A primer on torque, torque wrenches, threads & fasteners With bicycle and component materials getting lighter and more exotic, it's more important than ever to fasten them together properly. In this primer, we look at threads, torque and the use of torque wrenches, thread lockers and anti-sieze to keep everything together properly. This article will discuss the basics of torque; torque wrench use; and the theory of threaded fasteners. A table of bicycle specific torque values is available as an Adobe Acrobat® PDF file. Click HERE for the torque table. Threaded fasteners (nuts and bolts) are used to hold many components to the bike. As a fastener is tightened, the fastener actually flexes and stretches, much like a rubber band. This stretching is not permanent, but it gives the joint force to hold together, called "preload", or tension. Each fastener is designed for a certain range of tension. Too much tightening will deform the threads or the parts. Too little preload will mean the fastener will loosen with use. This can damage components, such as a crankset used with a loose bolts. Loose bolts and nuts are also generally the source of various creaking on the bike. Tension in the fastener depends largely upon the amount of torque (tightening) and the size of the thread. Generally, engineers will specify a thread size large enough to handle the anticipated stresses. For example, the M5 bolt of a water bottle cage bolt would not be a good choice for holding a crank. Even if the bolt were as tight as possible, it would not provide enough force to hold the arm secure to the spindle. The crank-to-spindle interface receives quite a lot of stress, making a larger thread (M8, M12, M14) a better choice. The amount of pressure applied by a thread can be substantial in order to hold the joint secure. For example, a fully tightened crank bolt can provide over 14,000 Newton force (3,000 pounds) as it holds the arm in place. It is commonly believed that bolts and nuts often come loose for no apparent reason. However, the common cause for threaded fasteners loosening is simply lack of tension during initial assembly. Vibration, stress, use, or abuse cannot typically overcome the amount of clamping force in a properly sized and secured threaded fastener. As a simple rule of thumb, any fastener should be tightened as tight as possible without failure of the thread or the component parts. This means the weakest part of the joint determines the limits of tension, and hence, torque. Torque measurements Torque for mechanics is simply a twisting or turning motion around the axis of the thread. This resistance can be correlated to, but is not a direct measurement of, fastener tension. Generally, the higher the resistance to rotation, the greater the tension in the threaded fastener. In other words, the more effort it takes to tighten a bolt, the tighter it is. Torque is measured as a unit of force acting on a rotating lever of some set length. In the USA, the common unit used to measure torque is the inch-pound (abbreviated in-lb.). This is a force of one pound acting at the end of a lever (wrench) only one inch long. Another torque unit used in the USA is the foot-pound (abbreviated ft-lb.), which is the force in pounds along a one foot long lever. It is possible to convert between the two units by multiplying or dividing by twelve. Because it can become confusing, it is best to stick to one designation. The units given in the torque table here will be the in-lb. A more universally accepted torque measurement is the Newton-meter (abbreviated Nm). One Newton-meter is a force of one Newton on a meter long lever. Another option sometimes used is the kilogram-centimeter (abbreviated kgf-cm), which is a kilogram of force acting on a lever one centimeter long. It is possible to convert between the various systems: in-lb = ft-lb. × 12   EXAMPLE: 5.5 ft-lb × 12 = 66 in-lb in-lb = Nm × 8.851   EXAMPLE: 9 Nm × 8.851 = 79.7 in-lb in-lb = kgf-cm × 0.87   EXAMPLE: 300 kgf-cm × 0.87 = 261 in-lb Torque Wrench Types Torque wrenches are simply tools for measuring resistance to rotation. There is a corallation between the tension in the bolt and the effort it takes to turn it. Any tool, even a torque wrench, should be used with common sense. A cross-threaded bolt will not properly tighten even with a torque wrench. The mechanic must be aware of the purpose of torque, and what it is try to achieve in the joint. It is also important to consider thread preparation, which is discussed in detail at the end of this article. Torque wrenches available to general industrial work, including bicycle work, are accurate to plus or minus four percent. In other words, a torque wrench set for 100 in-lbs might tighten to 96 in-lbs, or 104 in-lbs. There are basically three types of torque wrenches, the beam type, the click type and the dial type. Beam type Beam-type wrenches are relatively simple. The socket head holds two steel beams, a primary beam and an indicator or pointer beam. The primary beam deflects as the handle is pulled. The separate pointer beam remains un-deflected, and the primary beam below flexes and moves with the handle. The reading is taken at the end of the pointer, at the reading plate on the primary beam. The handle is moved until the desired reading is attained. These wrenches rarely require recalibration. If the pointer needle is not pointing to zero when the tool is at rest, it is simply bent back until it does align. Fatigue in the steel is not an issue.      Click type Click-type torque wrenches use a pivoting head on top of a coil spring. The spring is compressed or relaxed for different settings. These wrenches usually have a ratcheting head. Rotate the handle on the barrel-body to the appropriate setting. Turn wrench bit on bolt/nut and both feel and listen for the head to pivot or "click". The click noise may be soft or even non existent. Stop at this point. After use, relax the spring by dialing the setting down to relatively low settings, but use care not to back down below the scale. The click-types do rely on moving parts, and will eventually require service. The internal parts may need cleaning and greasing, and the wrench may require re-calibration. It is best to send the click-type wrench to a professional service center for this work. Dial type The dial torque wrenches are similar to the beam types, except the beam is connected to a dial by gears. This is then housed inside a protective box-like cover. Simply turn the wrench until the dial points to the desired torque setting. Dial type wrenches are typically not ratcheting. The dial type wrenches have fewer parts than the click-types, and require relatively less service.

Thread Theory To effectively service fasteners, it is important to have a working knowledge of threads. A thread is a continuous heilical ridge formed on the inside (nut) or outside (screw) of a cylinder. This ridge is called the crest. Between each crest is a space, called the root. Threads are set at an angle to the axis of the bolt or nut. This slope is called the helix angle. The angle must be sloped, either upward to the right (for right-hand threaded screws) or upward to the left (for left-hand threaded screws). The thread forms a "V" shape between crest. The angle of this "V" is called the thread angle, and is determined by fastener engineers. For external threads (bolts), a right-hand thread slopes up to the right, but the internal right-hand thread slopes up to the left. For external left-hand threads, the threads slope up to the left, while the internal left-hand threads slope up to the right. The right-hand screw tightens clockwise (to the right). The left-hand screw tightens counter-clockwise (to the left). Left-hand threads on bicycles are seen on the drive side of bottom bracket and the left pedal. Notice the slope of the threads in the pedals threads below.     Threads are designated or named by the external thread major diameter and a pitch measurement. The major diameter is the outer diameter at the top of the thread crests. Thread sizes are given in nominal sizes, not in the actual measurement. The exact measurement is slightly below the named or nominal size. For example, a 6mm bolt may measure 5.8mm or 5.9mm, but it is called 6mm bolt. It is also common to use "M" before the bolt size, such as M6 for a 6mm bolt. Note: The wrench size for the head of the bolt or nut is not used to determine the size of the thread. For example, the common socket head cap screw in a 6mm x 1mm thread uses a 5mm hex wrench.

Thread pitch is the distance from the crest of one thread to another creat measured along the axis of the thread. Pitch is best measured using a thread pitch gauge. So-called "English", "Standard", "Imperial" or SAE threads are designated by the frequency of how many threads are counted along one inch. This is called "Threads per Inch", and is abbreviated as "TPI". Metric threading uses the direct pitch measurement in millimeters from thread crest to the adjacent thread crest measured along the thread axis. An example of an SAE thread is 9/16" x 20 TPI (pedal threads). An example of metric thread would be 10mm x 1mm (common rear derailleur bolt). NOTE: The term "Standard" threading is used primarily in the USA. The assumption in the USA is that the common SAE threading is the "standard". Typically, if a thread has a pitch designated as TPI, it is a SAE thread and the diameter is given in fractional inch sizes. If the pitch matches the metric standards, the diameter is given in millimeters. However, some thread standards will mix tpi with a metric diameter. Some Italian manufacturers use threads with a metric diameter and SAE thread pitches. For example, the "Italian" bottom bracket thread standard is 36mm x 24 tpi, and some Italian made rear axles are 10mm x 26 tpi.

Threads are sometimes identified as "fine" or "coarse". A fine thread will have a relatively small pitch measurement, and the threads will be closer together. A coarse thread has a relatively larger pitch measurement, and the threads will be further apart. Fine pitch threads are sometimes used to make adjustments. Derailleur adjusting screws are commonly a 0.75mm pitch. A quarter of a turn on a derailleur screw advances the screw end only 0.19mm. A fine thread will have less depth as compared to a coarse thread, and consequently fine threads are easier to strip. A coarse thread is more resistant to stripping but also less efficient in transmitting torque (turning) into thread tension. Generally, a fine pitch is easier to tighten in that tension is achieved at lower torques. In the image below, two bolts of the same diameter are magnified. Notice the relatively coarser threads are deeper as compared to the fine threads. Even when threads are properly sized, there will be play or slop between external and internal threads when engaged. This play is normal and disappears when the fastener is tightened. The thread diameter can be a bit larger or smaller than ideal, and yet the part will still function adequately. If tolerances are exceeded, the part may require excess force to install, or it may be quite sloppy and fail during tightening. For threads to interchange and match, both the diameter and pitch must match.

Thread Formation and Repair Threads can be made by cutting with taps or dies. Taps cut an internal thread, such as a bottom bracket shell in the frame. Dies cut an external thread, such as a steering column. Thread may also be cut using a lathe, or they may be rolled, such as threads on a spoke end, or on hub axles.    When a thread becomes damaged, there are some options for repair. Typically, when an internal thread becomes damaged, it is damaged at end of the threads, not the middle. If only minor damage has occurred, it may be possible to re-tap the thread. This assumes that enough undamaged thread is remaining to allow proper tightness. As a practical test, after tapping the thread, slightly over-torque from the recommended specification. If the thread is weakened, it will strip and not pass this test. If it does not strip, the thread is adequate, and should survive the use. Internal threads may sometimes be repaired using a coil system. Recoil® and Helicoil® Companies supply a tap, coil inserts, and coil driver. The damaged thread is drilled out to a specific size. New larger threads are installed with a specifically sized tap. The inserted coil has the outside diameter of the coil, and the inside diameter and pitch of the original thread.

Thread Preparation There is resistance to turning the bolt, as the fastener gets tighter. Some resistance comes from friction and rubbing between the internal and external thread surfaces. Because of this, it is common to prepare the threads with lubrication. This can take for form of liquid lubrication, grease, or an anti-seize compound. Even liquid thread-lockers provide some lubrication during tightening. As a simple rule of thumb, if the thread size is small, such as a derailleur pinch bolt, a liquid lubricant is adequate. If the thread is large or the torque relatively high, such as a pedal thread or bottom bracket, use a grease or anti-seize compound. There are situations, however, where a manufacturer may recommend no lubrication on the fastener. Thread-lockers are special adhesives used in many industries and in many applications. The commonly available thread-lockers are called "anarobic". These liquids cure independent of air, and will harden and expand. This hardening and expansion is what gives these materials their special feature. However, thread-lockers should not be used to replace proper torque and pre-load when clamping load is important. Cycling component manufacturers sometimes use an "aerobic", or "dry" thread-locker for their products, such as on brake caliper bolts. This compound acts primarily as thread filler. If the part is removed, the compound tends to break down, so use a liquid thread-locker to supplement.

Thread-lockers come in different grades of strength. The lighter duty lockers are considered “service removable”, and can typically be removed with normal service procedures. There are compounds that are stronger and extra procedures are often necessary when disassembling, such as heating with a heat air-gun. Most thread-locking compounds are designed for use with metals. They are usually not intended for use with plastic, and may both harden and weaken the plastic.

“Retaining Compounds” are intended for press fit applications such as pressed studs. The retaining compounds tend to have a higher viscosity than the thread-locking compounds. Many retaining compounds require special technique for removal, such as excess force and or mild heat. Retaining compounds can provide a useful repair on marginal press fits, such as a headset cup that is a poor fit to the frame.

Anti-seize compounds are typically a mixture of finely ground materials, such as nickel, graphite, lead, copper, aluminum, zinc, and molysulfide, mixed with mineral oils. These compounds provide a good insulating layer between metals, preventing galling in the threads. These compounds provide much longer protection in adverse and wet conditions as compared to grease. Use care when applying these compounds and follow the safety directions of the manufacturers.

Washers and Locking washers Washers are often used with threaded fastener. The washer distributes the stress around the bolted joint. Additionally, the washer reduces friction as the bolt turns. Generally, it is best to have the washer under the turning part of the bolt, either the nut or the head. An example of washer use is under the head of the crank bolt. The washer distributes the pressure on the aluminum arm, and allows the bolt to tighten fully when torqued.