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Keeping It All Together

How A Bolt Works

By Joseph C. Dille
BMWMOA #24754
Part 1 of 3

Except for a few taper and press fits, our BMW motorcycles are held together with threaded fasteners. Improperly installed fasteners can fail by loosening or through outright breaking, causing a dangerous situation. In this installment, I will describe the basics of how fasteners work and how they should be installed. I will also point out a few simple things to look for when installing fasteners.

A screw thread is an extension of one of the basic machines, the inclined plane that, has been wrapped around a shaft. When the thread is turned, it moves the mating part or nut up the inclined plane. When more turning force, or torque is applied to the shaft, the more force is exerted on the nut. This force creates a tension in the bolt, which clamps the mating parts together. Preload is the technical term for the tension caused by tightening the fastener that holds the assembled parts together. Generating sufficient preload force is the key to strong and reliable bolted joints that will not loosen or break under load. Figure 1a shows the forces that act on a bolted joint. Force Diagram For Typical Bolted Joint
Figure 1a, Force Diagram For Typical Bolted Joint

It is often helpful to think of the fastener as a spring (see Figure 2a). It may seem odd to think of your bike as being held together by a bunch of springs, but this analogy works to show what happens when a bolt is tightened. Rotating the bolt, which in turn stretches the spring, generates the preload force. The more the bolt is rotated, the more it stretches and generates more preload or tension. A Bolt Acts Like A Spring To Clamp The Parts
Figure 2a

The clamping force Fc is the difference between the preload force and the tension force Ft on the joint. The clamping force is what holds the parts together. This translates mathematically to: Fc=Fp-Ft.

When there are no tension loads applied to the joint, the clamping force equals the preload force. If the tension load is equal to the preload, there is no clamping force. If the tension load is increased beyond the initial preload force, the joint will separate. Even after the joint separates, it will continue to take increased tensile loads until the ultimate tensile strength of the fastener(s) is reached and the fasteners break. As a practical matter, joint failure occurs well before the fasteners actually break because the parts that are being held together will loosen and not function properly.

Joints are loaded with shear force, tension force or a combination of both. In a joint loaded in tension the joint separating forces are opposed by the preload force on the bolt. A good example of this is a cylinder head. It is important to note that for a joint with stiff mating parts the load on the bolt remains constant (at Fp) until the tensile load is greater than the preload force. A simplistic view is that the ultimate strength of the joint is limited by the strength of the bolt. However, the higher the preload force the better the joint, because it will prevent the assembled parts from moving and the joint from loosening. A highly preloaded joint is also more resistant to cycling loads since less of the cyclic portion of the load is experienced by the fastener. In general, the preload force determines the strength of the joint. Joints are stronger and more fatigue resistant with greater preload force.

It is important that the preload force be maintained in the fastener during operation. Highly loaded or critical fasteners tend to be long, and they need to be stretched a relatively large amount to generate the preload force. This allows them to maintain their preload, even if they expand a little or the mating parts shrink. Examples of this include connecting rod bolts, flywheel bolts, K-bike rear wheel bolts and, airhead cylinder studs.

The other type of joint is loaded by shear force (Fs). In a joint loaded in shear, the friction between the parts keep them from moving when subject to a shear force. The friction between the parts carries the load, not the fastener. An example of this type of joint would be a shock absorber mount or the driveshaft flange on an airhead. The greater the preload force, the greater the clamping force, the greater the friction, and the stronger the joint. With a properly designed and tightened joint, the bolt will not experience a direct shear load.

The mating parts also act like a spring, but a much stiffer spring. In the ideal case, the mating parts are much, much stiffer than the fastener. Engine-mounting studs on an airhead boxer are close to the ideal because the engine case and frame are much stiffer than the studs.

Joints with soft gaskets such as the oil pan or valve covers are an exception to the More-Preload-is Better rule. High loads can deform the gasket or mating surface, which causes leaks.

Proper preload is the key to reliable bolted joints (see figure 3a).

Who Determines The Strength of A Bolt?
Figure 3a
(Courtesy of SPS Technologies, Aerospace Fasteners Group)

If preload is so important, how is it set? This is the 102,000dm question. The ideal way is to measure it directly with strain gauges or some other force-indicating instrument. This is impractical and unnecessary for all but the most critical applications such as aircraft turbine engines. Tightening the fastener creates the preload. Ideally, the preload is related to the applied torque by the torque-tension relationship:

Fp = T/(D *K)
where: Fp = the preload
T = the torque
D = the thread diameter
K = a “tightening factor” or “K-value” specific for the assembly conditions

For many industrial and automotive applications, the torque is used to establish the preload. Angle torquing is a more accurate method in which the joint is tightened to a low torque to take up slack in the joint. The fastener is then tightened a specified amount of rotation with the help of an angle indication tool. Angle torquing is sometimes called the “Turn of Nut” method in industrial publications. Special washers are also available that produce an indication when the desired preload is achieved. Since the fastener is essentially a spring, the preload force can be ascertained by measuring the elongation of the fastener. The most direct method is to mount a strain gage on the fastener or mating part to indicate the preload force while tightening. Table 1 gives the Industrial Fastener Institute’s estimate of the effectiveness of various preloading methods.

Preload Measuring Method Error
Operator “Feel” +/- 35%
Torque Wrench +/- 25%
Angle Torquing +/- 15%
Load Indicating Washer +/- 10%
Fastener Elongation +/- 5%
Strain Gauges +/- 1%

Table 1

I suspect the +/- 35% variation listed for “feel” is conservative because it is developed in a production environment in which the same wrench is used every time. Wrench type and handle length and shape change the amount of torque applied. For example, I have a 10mm sockets in 1/4, 3/8″ and 1/2″ drive sizes. I am sure I apply more torque when using the 1/2″ drive than the 1/4″ drive. It is interesting to note that using a torque wrench gives a relatively small improvement over operator “feel.” However, using a torque wrench to set preload is still better than feel.

BMW and other manufacturers specify the torque of critical fasteners. BMW also publishes the DIN/ISO standard for all fasteners not specifically called out in the assembly instructions. However, Torque is a relatively poor way of generating preload because the value for K can be greatly influenced by the friction of the threads of the screw or nut on the parts and the friction of the bearing surface of the head. Friction changes with surface finish, material, hardness, and lubrication. Manufacturers are aware of this and use a safety factor when designing assemblies to account for the inconsistency in the K-value.

Attention to detail when reassembling improves the consistency of the K-value and improves the consistency of the preload generated by tightening. Threaded connections should turn freely with no binding when assembling. Test the bolt with the nut or threaded hole to make sure it turns freely. If it does not, inspect the threads for damage. (Bumps on our inclined planes reduce their efficiency.) Light damage can be cleaned up with the appropriate tap or die. Heavily damaged fasteners should be replaced. Dirt and debris in the threads can also cause them to bind. They can be cleaned with spray cleaner and compressed air. Heavily encrusted threads may have to be cleaned first with a tap, or bolt with a flat filed about 1/3 of the way across on one side.

For those readers who may doubt that there can be a +/-25% variation in preload when using a torque wrench, I have experience that shows this to be true. At work, I did an experiment where I torqued 36 screws to the point where they yielded (stretched). I assumed that the preload required to yield all screws was the same because they were from the same production lot from a reputable manufacturer. For the test, I used the same parts, the same torque wrench, the same lubrication, and the same technique. My results gave a perfectly normal distribution of torques with a standard deviation of 7.9%. This statistic indicates that 99% of the screws would yield at the mean torque value +/-23.7%, which is darn close to +/-25%.

Angle torquing is a more accurate way of preloading fasteners. The fastener is tightened to a seating torque to take up the slack and then tightened by turning it a specified number of degrees regardless of torque using a tool to measure the amount of rotation. This method is only used for certain highly loaded critical fasteners where the application warrants the extra time and expense such as with automotive head bolts sometimes use this technique. BMW uses the angle torquing method for highly stressed fasteners such as K-bike connecting rods and R11/12 cylinder heads.

It is important to heed the manufacturer’s specification for thread lubrication if provided. One critical area where this is spelled out is in BMW’s documentation for the K-bike rear wheel bolts. The bolts should be assembled dry. It may be tempting to add an anti-seize paste to prevent rust and make them easier to remove later. The tightening torque specification is developed using a “K” value for dry threads. Lubricating with a high-pressure lubricant like anti-seize reduces the friction, resulting in a much greater preload than the nominal design value. In the extreme case the bolts stretch when tightened to the recommended torque. If anti-seize is used, the tightening torque should be recalculated to account for the lower thread friction.

The threaded fasteners in our motorcycles work by stretching when tightened to produce a tension that clamps parts together. Tightening to a specified torque is the most common way of tightening/preloading fasteners. However, this produces a preload force with a potential for a +/-25% error. The good news is that there is a conservative amount of safety factored into the joint design. Fasteners have a good margin to allow for this variation.


Deutsche Industrial Norms
International Standards Organization
The tensile (pulling) force generated by tightening the fastener. See figure 1a
The twisting force applied to a fastener usually expressed an Newton-meters (N-m) or foot-pounds (ft-lb).
Torque Wrench-
Special Wrench used to tighten a fastener to a specified torque.
Threaded fastener tightened by applying torque on the head.
Threaded fastener tightened by applying torque to a nut.
The condition where sufficient tensile force has been applied to cause the fastener to permanently deform. (Our spring has stretched.)

Keeping It All Together

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.

Basic Screw/Bolt Dimensions
Figure 1b, Basic Screw/Bolt Dimensions

Screw/Bolt Types
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 bolt’s 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
N/mm2 psi N/mm2 psi
2 none Steel 510 74,000 393 57,000
5 3 Steel 827 120,000 634 92,000
8 6 Alloy Steel 1030 150,000 896 130,000
SHCS none Alloy Steel 1240 180,000 965 140,000
18-8 none 302 Stainless 690 100,000 448 65,000
316 none 316 Stainless 690 100,000 448 65,000
Metric Class Marks on Head Material Tensile Strength Yield Strength
N/mm2 psi N/mm2 psi
8.8 8.8 Steel 800 116,000 640 93,000
10.9 10.9 Steel 1040 151,000 940 136,000
12.9 12.9 Alloy Steel 1220 177,000 1100 160,000
A2-70 A2-70 304 Stainless 700 102,000 450 65,000
A4-80 A4-80 316 Stainless 800 116,000 600 87,000

Table 1b,Fastener Property Comparison

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.

Oh, Rats!

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 don’t 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 M5.9×1.1 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.


The force/load on a material divided by its cross section area. Normally expressed as pounds per square inch (psi) or Newtons per square millimeter (N/mm2).
American Society for Testing Materials
Deutsch Industrial Normals
International Standards Organization
Original Equipment Manufacturer. Parts made to the specifications used on a new vehicle


Keeping It All Together

Torque Wrenches- How Good Are They?

Part 3 of 3

It is important to have a reliable, accurate torque wrench to properly tighten fasteners to specification. A torque wrench is probably be one of the most expensive hand tools in your collection. In this, the final installment, I explain the differences between the two common types of torque wrenches and explain how to use them. I also share data I obtained by testing a bunch of wrenches.

There are two common types of torque wrenches for home shop use; the “beam” type and the “clicker” type. The beam type torque wrench is shown in Figure 1c and is the least expensive torque wrench. The beam wrench works by the beam bending in response to the torque applied as shown in Figure 2c. This type is very simple, reliable, and accurate, and there is little that can go wrong with it when used properly. When tightening a bolt, make sure to only apply force in the center of the handle. This allows the beam to bend in the manner it was designed to indicate the correct torque. Do not over torque the wrench or the beam may bend permanently. Do not drop the wrench because rough handling can bend the pointer arm or pointer. If the pointer is bent, it can be bent back to the center without affecting accuracy. If the beam is bent it cannot be bent back.

Parts of A Beam Torque Wrench
Figure 1c, Parts of A Beam Torque Wrench

Beam Torque Wrench Operation
Figure 2c, Beam Torque Wrench Operation

Figure 3c shows the clicker torque wrench, which is sometimes called a digital wrench. A clicker torque wrench works by preloading a “snap” mechanism with a spring to release at a specified torque. When the mechanism releases the ratchet head it makes a “click” noise as shown in Figure 4c. The torque is set by rotating the handle until the desired torque is shown in the window. Older clicker wrenches have a micrometer style scale along the handle instead of a window. The clicker wrench is much easier to use because it is easy to set the desired torque and just pull until you feel the click. The ratchet head also makes it easy to use in confined spaces. It is good practice to set a clicker wrench to its lowest setting before putting it away to prevent the spring from taking a set. Avoid rough handling and dropping because it can damage the mechanism. Do not use the torque wrench to loosen tight fasteners since this may damage the calibration.

Parts of A Clicker Torque Wrench
Figure 3c, Parts of A Clicker Torque Wrench

Parts of A Beam Torque Wrench
Figure 4c, Clicker Torque Wrench Operation

I always wondered about the accuracy of torque wrenches, so I made my own torque wrench tester. The tester consisted of a lever arm that lifted a series of weights off the floor. The torque tester is shown in operation in Figure 5c. By changing the position of the weights on the lever arm and changing the weights, I could obtain torques from 3 to 105 ft-lb. The weights were barbell weights that I determined the exact weight using a digital shipping scale. I then calibrated the lever arm by using a precision electronic torque wrench and then back calculated the lengths using my known weights. A known torque exists when the arm is horizontal and the weights jar lifted off the floor. I made a table of lever arm lengths and weight combinations so I could determine the applied torque in any situation. I estimate the accuracy of my home-built instrument to be +/-3% of the calculated torque.

Torque Tester
Torque Tester
Torque Tester in Operation
Joey Dille Demonstrating the Torque Tester

Figure 5c

Using the torque tester is straight forward. The pivot tube is placed in a vise and the desired weight is placed on the holder. The lifting chain/cable is then adjusted so the lever arm is parallel to the floor and located at the desired length per the torque table. The torque wrench is then inserted into the tester so it is approximately even with the lever arm. For beam wrenches, the wrench is rotated until the weights come off the floor and the indicated torque and applied torque are recorded. For clicker wrenches, the procedure is a little different. The wrench is set for torque slightly below the calculated applied torque. The wrench is then inserted in the tester and rotated slowly until it clicks. The wrench is set for the next higher torque increment and tried again. Successively higher torque settings are tried until the weights can be lifted from the floor without the wrench clicking. The highest setting where the wrench still clicked was recorded along with the applied torque. I learned that it was important to rotate the wrench slowly to avoid premature clicking caused by the inertia of the weights.

Once complete, I decided to use my new toy to check a bunch of torque wrenches to see how good they really were. I asked my friends in the Mac-Pac* to bring their torque wrenches to one of our wrench sessions at Bruce’s garage. I was able to test a total of 13 wrenches, 3 beam, and 10 clickers. The results of my testing is shown in Table 1c and Figures 6c and 7c. I found the clickers to be much more repeatable than I expected. The beam type and clickers were both fairly accurate and linear over their range. The bottom line is torque wrenches, even inexpensive ones, were quite good.

Owner Size Type Range
Maximum Error Average Error
ft-lb % ft-lb %
Joe 3/8 Clicker 5-75 -7.3 -12.3% -5.0 -11.0%
Joe 3/8 Beam 2-50 1.3 0.3% 0.6 0.2%
Roger 3/8 Clicker 2-21 -1.6 -13.9% -0.8 -8.6%
Bruce 3/8 Clicker 2-21 1.0 5.3% 0.8 1.7%
Mike D. 3/8 Clicker 10-80 1.2 0.6% 0.2 0.1%
Wayne 3/8 Clicker 10-75 -2.8 -6.8% -1.6 -4.0%
Joe (new) 1/2 Clicker 10-150 -4.3 -4.7% -1.7 -2.4%
Joe (old) 1/2 Clicker 10-150 4.7 -10.7% 0.2 -0.6%
John 1/2 Clicker 10-150 -3.3 20.0% -1.0 1.1%
Mike D. 1/2 Beam 5-150 7.8 28.1% 4.9 12.7%
Ron 1/2 Beam 5-150 -4.3 -6.8% -2.5 -4.3%
Bruce 1/2 Clicker 25-250 -1.8 -3.4% -0.8 -1.3%
Mike B. 1/2 Clicker 10-150 2.7 3.9% 1.2 2.0%

Calibration Results for 3/8
Figure 6c

Calibration Results for 1/2
Figure 7c

If you wonder about the accuracy of your wrenches, you can get them calibrated by a Snap-On tool dealer or a local metrology lab. Griot’s Garage can also calibrate torque wrenches on a mail order basis for $25 plus shipping.

Torque Wrenches and Accessories

From time to time people have asked me if it is OK to use an extension with a torque wrench. The answer is yes. Using an extension or reducer with a torque wrench does not affect the accuracy. Others have asked if it is OK to use a universal joint with a torque wrench. The answer is NO. Universal joints change the torque as the drive angle increases. I checked this out with my torque tester. The results are shown in Figure 8c. Don’t use universal joints with torque wrenches.

Effect of Universal Joint on Torque Reaction
Figure 8c

Torque extensions are sometimes required to tighten fasteners in locations where the torque wrench will not fit such as the drive shaft flange on older airheads. Figure 9c shows the extension I made for this purpose. To work correctly, one must understand how the position of the extension affects the torque as shown in Figure 10c. There is a formula for relating actual bolt torque to the wrench torque based on the length of the wrench and extension and the angle between the two. It is best to keep the two at right angles so the torque will be the same.

Home-Made Torque Wrench Extension
Figure 9c, Home-Made Torque Wrench Extension

Proper use of Torque Wrench with Extension
Figure 10c, Proper use of Torque Wrench with Extension

I wish to thank fellow BMW riders:

  • Roger Albert
  • Brian Curry
  • Bob Spena
  • Wayne Woodruf

for their help editing the article and providing encouragement. I hope this article has removed some of the mystery from the little things that keep our bikes together. I will close with a list of useful fastener-related references.

Fastener References:

Mr. Metric Inc.
SAN JOSE, CA 95112
(800) 944-1897
Stainless and Plain Steel Fasteners
Nice Catalog
Maryland Metrics
P.O. Box 261
Owings Mills MD 21117
(800) 638-1830
Stainless and Plain Steel Fasteners
Great technical section on web site
Gardner-Wescott Co.
10110 Six Mile Road
Northville MI 48167
Stainless and Plain Steel Fasteners
No minimum order
Barnhill Bolt
2500 Princeton NE
Albuquerque, NM 87107
(505) 884-1808
Stainless and Plain Steel Fasteners
Good online FAQ
Metric & Multistandard Corp.
120 Old Saw Mill River Rd
Hawthorne NY 10532
Stainless, Plain Steel Fasteners and Tools
Good catalog and technical information
Internet BMW Riders
Links to Many Fastener Suppliers
Rocky Point Cycle
2509 Linebaugh Rd
Xenia OH 45358-9512
Stainless Fasteners and Parts
Good stainless selection
Moto-Bins Ltd
16 Surfleet Road
Lincolnshire PE11 4AG UK
+44 (0)1775 680881
Stainless Fasteners and Parts
Rare stainless bits
Emhart Fastening Teknologies
510 River Road
Shelton, CT 06484 USA
HeliCoil Thread repair kits
Loctite Corp.
10 Columbus Boulevard
Hartford Square North
Hartford, CT 06106
Thread locker and thread repair components
2801 80th Street
PO Box 1410
Kenosha, Wisconsin 53141-1410
1-800-TOOLS-4-U (1-800-866-5748) Tools and Torque wrench calibrations
Airheads Beemer Club
Great Article on Heli Coilstm
Griot’s Garage
3500-A 20th St. E.
Tacoma WA 98424 800-345-5789
Tools and Torque wrench calibrations
WTI Fasteners LTD
British manufacturers of Helical wire thread inserts
Alfa Romeo Owners of Oregon
International Fastener Guide 🙂
The Bolt Depot
286 Bridge Street
North Weymouth, MA 02191
Fasteners and lots of good tech info
Tegger’s Torque Wrench FAQ
A great exploded view and explination of how a clicker wrench works.
Stainless Fasteners Handbook
A design guide by the Stainless Steel Industry of North America. This is a great compilation of information. The technical information is spot on, but the material comparisons are a bit biased. Read with care.
Flexible Assembly Systems Inc.
8451 Miralani Drive, Suite N
San Diego, CA 92126
(800) 696-7614
High end industrial torque wrenches, calibration services and FAQs
Thanks for visiting!

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