Think back to the first time you began to use tools. What was the first thing you took apart? Perhaps it was a toy or a bicycle, or maybe even something you weren’t supposed to, just to see how it worked. Regardless of what is was, when you put it back together, one question that we all asked was “how tight should I make this?”
We all learned by doing, and in the process, left bolts too loose or made them too tight; we’ve all done it. But it doesn’t take long to discover the consequence of both, and eventually you figure out that how tight you make something varies depending on the size of the fasteners and the materials involved.
Over time, you develop a “feel” that allows you to attain a fair estimation of torque by hand, but when you enter the world of a professional auto technician, you realize quickly that this is not good enough. Your torque wrenches should be at the top of your list of valued and most used tools. If they are not, now is the time to reconsider.
Torque, simply put, is a measurement that specifies how tight to make a fastener. A torque wrench, when properly used, is a tool that allows you to precisely secure a fastener to its recommended specification. Mathematically speaking, torque is force multiplied by distance. In other words, if a torque specification of 50 ft.-lbs. is called for, it will require 50 pounds of weight at the end of a one-foot bar to achieve 50 ft.-lbs. of torque at the axis of rotation (at the fastener).
European countries and their resulting service manuals use the Newton-Meter (Nm) specification instead of foot-pounds (ft.-lbs.), but most will list both side by side. If not, the internet is loaded with conversion calculators.
What makes torque so important? When bolts are tightened, they stretch. This is not an immediately visible effect, like stretching a rubber band, but the idea is the same. The metal of the bolt resists stretching and acts like a spring with a natural tendency to return to its original shape. The tighter the bolt, the more it stretches and the greater the “spring” pressure it puts on its mating components such as a nut or threaded hole.
The force resulting from this pressure is called the clamping force of a fastener and this is what keeps components tight and prevents the fasteners from loosening. When a bolt is under-torqued, the proper clamping force is not attained, leaving the likelihood that the fastener (and the component it is securing) will loosen up.
When a bolt is over-torqued, it is stretched beyond its yield point, and its integrity as well as that of its mating components is affected. It no longer has the ability to “spring” back to its original shape and provide the clamping force that it is designed for. The result, if not a broken bolt or damaged threads, is again the likelihood that the fastener and the component it is securing will loosen up.
A properly torqued fastener will not loosen or fail, and with the exception of torque-to-yield fasteners, can be removed and reinstalled many times.
There are many factors that affect the torque specification assigned to each and every fastener including size, the material it is made of, its mating component, its design and thread pitch and the application for which it is intended.
The science behind torque is as complex as it is interesting, but for sake of this article, there’s no need to get too deep into that end of it. Steel, stainless steel, cast iron or aluminum are among the most common materials we work with. Even though you almost certainly already know which of these require greater care, regardless of what you are working with, the rules of torque apply in exactly the same manner and following them precisely should be part of your normal repair routine.
Now that you understand the reason for torque specifications, let’s look at the factors that affect the accuracy of it.
Dirty, rusty or corroded threads will create increased friction and heat buildup when threading in a fastener. The increased force required to install the fastener can result in the torque specification being reached before the fastener attains its intended clamping force. In severe cases galling can occur, most commonly with stainless steel fasteners, resulting in hardware that must be replaced. Always clean the threads using a wire brush, thread chaser or tap if necessary.
Lubrication, or the lack of it, affects torque. During the design and engineering process of any component or part of a vehicle, the fasteners are chosen based on the job they must perform; the heat or cold they must endure, their exposure to vehicle fluids and their exposure to the elements. The fastener and mating materials, their hardness, their location and any coatings they may have as well as any sealer that must be applied to the threads all affects torque.
For these reasons, some fasteners are to be installed dry, others with lubrication or sealers, some on the threads, some under the head of the bolt. Following the manufacturer’s specification for this process is the only way to guarantee the proper torque.
Thread pitch or thread count, a smooth under-head surface, a ribbed under-head surface, a tapered seat, a conical seat, flat washers, lock washers and many other design factors including the mating surface and the component that is being secured are taken into account when a torque specification is established. Don’t underestimate the importance of using the correct bolt and hardware with the correct application.
Torque-to-yield fasteners, sometimes referred to as “stretch” bolts, are used in order to achieve a very high preload and commonly used in areas that don’t require routine service such as suspension components and cylinder heads. They are able to achieve this high preload by being tightened to the point at which they are in effect stretched right up to their yield point. It is most critical to follow proper procedures when torqueing these bolts since they generally begin with a torque specification then follow with a final angle of rotation.
Torque-to-yield fasteners must always be replaced if loosened even slightly or removed. Since they are torqued to the point of yield, they will not spring back to their original shape when loosened, so they no longer have the ability to stretch and apply the proper clamping force they are designed for.
The same rules for torque apply to all fasteners. Clean the threads, lubricate only when and where it is called for, use the correct fasteners and hardware and follow all manufacturer’s specifications. Torque slowly for an accurate reading and don’t forget to get those torque wrenches calibrated from time to time.
A Word About Wheels
What’s the longest lug wrench you have ever seen for a car? When you think about it, they are all actually fairly short, generally in the range of 12- to 16-in. long. But before I continue with this thought, let’s revisit the definition of Torque. Torque is force multiplied by distance.
So, if the recommended torque of the lug nuts or bolts on your car is 80 ft.-lbs. (108 Nm), and the lug wrench is 12-in. long, it means you only have to put 80-lbs. of force on the end of it in order to reach the proper torque. Now, how much do you weigh, 180-lbs., 200-lbs.? If you’re leaning all your weight on the end of this lug wrench, do you think you have reached the proper torque? At least. What if you are standing on the end, have you exceeded the proper torque? Absolutely!
In other words, automobile engineers have designed lug wrenches to provide the average person enough leverage to properly tighten (and loosen) lug nuts or bolts, without over-tightening them. These engineers are masters at the properties of metal and fasteners, and do you think they know what they are doing? I think you see where I am going with this.
I fully understand that it’s not realistic to torque every last little thing. You’re certainly not going to torque that washer reservoir or the cupholder on your dash. There are plenty of other things that realistically won’t require it either, but when it comes to wheels, they are one of the most critical things to get right. That’s not to undermine the importance of any other fasteners, but this is one of the most predominant safety issues on a vehicle.
When installing wheels, the lug nut or bolt threads must be clean and dry. The wheel hub and mounting flanges must be clean and free of corrosion buildup and the hub and mounting flange of the brake rotor or drums must be clean and free of corrosion as well.
Blasting the lug bolts on with an impact then checking them with a torque wrench does not count. Start them by hand and either run them in by hand or use a torque-limiting impact that is designed to seat lug bolts without over-torqueing them. Torque them to their final specification with the vehicle securely on the ground.
Loose lug bolts have obvious consequences, but over-tightened ones do their own share of damage. This can and will damage not only the lug seat on the wheel as well as the lug nut or bolt itself but can also warp brake rotors and damage the threads in wheel flanges.
Now, back to those factory lug wrenches. What if your customer has a flat in the middle of the night in the middle of nowhere? They might be perfectly capable and familiar with changing a tire, but what if they only weigh 150-lbs.? They won’t stand a chance against a powerful impact.
A final note regarding aftermarket wheels; some require lug nuts with washers. These washers appear flat at a glance but are slightly concave (and marked with the wheel side) to assist the clamping force of the lug nut. These manufacturers always warn to recheck the wheel torque after driving a short number of miles. Don’t overlook this request and make sure your customer doesn’t either.
That’s it in a “nut” shell. If you’re not convinced or would like more information about a certain aspect of it, feel free to drop me a line. TS