The methodology of threaded fastener use tends to be passed on by word of mouth rather than by any process of formal education. This has both its good points and its bad. The good side is that a lot of correct procedure does get passed along and it is liable to be effectively taught. The other side of the coin is that a lot of folklore gets passed along with the truth. Let’s look at some prevalent folklore.
Myth: An experienced and capable mechanic doesn’t need a torque wrench-—he can feel the correct tightness and repeat it as close as a torque wrench. This is obvious nonsense. The person who can feel any given torque within ten percent, let alone repeat it, has not been born. I am aware that people have been tightening bolts—-sometimes critical bolts-—by feel for generations. I am further aware that each of them will swear that they have never had a fastener either back out or fail in service. These people are either lucky or lying. To my mind, their success is a tribute to the over-design of the average piece of machinery, the inherent idiot-proof nature of the threaded fastener, and the boundless ability of the human being to forget what he does not wish to remember.
The corollary to the first myth is: “Every bolt on an aircraft or racing car must be tightened with a torque wrench every time.” This is an excellent idea. It is also so impractical as to be ludicrous. The truth of the matter is that every critical bolt and nut that is loaded in tension and subjected to vibration or reversals of load should be tightened to a predetermined level of stress. Determining which bolts and nuts are critical is the job of the designer and of the engineer (in the British sense) in charge. In my own case, I determine which installations require the use of a torque wrench and insist on its use.
Myth: Bolts must always be installed with the bolt head up and facing forward so that, if the nut should fall off, gravity and the force of the airstream will tend to keep the bolt in place. There is nothing wrong with installing bolts in this manner. In fact, in the interest of standardization, I usually do. But to hope that gravity or air pressure will keep a bolt in place is unrealistic. There is a place in this world for dreamers—but that place is not in engineering. Neither is it anywhere near an airfield or a racetrack. There is absolutely nothing wrong with installing bolts wrong end up or backwards, when it is more convenient to do so.
Myth: Elastic stop nuts are single use items. If reused, they will not lock reliably. This one started in the infancy of the elastic stop nut when the locking collar was made from organic fiber rather than nylon and it wasn’t reusable. Those days are gone. Any of the current families of self-locking nuts can be reused many times.
In most applications the rule of thumb is: “If there is no visible damage to the thread and you cannot spin the nut with your fingers, it is OK to reuse it.” Obviously this is not true in the case of critical tension applications which require high tensile nuts. I do not reuse critical nuts, locking or not. In my world this includes all connecting rod and most cylinder head and main bearing cap nuts.
Myth: You cannot use an elastic stop nut on a bolt with a drilled shank. Why not? If passing over a deburred and chamfered hole in the bolt shank is going to destroy the self-locking feature of a nut then I, for one, don’t want toknow about that type of nut.
Myth: Always turn the nut, never the bolt. Again, it is easy to see where this comes from, and again, it is good practice. We have seen that bolts loaded in shear should be installed in close-tolerance reamed holes. When we turn the bolt in a closely fitting hole, we will produce friction between the bolt and the hole surfaces which can give a false tightening torque reading; damage the bolt surface, the hole surface or both; enlarge the hole; remove some of the plating from the bolt and if the bolt is titanium, the bolt will gall and weld itself to the wall of the hole.
The trouble is that there are many applications where it is easier to turn the bolt than it is to turn the nut. Some books state that, since we don’t worry about turning tension bolts installed in blind holes, we don’t need to worry about turning any bolt when it is more convenient to do so. Wrong! What is being overlooked is that tension bolts are properly installed in loose drilled holes where there will be little if any contact between the bore of the hole and the shank of the bolt. My rule is that convenience takes a back seat to damaging parts, I turn the nut whenever it is possible to do so. However, a primary rule of life is that we do what we have to do-—and that includes turning bolts and holding nuts when the need arises.