Threadlocker can be both a blessing and a curse for DIY motorcycle work – a curse because every time you use it on a fastener reinstallation, you must clean it all off. There’s also confusion as to which type to use among the many choices, as well as arguments about which brand is best. Many may not understand how threadlocker works or simply choose to ignore it in favor of mechanical forms of security, like locking washers or safety wire. This puts threadlocker up there with oil as a topic for discussion and debate.
As with many useful things we take for granted, threadlocker was discovered accidentally. During World War II, when scientists were searching for clear plastic to use in gunsights, they developed a chemical compound of extraordinary stickiness, the first of what are now known as cyanoacrylates. Initially they saw no use for it, but the compound’s potential as a fast-setting adhesive led to the founding of Loctite in 1958. Development and proliferation of similar adhesives have continued ever since.
About the same time threadlocker for vehicles appeared in the late 1960s, Gerhard Junker developed a theory regarding the loosening of fasteners (search Google or YouTube for “Junker test”), which blames this chronic problem on vibration. Junker test equipment indeed vibrates like hell, and practically every traditional solution that has been tried to prevent loosening fails. However, the test vibrations resemble a train going over a trestle, a steam ship at sea, or a paint shaker more than the vibration we Harley riders know.
Motorcycles have all three types of loads that affect fasteners: axial (like head bolts), where the load runs in the same direction as the bolt; transverse (like axles), where the load is perpendicular to the bolt or shaft; and radial (like crankshafts and gear shafts), where the load is transverse in rotation. The Junker test mainly evaluates the axial type. Most motorcycle fastener issues are in either axial or radial situations, and that’s where tension comes in.
Torque from tightening a fastener and tension on a tightened fastener are not the same thing. When it comes to torque versus tension involving critical fasteners, the most familiar axial example is the involved process for torquing cylinder heads on air-cooled Harleys, where stretching and clamp loads under tension must be considered, not just measured torque on the bolts.
Threadlocker works to prevent tension loss from vibration by filling in the gaps between threads in fastener joints. No matter how precise or high-grade nuts, bolts, and screws might be, the machining of the threads is never 100% perfect. Contact areas between male and female fasteners benefit greatly from the sticky “mortar between the bricks” that threadlocker provides. It also protects against rust and corrosion on the threads.
All brands, grades, and types of threadlocker are anaerobic – the stuff only cures in the absence of air. In the case of typical automotive or motorcycle applications, it also only cures when in contact with active metals, such as iron, steel, brass, bronze, copper, or aluminum alloy. Active metals are prone to losing electrons when exposed to moisture and air, resulting in corrosion, rust, or oxidization. If threadlocker is used on inactive metals, such as stainless steel, pure aluminum, bright plate/chrome, zinc, titanium, cadmium, or anodized metals, an activator/catalyst surface treatment is needed for curing.
For most applications, you can’t go wrong following the factory recommendations and instructions for every fastener on your machine. Many of them, particularly for critical applications, are offered as original equipment with a patch of threadlocker on the threads.
It is worthwhile to peruse, choose, and use threadlocker, especially on critical fasteners. It works better than mechanical locks and withstands temperature fluctuations and vibration very well.