The short answer to the question of "why threadlockers?" has to do with the fact that trying to properly tighten a nut or bolt to a specific torque value (let alone an unknown one) is an inconsistent, erratic process at best. Getting any fastener to do its job faithfully in spite of vibration and heat (meaning expansion, contraction, and outright distortion in dissimilar metals) can be very tricky.
First, we have to deal with the basic concept of how a bolt, screw, or practically any other threaded fastener really works. To come right down to it, they all function on the physics of tension (stretch and elasticity), not shear (sideways or lateral loading). In other words, the proper function of a bolt, let's say, depends on its ability to clamp parts together, not to hang 'em like a picture from a wall.
Coupled with this fact is the questionable practice of attempting to get the precise amount of stretch required for a particular fastener to do its job without fail (or failing!) by using a torque wrench. For instance, is that supposed to be "dry" or "wet" torque? To get the same amount of stretch from a given fastener, the difference between wet and dry can require anything from 20-percent more torque to more than 100-percent increases! Depending on the material the fastener is made of and how it's coated, it might even require a reduction in torque setting to work. Still, imperfect as it may be, using a torque wrench to snug up a bolt on your hog is the accepted method of getting it done the way the shop manual specifies it be done. Screws usually don't have torque values at all, and reusing worn or dirty fasteners of any kind just makes for way too much guesswork when it comes to critical fasteners, wouldn't you say? And on a Harley, what fastener isn't critical?
The Motor Company understands this issue pretty well, and is moving more and more toward sophisticated fastener-tightening techniques that rely on creating the proper stretch in everything from head bolts to compensators to pulley nuts, while relying less and less on basic torque settings.
Knowing that, consider what happens when you are bolting things to your bike the old-fashioned way. The reality, regardless of the reading on your wrench, can be anything from, say, 16-24 lb-ft (if it's even in that ballpark when you're finished), so it's not the most accurate gauge of fastener stretch. If you aren't using a torque wrench, there's simply no telling. Yet, in spite of vibration, heat, cold, and who knows what sort of demands on the fasteners, you hope things will stay put.
So, knowing it's not quite as scientific and precise a process as it should be, how do you cover your bets and go from hope to near certainty?
Arguably the best way, bar none, is to apply a little dab of anaerobic liquid resin. Invented in the basement of a Trinity College chemistry professor and his son Robert back in 1953, the stuff came to market in 1956 known by the name coined by Robert's wife-"Loctite." Today, there are plenty of companies making this magic in a bottle. Permatex and Loctite, however, are the most famous makers of these sealants, which harden and expand in the absence of air and come in several different strengths and for practically any automotive application you can name. What makes so-called threadlockers so indispensable is this ability to dry and adhere to metals, particularly threaded metals in microscopic spaces.
In virtually every case where sheer heat (more than 400 degrees or so) doesn't preclude its use, the factory recommends the use of a particular grade of threadlocker that's suited to the specific task. One major reason (aside from vibration-proofing) is that threadlocker will tolerate a little variance in torque and will help compensate for slight irregularities in stretch from one fastener to another.
It's simple: Using the right grade of threadlocker, in the right applications and in the right way, is the best insurance that your hardware will stay attached. It's a major help (and minor miracle). So sit tight and lock on as we give you the rundown on using threadlocker to tighten up your ride.