A torque wrench is an invaluable tool for the do-it-yourself mechanic. Fastening nuts and bolts can be done with any set of spanners or with a ratcheting handle and a set of sockets. The value that a torque wrench adds is that you can tighten them accurately to a specified tightness or torque setting.
Torque is defined as a twisting force that causes rotation. When tightening a bolt or a nut, this torque applied can be measured and used to standardize the degree of tightness that those fasteners need to prevent them from being loosened inadvertently. Critical car components, like the wheels, need to be bolted on precisely so they stay tight even under severe vibration, movement, or temperature changes. Torque specifications are determined by the size of the bolt or nut, as well as the material used for it and its designed application.
A properly torqued bolt or nut has the threads stretched optimally to act as a tight spring, preventing movement of the mated surfaces. If, however, it is torqued beyond the capacity of the material to stretch, the threads will break apart. If it is not stretched because the torque applied is too low, it will not have adequate tension to remain in place. Imagine if part of your suspension system hasn’t been tightened correctly: You’ll feel rattles first as it loosens, before it breaks apart and collapses while you’re driving!
It cannot be overemphasized that the torque specifications recommended by the specific car’s manual should be followed. The manufacturer has made exact calculations and rigorously tested them in their intended functions to determine the correct torque for each bolt and nut.
Clicker-type torque wrenches, the most popular kind, look like ordinary ratcheting handles except that they have a longer handle and have a rotating grip/dial on the end of the handle to set the torque. They come in a variety of handle lengths and socket drive sizes based on the torque range they are intended for. Shorter torque wrenches come in quarter-inch drives for lower torque settings such as those for spark plugs, at 8lb-ft, or small engine bolts at 10-20lb-ft. Bigger half-inch drive wrenches with long handles are meant to tighten hub axle nuts to over 200lb-ft.
Being purely mechanical devices, torque wrenches are simple to use. Unlock the dial grip by twisting the knob at the end of the handle. Now, rotate the graduated dial to the desired torque number etched on the long handle. It may have two scales on opposite faces: the commonly used pound-foot on one side, and its metric equivalent Newton-meter on the other. Once you’ve precisely set the torque, lock the knob at the end. It’s important to hold the handle exactly in the grip, and on the grip alone, when torquing. The length of the handle is factored in as the lever for the torque force.
Attach the appropriate socket to the drive head and tighten the nut or bolt until you hear and feel a click. That indicates that you have twisted to the set torque tightness. Once you’re done, dial it back to zero or near zero, then tighten the locking knob. Store the torque wrench in its protective case, as it is sensitive to knocks and drops. Easing it off to zero relaxes the spring inside the handle. If the spring is stored for a long time under tension, it may lose its accuracy and will need recalibration sooner. Manufacturers recommend annual calibration as the only maintenance needed, but if you’re meticulous enough to count the click actuations, have the wrench calibrated after 5,000 clicks.
Another common type of torque wrench is the deflecting beam-type. It is just a long bar with an attached pointer that indicates on a scale the twisting force applied to it. You read the scale as you apply the torque. A more sophisticated torque wrench is one with a rigid case and a dial indicator. This type is the most accurate and compact. Torque sticks look like socket extenders and can be used with the usual ratcheting socket handle. A set will come with different torque settings, and usually color-coded and labeled with the indicated torque. When in use, excess twisting force will not be transmitted to the attached socket. So, if you use a torque stick rated for 100lb-ft, even if you apply twice the torque, it will only twist the socket with 100lb-ft of force.
Regarding the use of socket extenders, which is sometimes necessary to access a bolt head, these extension rods are stiff enough, but may still diminish the torquing force applied. Similar to how the torque sticks work, the thinner and the longer the socket extender, the more it decreases the torque delivered.