For thread preparation, three choices exist: leave the thread dry (unprepped), treat the threads with a lubricant, or treat the threads with a thread-locking compound. With almost no exceptions, leaving threads untreated is a mistake, so for a good mechanic the question comes down to whether to lube it or to “loc-it” (use a thread-locking compound).
Lubrication provides three benefits. First, the lubrication reduces galling (the tearing of the surface of a material as a result of friction). Second, through the reduction of friction the lubricant allows the threaded fitting to turn further at a given torque, thereby increasing the tension on the fitting (it’s the tension that ultimately keeps the part from unthreading). Finally, lubrication resists contamination of the thread by moisture, which reduces the potential of corrosion developing.
Thread-locking compounds also provide several benefits. In the case of liquid thread lockers applied at the time of final assembly, they also have some lubricating properties. These lubricating properties reduce galling and reduce friction. While these properties are in common with lubricants, lubricants are superior at doing these things. Second, thread-locking compounds (I’ll just call them “thread lockers” from this point on), cause a part to hold more securely at the same torque compared to using a lubricant. Finally, thread lockers (particularly the liquid ones) have the maximum potential to prevent corrosion.
So at first glance, it appears as though thread lockers come close to matching lubricants in some ways, and offer superior results regarding security and corrosion prevention. But that’s not the whole story, because each of these choices has certain drawbacks.
With lubricants (which include oil, grease, and anti-seize compounds), ease of application varies depending on the material and the circumstance. When parts are fully disassembled, it’s pretty much a toss-up which of these three categories of lubricants are easiest to apply (though there are small differences). But when working with partially-assembled parts (for example, lubricating a cable pinch bolt that is already installed in a derailleur or brake), the ability of oil to seep into the threads gives it an advantage over grease and anti-seize, which both require full dissassembly to apply. On parts that are not assembled, grease and anti-seize are equally easy to apply, but anti-seize is notorious for making a mess that’s no fun to clean up.
Thread lockers exist in two basic categories: dry “pre-applied” compounds (which are on the threads when the part leaves the manufacturer), and liquid compounds (which the mechanic applies immediately prior to assembly). With liquid thread lockers, one must always disassemble the fitting (if pre-assembled), which is harder than using oil, but the same ease of use as grease. Dry thread lockers take zero effort to apply (since they are pre-applied at the factory), but since they cause the part to have high resistance during installation, ultimately they are the least convenient of all the options.
The dry thread lockers excel at the locking job, but since they are rarely applied to the full length of a threaded item, they leave some threads exposed to moisture (and the potential for corrosion). Properly applied, liquid thread lockers will cover 100% of the thread, thereby maximizing the corrosion resistance.
There are three common misconceptions about thread lockers. First, some think that thread lockers cause problems with disassembly, possibly to the point that a part may be destroyed in the attempt to remove it. This misconception results from using the wrong grade of thread locker for the application. Even when misuse has occurred, heating the parts with a hot-air gun will cause the thread locker to soften enough to eliminate any removal difficulty.
The second misconception is that as soon as one breaks loose parts retained with a thread locker, that all the benefits are lost unless fresh thread locker is applied. In fact, with both the dry and liquid thread lockers, only a small percentage of effectiveness becomes lost with each dissassembly repetition. That is because both varieties are reactivated by heat. The very resistance that occurs from re-installing a part with used thread locker on it causes a repetition of the curing cycle. This does not mean that one should ignore re-applying thread locker when parts have been fully separated; more relevantly, when a mechanic wants to just loosen and retighten an adjustment where thread locker has been used, it shouldn’t be considered necessary to fully remove the part to reapply thread locker.
The third misconception about thread lockers are that they are some form of an adhesive. In the case of liquid thread lockers, the liquid consists of an anaerobic compound that changes from a liquid to a solid with reduced exposure to oxygen (which is why brand-new bottles come about half full). In converting to a solid, these anaerobic compounds also expand, thereby creating more pressure between the threads (which explains why the parts end up more secure at a given torque than would have been the case with dry or lubricated threads). This solid cannot be dissolved by water, and it fills all the air gaps between the threads, which explains why nothing beats the anti-corrosion benefits of liquid thread lockers (when applied to cover all of the threads). Dry thread lockers are more of a plastic that softens with heat (which comes from the friction of assembly), then the thread locker re-hardens with cooling.
Oil, grease, and anti-seize are all closely related. In fact, grease and anti-seize are simply oil with additives.
Of these three types of lubricants, oil, being a liquid, remains in place for the least time. It diminishes through leaking, evaporation, and “wash-out” (which is the result of repeated exposure to liquid water). Grease is oil that has been stabilized by the addition of some form of wax. This reduces the rate of leaking (leaking still occurs, particularly in warm environments, because the wax and oil tend to separate), evaporation (which still occurs for the same reason as leaking), and washout (because the semi-solid state of grease resists displacement by water better than liquid oil). The better stability over time gives thread lockers the advantage over lubricants in regard to corrosion resistance.
Anti-seize is a high-wax-level grease with the addition of additives that chemically reduce corrosion by changing the exchange of ions between two adjacent dissimilar metals. Without the neutralization of this ion exchange, the two dissimilar metals become chemically bonded by the creation of a new compound between where the two materials touch. Since this ion exchange does not happen uniformly, the two initially-smooth metallic surfaces become very textured, with the new compound acting very much like a mortar between the very rough surfaces of two concrete blocks. Note that galvanic corrosion only occurs between dissimilar metals, so using anti-seize between two pieces of steel, two pieces of titanium, or two pieces of aluminum is not very different than just using grease. The high wax level reduces the tendency for leaking, evaporation, and “wash out”, but because anti-seize can still degrade over time, properly used liquid thread lockers still provide the ultimate corrosion resistance.
Given all this, it seems as though using liquid thread locker would always be the best choice, but this misses a few points. First, since liquid thread locker doesn’t lubricate as effectively as grease or oil, when a manufacturer recommends a torque based on the assumption that the threads are treated with a lubricant, the mechanic must account for this. With a thread locker, when the recommended torque is a range of values, use the bottom of the range to prevent the possibility that the extra resistance to “break-free” caused by the thread locker won’t lead to deformation of the wrench flats. Second, parts treated with thread locker don’t just spin effortless after the “break free” point has been overcome, leading to a more tedious and time-consuming breakdown. Finally, when lubricating threads, the exact choice of lubricant is not terribly important, but when using a thread locker, the thread diameter, the thread pitch, and even the length of thread engagement can influence which grade of thread locker works best.