The Nuts and Bolts of Bolting
Part 2 of 3
Some people may wonder why there are so many types of screws and bolts. Others may wonder what the markings on the head mean. In this installment I discuss the different fastener types used on our BMW motorcycles and some common fastener-related problems.
Threaded fasteners are classified by shape, material and finish, which are specified by industry standards. In the United States, the American Society for Testing Materials (ASTM) sets the standards. In Europe, the International Standards Organization (ISO) sets the standards. The metric fasteners on BMW bikes are specified by the ISO.
What Size is it?
For metric screws, the basic dimensions are given as MaXb, where a denotes the nominal thread diameter in millimeters, and b denotes the pitch, in millimeters. A common thread on our bikes is an M6X1, which is 6mm in diameter with a 1mm pitch. (These are the ones that require a 10mm hex wrench or a 5mm Allen wrench.) Figure 1b shows the basic dimensions for a screw or bolt. Figure 2b shows the various fastener types. Note how the length is specified differently for countersunk style screws.
Figure 1b, Basic Screw/Bolt Dimensions
Figure 2b, Screw and Bolt Types
How Strong is it?
One important consideration in applying a bolt is its strength. The bolt material strength is determined by the alloy and processing method (for example, cold working and heat treating.) The two important material properties are the tensile strength and yield strength. The tensile strength, sometimes called the ultimate strength, is the stress level where the material breaks. The yield strength is the stress level where the material yields or permanently deforms. When operating under any normal load fasteners should be below the yield stress. The tensile strength is always higher than the yield strength. Materials with a large difference between the yield and tensile strength are considered ductile, meaning they will stretch substantially before breaking. The load a fastener carries is calculated by multiplying the material strength by the nominal cross-section area of the thread. For inch-size fasteners, the material strength is specified by the "grade." A grade 8 bolt is stronger than a grade 5, which is stronger than a grade 2. The grade is indicated by a series of marks on the boltís head. For metric fasteners, the term "property class" is used and is stamped directly on the head. The property class for steel fasteners is given in the form X.Y, where X is 1/100 of the nominal tensile strength in newtons/mm2, and Y is 10 times the ratio between the yield strength and tensile strength. The multiplication of these two numbers gives 1/10 of the yield strength in newtons/mm2 . For example, a fastener with a property class of 8.8 has a nominal tensile strength of 800 newtons/mm2 (116,000 psi) and a yield strength of 640 newtons/mm2 (93,000 psi).
There are two common types of stainless steel fasteners: corrosion-resistant stainless steel, ASTM 304 (a.k.a. 18-8) or DIN/ISO A2, and acid-resistant stainless steel, ASTM 316 or DIN/ISO A4. A2 is by far the most prevalent material, and is what is normally supplied for stainless metric fasteners. The BMW OEM Mareg battery comes with A4 screws for improved acid resistance. There are three typical property classes (strengths) for in the metric system: 50, 70, and 80. The class equals the tensile strength divided by 10. The metric property class is a dash (-) number after the alloy designator. For example, a screw marked A2-70 is a 304 stainless steel screw with a 700 N/mm2 tensile strength. Both alloys come in all property classes, but A2-70 and A4-80 are the most common.
Table 1b compares the material properties of typical inch and metric fasteners. The inch-size socket-head cap screws (SHCS) are included in the table which have a higher strength than graded fasteners, but no specific markings except for their shape. I have also taken some liberties with the terminology for the sake of simplifying the comparison.
|Inch Grade||Marks on Head||Material||Tensile Strength||Yield Strength|
|Metric Class||Marks on Head||Material||Tensile Strength||Yield Strength|
Note that the strength class specifies much more than the strength of the fastener and includes properties like the alloy, manufacturing method, hardness, and heat treatment.
Handy Tip: When sorting through a mixed pile of inch and metric fasteners the metric ones can always be identified by the marking on the head, that is 8.8, 12.9 A2 etc.
Rust Never Sleeps
From a strength and preload standpoint the ideal steel fastener would have a plain black finish, (sometimes called a light oil finish). This finish produces a fairly consistent K-value and does not compromise the strength of the fastener. This finish would be unacceptable on a bike since it corrodes easily. The common solution is to apply a zinc or cadmium plating to prevent corrosion, and apply a conversion coating such as chromate to keep the finish looking nice. If a more decorative finish is desired, the fastener is usually polished and chrome plated. Plating causes problems with high-alloy steels due to hydrogen embrittlement, if appropriate measures are not taken after plating to "bake out" the hydrogen. This is especially true of chrome plating which tends to lock in the hydrogen. Plating does not adversely effect the mild steel used for 8.8 fasteners. The torque-tension relationship is greatly affected by plating due to its effect on the friction coefficient. Cadmium plating reduces the friction by 25% and zinc plating increases the friction up to 40%. This requires a corresponding 25% reduction or 40% increase in required torque for the same tension. Stainless steel fasteners have a friction coefficient about two times the corresponding plain steel fastener. This does not mean that stainless fasteners require double the specified torque since they usually cannot achieve the strength of a steel fastener.
Thread lubrication is another variable that affects the torque-tension relationship. I performed experiments on approximately 20 lubricants and found that the lubricant can change the torque to achieve a given tension by a factor of two (up or down!) I found that super clean fasteners or those lubricated with light lubricants like WD-40tm require a high torque to achieve the desired tension. Fasteners lubricated with oil such as motor oil and the oil found on black fasteners require a medium torque. Fasteners lubricated with extreme pressure grease or anti-seize paste require the least torque. Interestingly, I found that Loc-titetm has about the same lubrication action as light oil. I discussed this with the manufacturer and they said this was by design so that the torque-tension relationship would be approximately the same as plain steel fasteners with normal manufacturing oil. Cool.
Shake, Rattle and Roll
Even our impeccably smooth-running BMWs vibrate to some extent. This can cause fasteners to loosen over time and parts to fall off. It is important to take anti-vibration precautions where needed. The most important and easiest measure is to keep fasteners tight. The tension holds the parts together and prevents relative motion, which leads to loosening. Another method is to use prevailing torque fasteners such as those with the nylon imbedded in the nut, commonly known as Ny-loktm. The nylon serves to keep tension on the screw threads even after the tension in the bolt is gone. The friction of the threads and the nylon keep the nut from loosening, even if there is little or no tension in the fastener. These are particularly useful for fender mounts where the rubber mounting prevents significant tension from being applied to the fastener. It is important to replace worn or damaged prevailing torque fasteners with the same type. The BMW service manuals show the minimum acceptable torques for prevailing torque fasteners. In any case, if you can thread a Ny-lok nut on with your fingers it is worn out and should be replaced.
Thread-locking compounds such as Loc-Titetm are another popular anti-vibration measure that can be applied to any fastener that has a propensity to loosen. These compounds are anaerobic adhesives that cure in the absence of air. The adhesive cures in the spaces of the threads when the fastener is tightened and drives out the air. The adhesive comes in several grades depending on the desired strength. Thread-locking compounds do increase the disassembly torque. Usually fasteners can be removed with normal methods, but heating is sometimes required to weaken the bond when the highest strength compound is used (for example those used to hold the brake disk on the K-100 rear drive).
Nylon-based prevailing torque fasteners and thread-locking compounds cannot be used for high-temperature applications because they melt. For these applications, a fastener with a deformed thread is used. The nuts that hold the exhaust pipes on a K-100 engine are of this type. Of course, never replace these nuts with plain or Ny-loctm nuts because they might loosen.
The most common anti-vibration measure is a simple lock washer. BMW often uses the wave-type washer, which is a plain flat washer that has been deformed to a potato chip shape. The most important thing the washer does is present a smooth surface with known friction characteristics to improve the preload when tightening. The washer also acts as a little spring to keep some tension on the fastener, even after the parts have loosened. Wave washers should be replaced if they become flat.
Every now and then we make a mistake and strip a thread. This is easy to do on a bike because there are a lot of things that are threaded onto aluminum. One of my favorites is torquing to the specification in N-m using the ft-lb scale on the torque wrench. (Always "feel" for tightness even when using a torque wrench.) Misprinted torque specifications, corrosion, and cross-threading are other common ways to ruin threads. There are four ways to repair damaged threads. One way is to weld the hole closed and then drill and tap a new thread. In many cases, this is not practical or cost-effective. Sometimes the damaged hole can be drilled to the next larger inch or metric size, and a larger fastener can be used.
A more elegant solution is to use a threaded insert, which works by first drilling and tapping the damaged hole to a larger size and then installing a steel insert that has the new larger threads on the outside and the original threads on the inside. Popular brand names include Helicoiltm, Keenserttm, and Timeserttm. Thread repair insert kits are available at larger automotive stores. The kits include a drill, special tap, insertion tool, and a few inserts. At $25-$35 they are not cheap, but they can represent a substantial savings compared with a new part. Inserts have the advantage that the new threads are steel. The inside surface stands up well to repeated use. The outside threads that bear on the aluminum are larger and spread the load out to make a stronger assembly. A handy place for a threaded insert is the ground strap bolt on an air head twin because they tend to strip with use.
Another technique is a form-in-place repair where an epoxy-like material is used to form new threads. This repair comes as a kit and costs about $12. The first step is to spray the fastener with a special release compound. Be sure to coat it well as you donít want the fastener to get epoxied in the repaired hole. While the release compound is drying, clean the stripped hole with solvent and compressed air. Mix the thread-forming compound and fill the hole per directions. Next thread the coated fastener into the compound-filled hole and wipe off the excess goop that squeezes out. The coated fastener is removed after the compound has achieved the initial set and trim off any excess that sticks out above the repaired hole. The compound is fully cured and ready to use in 24 hours. The repair is about as strong as the original aluminum. I successfully used this technique when I stripped the left-handed thread for the mirror on my R75/5. The repair worked perfectly for 25k miles before I sold the bike.
One Use Fasteners
Certain critical fasteners such as connecting rod bolts, clutch cover screws, and drive shaft bolts are designed to be used only once. Because of their critical nature and high loading, high torques are specified for these particular applications. The high torque provides a high clamping force but may stretch or yield the fastener in the process. This is okay for the initial installation and is even desirable. However, the M6x1 thread that we started out with has now stretched to something like a M5.9x1.1 thread. If this fastener is reused, the threads will bind in the mating hole and will fail to properly clamp the parts even after they have been torqued to specification. In my opinion, installing new bolts is cheap insurance.
What About Stainless?
Replacing the OEM-plated steel fasteners on the outside of the bike provides a good-looking, low maintenance cure for rusted nuts and bolts. When contemplating replacing OEM fasteners with stainless steel, remember that common stainless steel does not have the strength of common alloy steels and should not be used for critical, highly stressed applications. The stainless steel also has higher friction than steel does which gives less preload at the same torque. When in doubt, keep the OEM steel stuff for critical applications.
The Nuts and Bolts of Bolting
There are many different sizes, materials, and types of threaded fasteners. Each nut, bolt, and screw on our BMW bikes is specific to the use and its environment during use. Take time to look at the fasteners that you remove while doing repair or maintenance work. Check all fasteners for condition and replace any worn or damaged ones with new ones of the appropriate type. When installing accessories use fasteners that correct for the application. Use the markings on the fastener to determine its type. Generic fasteners often found at the hardware store may not be an appropriate replacement.
Continue to Part 3, "Torque Wrenches, How Good Are They?"
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