A look at the fastening and sealant applications that are responsible for keeping your motorhome together.
By Mark Quasius, F333630
February 2013
Motorhomes face unique challenges that residential homes do not. When you consider that today’s roads aren’t exactly as smooth as glass, it’s something of a miracle that any motorhome makes it to its destination without falling apart. Yet, it’s inevitable that something will need to be reattached or replaced every now and then.
The constant pounding and jarring eventually take their toll, and things will need to be repaired. Duct tape does wonders, and it may work as a temporary fix, but ultimately the item will need to be repaired properly using the correct fastener. There are many methods used to hold things together, but each has its own characteristics and uses, so it’s important to match the proper method to each application.
Welding
Welding is the process of heating two metals and adding a third metal to bond the two pieces together. When done properly, the result is that the two pieces are joined as one, with no seams or cracks remaining.
Electric welding involves creating an electrical arc that heats up both the electrode and the material being welded. The electrode then melts into the cavity and flows into the two pieces that are being joined. This can be done with a stick welder or a wire-fed welder. In either case, a flux is used to provide a shield to allow the weld to cool and protect it from corrosion during the process.
It’s important to disconnect any electronic control module (ECM), batteries, or other computerized circuits prior to welding. This will prevent damage that can occur when an electrical charge is passed through the frame ground. Arc welding is great for trailer hitches and other heavy-duty structural metalwork.
Gas welding involves the same principle, but a torch is used to apply heat to both the surface and the electrode. It doesn’t provide the same strength as an arc weld does, because the penetration isn’t as deep. This type of welding used to be popular with sheet metal and other light-duty jobs, but advancements in electric welding technology have nearly eliminated gas welding as a practice.
A lesser-known task is plastic welding. Plastic welders use heat to melt plastic and a selection of various plastic filler rods to fill in and join the pieces together. This type of welding uses the same basic techniques as electric welding, with both the damaged area and the filler rod melted to fuse together. The only difference is that you are using plastic and a hot air gun to perform the task. If you have a cracked fan shroud, an ABS or urethane bumper, or even a stripped screw hole in a plastic dash component, a plastic welder can be a real lifesaver.
Soldering
Soldering is a bit different from welding in that you don’t fuse the substrate and electrode together. Heat is applied to both the substrate and the soldering wire. Once the substrate gets hot enough, the solder will flow and stick to it. But the solder doesn’t become part of the substrate. It’s more like hot glue that attaches two items together. Soldering is most often used in electrical work where conductivity between the parts being joined is required. Two copper wires easily can be joined together and then covered with a urethane shrink tube to create a trouble-free bond. Most solder is a blend of tin and lead, although lead-free solder is available for plumbing. Radiator repair is another common task for solder.
Glues
Not every item is metallic, so situations could arise where soldering or welding just won’t work. In many cases, choosing the correct glue will be your best solution. Glues are available in many different compositions, and it’s important to select the proper glue or adhesive for your particular task. Basic glues such as woodworking glue, spray-on adhesive, and instant glue involve a one-part process. You simply apply the glue to the surface, clamp both items together, and wait for the glue to dry. For tougher jobs, two-part glues such as epoxies are a better choice because of their increased holding power. Epoxies work by mixing the resin with a hardener. Once the parts are mixed, it doesn’t take long for the mixture to set up, so you have to work fast. Don’t mix any more than you can use in a short period of time. Glues work well for light-duty tasks that cannot be welded, but they do have a limited range of applications, so be sure to check the label to determine which one is suitable for your particular application.
Nuts And Bolts
The vast majority of fastening tasks involve the use of a fastener to hold two items together. A bolt or a bolt and nut combination are used to create the clamping force. Bolts can be run into a prethreaded blind hole, such as an engine block, to hold down cylinder heads or various accessories, or they can be dropped through a larger hole where a nut is added to provide the clamping force. Bolts come in a wide range of diameters and lengths to fit most any application. Bolts are also graded, with a Grade 5 bolt being a good general-purpose fastener and a Grade 8 bolt being used for higher-tensile-strength applications. These ratings can be identified by the number of raised hash marks on the bolt head. Three marks will indicate a Grade 5 bolt, while six marks indicate a Grade 8 fastener. If there are no marks, it’s safe to assume that it is a lower-quality fastener and should not be used for anything critical.
Most bolts are inserted into a predrilled hole and a nut is threaded to the opposite end. This provides the clamping force necessary to secure the joint or attachment. If the nut is not tightened adequately, it will loosen and the joint will fail. If the nut is overtightened, the threads can be damaged and strip off, so it’s important to tighten these fasteners to the proper torque as indicated on a spec sheet or torque chart for common fasteners.
In most cases, a nut by itself will not retain its torque without some help, so various measures can be taken to prevent the nut from loosening. Lock washers are the most common way to prevent a nut from backing off by biting into the metal beneath the fastener. In some cases, a bolt is used as an electrical ground stud, so a star lock washer provides more area to conduct electricity when electrical terminals are connected to it. Fender washers can be used to spread the clamping force of the bolt to a larger area around the hole. Rubber washers may be appropriate in areas with moisture.
The ideal solution for keeping a fastening combination tight is a nylon lock nut. These durable fasteners have a nylon ring attached to one end of the nut. This undersize nylon ring grips the bolt’s threads to prevent it from loosening. Another advantage of a nylon lock nut is that it remains tight even when tension is removed, unlike a typical lock washer, which requires tension in order to hold. Lock nuts are a perfect solution for anything that moves, swells, or shrinks.
| Standard Bolt Torque Specifications |
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In some applications, it may be advisable to use a chemical compound to prevent fasteners from working loose. Brands such as Loctite offer a wide range of products, but three of the company’s products are most apt to be used in a motorhome application, and they can be identified by their color. Loctite Threadlocker Red 271 is the toughest of the company’s thread-locking compounds and is designed to hold up under high heat and torque. It will require some extra muscle to remove fasteners that have been installed with Loctite Red, because of its holding power.
Loctite Threadocker Blue 242 isn’t as strong as the Loctite Red, but it still has respectable fastening power and will hold up to temperatures as high as 300 degrees Fahrenheit. One common use for Loctite Blue is in a stud-and-nut combination. Use the Loctite Red to hold the stud in place and the Loctite Blue to lock the nut to the stud. That way when it’s time to remove the nut, the stud should stay in place.
Loctite Threadlocker Green 290 is unique in that it wicks into threads, because the fluid isn’t as thick as the Red or Blue products. It’s suited for smaller fasteners that won’t hold up to higher torque when they need to be removed. These color definitions also carry through to competitive brands such as Permatex, so finding a nearby supply of thread-locking compound shouldn’t be a problem.
With any thread-locking product, it’s important to torque the fastener while the product is still moist. Once the compound is dry, it will become difficult to obtain an accurate torque reading. It also should be noted that once you break the seal after it has set up, the thread locker is rendered useless unless you reapply. Thread lockers are a great choice for fasteners that screw into blind holes, such as engine bolts.
When dealing with electrical connections, dielectric grease or battery terminal spray should be used to avoid future problems.
Screws And Rivets
Not every application requires the strength of a bolt and nut. Many light-duty jobs also have blind holes and do not have access on the back side to attach a nut. For many of these applications, some type of screw works well. Screws also depend on clamping force to hold, but they rely on threads that tap into the material being held. The actual pullout factor is going to depend on the thickness of the material that is being fastened and how soft or hard it is. Thicker materials have more room for thread and will have a higher holding power than thinner material, and materials with a higher density will deliver greater holding power, whereas a lighter-density material will strip threads much earlier. The biggest benefit is that screws do not require a nut, so they can be installed without needing access to the back side, which is why they are very popular in motorhome assembly lines. Standard screws require a small pilot hole, but self-tapping screws come with a drill-bit tip to create the hole. Ideally, these screws will have a hex head, a Phillips head, or a square-drive head to allow power tool installation without slipping.
If the material is too thin or too soft, you should avoid using screws and instead opt for a rivet. Rivets expand when tightened in the installation tool and knob up on the back side rather than cut threads into the material, so they achieve their clamping force similar to a nut and bolt. They can be installed from one side without requiring access to the back side. This gives them the ability to maintain good clamping force in thinner materials.
In some cases where a rivet is not desired or aesthetically pleasing, you may be able to use a screw anchor. Anchors function by installing into thin material and expanding, very similar to a rivet. However, the anchor will then accept a screw and will provide additional room for the threads.
Sealants
Even the best fastener won’t prevent water from penetrating areas that are subject to rain and moisture, such as an RV’s roof and sidewalls. For that, you’ll need to add a sealant. Sealants come in various compositions, and, similar to fasteners, the type of sealant that should be used depends on the job.
The most common application is a motorhome’s roof. A one-piece fiberglass roof might appear to be a near-perfect material to keep things dry inside, but when you consider the various plumbing vents, air conditioners, antennae, skylights, fan vents, and other hardware installed on the roof, you’ll quickly realize it’s really quite full of holes. Specific sealants are designed for EPDM rubber roofs to seal up seams as well as any penetrations. Butyl rubber sealant is very popular for fastening and sealing up accessory flanges.
Fiberglass roofs generally rely on a self-leveling sealant, which comes in tubes and can be dispensed from any standard caulking gun. Generally, a self-leveling sealant will flow to cover screw or rivet heads as well as brackets or flanges fastened to the roof. A bead of sealant is applied beneath the item that is being fastened to help provide a barrier. After the item is fastened, additional sealant is applied over the top of any fastener heads as well as along the edge of the flange.
Window replacements and furnace exhaust flanges generally receive a ring of rope caulking prior to being installed. The bead is applied beneath the flange and compresses when the flange is fastened to the sidewall. An additional bead of clear silicone caulking is applied to the upper edges of the window to prevent any water from seeping in.
Another popular sealant is Eternabond tape. This tape is considered magic by many RV owners and can be used as a permanent sealant as well as a temporary fix, which keeps it out of the duct tape category. Eternabond will stick to just about anything and doesn’t come off easily. It can be used as a temporary repair to deal with a water leak, or it can be used as a more permanent repair to fix a leaking seam. It is also available in a double-stick version to help mount or seal accessories to an existing surface.
Choosing the correct fastener and sealant will ensure that your repair is permanent so that you can concentrate on enjoying your motorhome. Even so, I wouldn’t leave home without that roll of duct tape.