bolts are lower-grade bolts. It is critical that all bolts are replaced with equivalent bolts. A common misconception is that the harder the bolt, the better it is. With collision energy management designed into vehicles, the failure mode of a fastener becomes a critical design element. Higher-grade bolts are very strong. Yet, as most collision repair technicians have found, they can be very brittle. They fail by breaking suddenly. Softer-grade bolts stretch and tear relatively slowly in comparison. Engineers have determined precisely how strong the various fasteners need to be. We should not redesign the vehicles during the repair process by substituting bolts. Often, this will mean acquiring the bolts from the vehicle manufacturer for replacements.
The shape of the fastener may also be critical to its function. Shoulder bolts are many times used with plastic parts so the plastic is not crushed as the bolt is tightened. Counter-sunk heads spread a load over a large area while staying flush to the surface. Bolts with large flange areas are designed to spread the load as well.
Washers are like those flange areas because they allow the load to be spread over a larger area. Flat washers should be replaced with washers of the same diameter and thickness. Some flat washers are stamped from sheet steel, producing one side that is rounded and one that is relatively sharp. Always place the sharp side against the part. The sharp edges can cut into the bolt head, making it weak and subject to failure. Other washers also assist in keeping the bolt tight. These lock washers come in many forms, and again, should always be replaced with equivalent pieces.
Using lock nuts is another way to keep things in place. These either have a distorted thread to lock in the threads of the bolt, or a nylon ring smaller than the threads. Most of the nylon lock nuts may be used a few times before replacement. The nuts with distorted threads will require new nuts and bolts. Castle nuts are used with cotter pins. These are commonly found on suspension parts. The nut is torqued in place, then turned additionally to align the slots with a hole in the bolt or stud. A cotter pin is installed, and the ends are thenbent over. Even if the nut should become loose, the pin will keep it from turning and coming off.
Torque
Torque is a topic for another story, but it is very important that all bolts be torqued properly. As engineers design parts more precisely, it becomes more important that exact torque is applied to fasteners. Bolts stay tight because they are stretched a little. Too little and they can become loose, too tight and they can reach the yield point of the steel, allowing them to relax and actually become looser than when installed. OEM recommendations are that all power train bolts, wheel bolts and nuts, many suspension fasteners and all restraint system fasteners be torqued when installed. In recent years, a wide array of torque tools has appeared, including screwdrivers with torque ability.
One-time use fasteners are a special case. They may be one-time use because of thread distortion during installation. Some nuts are purposely distorted so the threads will be locked in place with or without the bolt stretch. After one insertion and extraction, the threads are worn down so much that they are not as strong as they need to be. Other bolts are one-time use because they are designed as a torque-to-yield fastener. This means the torque applied to them has permanently distorted the metal. This can only be done once. The difficulty comes in identifying these one-time use fasteners so they may be ordered and replaced. Some parts will come with new bolts when purchased. However, this does not help if the part is being removed for access and then re-installed. When in doubt, inquire with the OEM parts department or look in service manuals.
Rivets
Like screws and bolts, rivets typically hold two or more parts together by passing through a hole and applying a clamping force. Rivets come in two common types. For instance, solid rivets are used to attach frame parts together on trucks. They are designed to be held solidly in place on one side and driven into a mushroom shape on the other side. This installation process expands the shank inside the hole through the parts with the first hammer strike. Rivets have this advantage over bolts: they hold parts from moving with a very tight fit as well as with the clamping force. Because this is not easily replicated in the collision repair shop, OEM recommendations are usually to replace frame rivets with bolts. Specific bolts purchased from the factory must be used. The bolts have special shanks that must be forced into place. This allows them to keep the parts from moving just as a rivet would.
Blind rivets are designed to be inserted from one side. Commonly called pop rivets, these have a nail that is pulled back through the hole to expand the lower portion of the rivet. It is important that the proper length rivet is selected. If it’s too long, the mushroom on the end will distort and not apply even pressure to the inside surface. Too short and the mushroom will be too small. Rivets are made from steel and aluminum and should always be replaced with the same type. Drive rivets work by forcing a spike through the rivet, which expands the lower portion of the rivet. Plastic rivets have been developed as well. They do not have as much strength, of course, but are a useful way to install plastic parts.
Clips
While dozens of screw and bolt designs exist, there are thousands of clip designs. Originally, metal clips were developed to speed assembly and allow invisible attachment for trim pieces. Windshield moldings were an early application. But plastic clips are cheaper, easier to make, and have no corrosion issues. Plastic clips are used to hold on almost everything but the sheet metal. Because they are specifically designed for each application, only exact copies will work. As they are hidden behind or inside the part they are holding in place, it is often difficult to know if they need to be replaced or not. Clips are not easily reused because they do not come out without breaking. Some clips hold with simple friction, while others have arrow headed designs that snap into place against the back side of a panel. Special clip removal tools may help in removing them without damage.
Adhesives
A popular alternative is adhesives. These are available in a wide variety, from the light-tack acrylic used on sticky notes and masking tape, to high performance epoxies used to hold aircraft together. In the collision repair shop, acrylic adhesive on two-sided tape is a common attachment. These can be removed with heat, which allows the adhesive to soften and release from the surface. Another tool is a wide steel blade used with a lubricant. Often access, or simply the soft composition of the foam, results in the foam tearing off the molding or emblem. When this happens, the tape will need to be completely removed and reapplied, or a new part will need to be ordered. Complex emblems may be almost impossible to re-tape. If the foam tape comes off intact, free film adhesive can be applied to re-install the part. Free film adhesive is simply adhesive sprayed on a non-stick sheet. It comes in sheets and as a series of very small dots. A peel and stick process gets the adhesive on the part, which can then be installed much like a new part.
Special tools may prevent breaking clips that can be difficult to get. Torque drivers of several types need to be used regularly and serviced annually to maintain accuracy. Service manuals can provide needed information about these important parts. With all the various types of fastening systems in use, technicians need to remain diligent in identifying replacement parts during the repair process to avoid any liability or safety issues and to speed the repair process.
