According to Wikipedia.org:
"A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force.
A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread.
A screw thread is the essential feature of the screw as a simple machine and also as a fastener.
The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation."
'Stripped threads' is a condition where the threads on a screw, in a nut or in any other threaded hole, shear away from the base material either partially or completely. The result is that a fastener cannot be tightened and therefore will not be able to hold parts together. Depending on the application, this can be dangerous or can result in other long term damage to the parts that are fastened together.
Stripped threads can be a result of any of the following. Note that some of these conditions can also result in snapped fasteners:.
Over tightening can cause stripped threads by producing excessive tensile loads. When the load on a screw is too great, it can break or the threads can yield. This is one reason why following torque specifications is so important.
This may seem absurd at first and granted it depends on the application. In some cases, if a fastener is not sufficiently tightened, the fastened parts can hammer back and forth causing impact loading on the fastener potentially leading to failed threads or a broken fastener. Mating parts can also be damaged.
High maintenance items that require frequent disassembly and assembly can be problematic. Stripped threads occur when a screw has been loosened and tightened so much that the threads simply wear out. This is particularly true when steel fasteners are used in nonferrous materials such as aluminum. A problem area on a dirt bike might be the seat mounting bolts because the seat is typically the air filter access which requires frequent maintenance.
Fatigue occurs when a material is subjected to repeated unloading and loading. This is analogous to vibration. The higher the load and frequency, the greater likelihood of thread failure.
This is similar to over tightening. However, it is possible to properly tighten a screw and then overload it in use. A good example would be an eyebolt used for hoisting an object. If the object is too heavy for the eyebolt, the threads can give way.
In the case of corrosion, the thread material can corrode to the point there is no holding power. Often, the corroded surfaces will bond together (ex. nut on a screw). The manner of failure is dependent on other variables such as the fastening materials used. Typically, the corroded threads will strip when screw removal is attempted.
For strength, material is needed. If a male threaded fastener does not engage enough threads into its female counterpart, the threads will break out. This can occur on either part depending on what material the parts are made from.
Dry bolts produce friction as they are tightened. Similarly, defects in threads such as dents, high spots or whatever can cause sufficient friction resulting in thread failure. The cleanliness of threads is important as well. Sand, iron grit or whatever can cause threads to heat up and lock together, ending up stripped.
What you need to do depends on the situation. If a nut has stripped, then the solution is to replace the nut. No repair in this instance is practical. The mating bolt or screw should also be replaced even if the threads appear to be good.
Repairing stripped threads generally implies that internal threads in a machined part are damaged. Threaded holes in engine cases, fork clamps, frame weldments etc. are examples of such. Within this context, there are still different possible repair solutions.
This is the expensive option, but a good choice actually. A trained, professional machinist has the skills, knowledge and equipment that enable him to make such repairs. Choose this method for parts that have risky consequences for the do it yourselfer. Sometimes, the expensive option is the cheaper option in the long run.
Occasionally, there are circumstances where you can drill out the stripped threads and use a nut on the back side (a longer screw will also be needed). You can do this if there is room to access a nut. Additionally, there must be a flat surface for the base of the nut to sit on. If the back surface is sloped, a tightened bolt will tend to bend. When drilling out a hole that is stripped, choose a drill size just big enough to clean out the bad threads. A flat washer and or lock washer should be used on the nut side. If the base material is aluminum or other soft material, a flat washer is needed to prevent damage to the base material.
Going over sized means threading the hole (called tapping) to the next standard size larger thread. While this is an option that works well, consider it carefully. Going over sized could compromise the integrity of the fastened parts. In many instances, there is not enough material around a screw hole to accommodate larger sizes. Usually, when converting to a larger thread size, the through hole on the mating part would also need to be drilled over sized for the new screw to move through freely. Some engine case covers can be exemplary of when you shouldn't do this. Drilling could leave little material remaining around a hole which could result in cracking. Also, a drill bit could break through the side of the hole if there is not enough material to sustain the forces of drilling (the hole walls being too thin). To use an over sized screw, you need to know how to drill and tap a hole and there are some basics of tapping that are important. Below are some good videos about tapping to help you.
Thanks to projectsinmetal.com, here is a three part series covering the basics of tapping. Much is discussed ranging from the available types of taps (Part 1), lubricants (Part 2), and tools used (Part 3):
This next video shows the drilling and tapping process:
For tap drilling shallow closed end holes, the proper drill diameter must be cut to the minimum depth required for the threads. Sometimes, this cannot be accomplished with just a standard drill. After a standard drill has been used, it is possible to follow with a flat bottom drill as explained in this video (unless you have the appropriate end mill):
You can grind a general purpose tap so threads can be cut deeper into a closed end hole. This is not necessary for every closed end hole, but some holes can be shallow, such as fork guard mounting holes. Taps can be purchased that are made for closed end holes:
Threaded inserts are a great invention and they work very well when used properly. An insert such as this is basically a larger diameter screw than the original. The inside of the insert is threaded to accept the original fastener. To install an insert like this, the stripped hole must be drilled and tapped to accept the insert. Furthermore, thread locker should be used to secure the insert in place. However, some inserts have built in stakes that get driven in to secure the insert once it has been threaded in. One limitation of a threaded insert is the diameter. This is basically the same limitation discussed in the previous "Go over sized" section. Refer back to that section to learn about tapping threads.
If you would like to learn about threaded inserts, please watch this video:
There are a few brands of helical inserts, one of which is Heli-Coil (my favorite thread repair option). A helical insert is a precisely wound thread shaped wire. This option is great because it requires minimal change in diameter of the damaged hole in order to restore it to the original size. Initial cost because can be a little expensive because a special tap size and drill specific to the insert are required ass well as the installation tools. However, inserts can be purchased separately should you already have the special tooling needed. A properly installed helical insert makes an excellent repair as good as or better than the original thread. Kits are available in many sizes and configurations.
Hopefully this article can help you choose a wise plan of action. Regardless of what option you take, complete the repair so the final outcome is identical to the OEM size and quality. When in doubt or if you don't feel as though you have the skills needed to make thread repairs, find a local machinist.