Everything you wanted to know about screws

Are you missing a screw?
Screws, nuts, studs, and threaded holes—our motorcycles are full of them. We're so used to seeing them that they're familiar to us, and we think we know everything about them: you tighten them, and that's it, they do their job by mechanically joining two or more elements together.

But beneath this simplicity lies a wide variety of shapes, threads, diameters, and torques that are worth knowing. Let's talk a little about them.

How do they work?
When I've had to explain how a screw works to my mechanics students, I always used the same example: A screw is nothing more than a spring. A very rigid spring, but with a certain elasticity, which we tighten when we tighten it. This tension creates the force that holds the parts together, and the screw in place, thanks to the mechanical friction that occurs when tightening.
In fact, as we tighten the screw, it stretches. We can easily check this by tightening a screw to a certain torque, having previously measured its length with a micrometer , tightening it with some washers or pins, and then measuring it again.

We will then check that it has stretched a little, due to the tension we have applied: exactly like a spring.

Press but, but without going too far
The most commonly used way to measure the tightening or tension that we exert on a screw is by using a torque wrench .
This tool will allow us to screw the screw to a predetermined tension, which will notify us with an audible "click",

a sound or vibration if it is electronic

or simply checking the needle on a dial.

This means that we will be able to control the tightness of that “spring,” so as not to leave it loose or overtighten it.
Although in mechanical engineering there are a series of typical torque values depending on the diameter and material of each bolt, it's best to follow the torque specifications in the manufacturer's manual for the vehicle you're working on. Sometimes these specifications consist of a torque plus a specific angle of rotation of the bolt, as for example, on Harley-Davidson engine cylinder head bolts, pulleys, or the primary chain balancer.

This tightening method became popular during World War II, and was first used in fighter aircraft engines. It appears to be more accurate because it doesn't rely as much on friction as simple torque tightening, but it does require an angle gauge to achieve the desired precision.

Shapes and sizes
Generally, when designing an engine, the use of the smallest possible screw/nut in diameter and length is taken into account, both considering the lightness of the components and (above all) the minimum cost for the function it must perform.
As a general rule, the maximum force that a screw can exert is when a part equal to its diameter is threaded; that is, if a screw has a diameter of 5 mm, a nut 5 mm deep would be enough to obtain the maximum force.
This rule doesn't work exactly with aluminum, bronze, and other non-ferrous materials. For example, the bolts holding the primary cover on your Harley need to be threaded at least twice their diameter to properly perform their function.
Screws are classified by their elastic strength or grade, the higher the grade the more stress the screw can withstand.

Metric or inches?
Here comes another of the big problems when we deal with the screws of our American, English, European, Japanese machines...
Generally, Anglo-Saxon brands use the Imperial measurement system (in inches), while in Europe and much of the rest of the world, the Metric system is used. Due to this duality, we have screws and nuts of both systems, and within them, there are fine and coarse thread pitches.
As a general rule, fine threads are used in applications subject to high stress, regardless of the material used, and coarser threads are used in more common applications. Coarse threads are also often used when the female part being threaded is weaker than the male part, and vice versa with fine threads.
If all this seems like little, you should know that there are also "left-hand" threaded screws, which are tightened and loosened in the opposite direction to the conventional screw. They are typically used on rotating parts, helping the rotation prevent the screw from loosening, although I personally think engine designers use them to drive mechanics crazy...

Fixings
Many times, especially when mechanical parts are subject to vibrations (does this sound familiar?), it is necessary to provide the screw/nut with some kind of retention system, either with washers, nylon, pins,

and also with a system widely used in competition and aeronautics, the safety wire .

It's important to reuse them in the same place where they were; if someone put them there, they're definitely necessary.
On other occasions, the manufacturer will recommend using screw locking agents, which are nothing more than anaerobic sealants, a liquid that solidifies in the absence of air, holding the screw in place. It's important to know that these come in different holding capacities, depending on the size of the screw and the type of work being done.

Conclusion
After all this information, you'll surely better understand the importance of tightening your motorcycle's bolts and nuts using the torque values and procedures recommended in the manufacturer's manual. Along with a good torque wrench and a can of Loctite, the workshop manual is one of the tools that shouldn't be missing from any garage, whether serious or amateur.

Frank Burguera

#harleymechanics