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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.
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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.
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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.
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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.
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 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.