Why You Need a Torque Wrench

Photo credit: Getty Images
Photo credit: Getty Images

From Car and Driver

Torque is a twisting force. The muscle you apply to the top of a screw-top jelly jar is torque. And when you use a wrench on a threaded fastener (such as a bolt, nut, screw, or stud), you're applying torque to either tighten it or loosen it. But how tight is tight? Certain types of fasteners—many of which are used on a variety of mechanical devices, including automobiles—require tightening to a specific amount so that the component operates properly. That's where a torque wrench comes in. What's the best torque wrench? That's not easy to say. The simplest way to answer that is to point out that there are numerous good ones (see the list at the end of this article for a few suggestions), so it's important that you know what a torque wrench does and what you need to know to find the best torque wrench for you.

How Does a Torque Wrench Work?

The difference between a basic wrench and a torque wrench is that the latter indicates—with either an electronic window, a clicking noise, a needle, or a gauge—exactly how much force is being applied to its handle. It's important, but not well-known, that many fasteners require a very specific tightening specification. When it comes to certain nuts and bolts on cars, guessing it's "tight enough" is almost never right.

Why Do I Need One?

Why is it important to know precisely how much force or torque is being applied? Too loose and the fastener might back off, causing an obvious issue. Too tight and the fastener or the fastened part might be damaged or distorted. Overtightened lug nuts on a wheel, for example, can lead to brake-rotor warping, less effective stopping, premature brake wear, and the significant inconvenience of stuck lugs. You could even cause a wheel stud (the threaded part) to snap off. Not tightening those lug nuts enough, on the other hand, could actually cause your lugs to loosen, fall off, and cause a wheel to fall off. That's why most owner's manuals have a torque specification for lug nuts.

Photo credit: Getty Images
Photo credit: Getty Images

If you plan to do any major work on your engine or on some major powertrain components, you absolutely need a torque wrench. Overtightening cylinder-head bolts, for instance, can easily cause expensive damage and catastrophic coolant loss. Too-tight exhaust manifold bolts can cause the manifold to crack. Torque specs are so critical to proper car repair that most shop manuals provide them along with repair steps and then also include them in back-page tables with fastener location, designation, and individual specs. Torque force can be measured in units of meter-kilograms (mkg), newton-meters (Nm), and pound-feet (lb-ft). If you're buying a new wrench, make sure it's calibrated with the same units used in your repair procedures or have a conversion table handy. Most vehicles sold in the United States have torque specs expressed in lb-ft.

How a Torque Wrench Works

There are decent torque wrenches at various price points ($40–$140) with socket-drive sizes of ¼, ⅜, ½, ¾, and 1 inch. The smaller ones are generally used for things like installing delicate temperature switches in fragile cast housings. The larger ones are used to put big twist on crank-pulley or transmission-gear nuts. As with socket wrenches, you can handle most major repair needs with a ½-inch wrench. (That's the size we use in the Car and Driver test garage to torque lug nuts.)

Photo credit: Getty Images
Photo credit: Getty Images

Types of Torque Wrenches

There are four basic types of torque wrenches, each with slightly different operating principles: beam or deflection, dial indicator, clicker, and digital. The beam- and dial-indicator-types use a simple mechanical system that connects to a pointer that hovers above a torque scale attached to the handle or to the dial indicator. When force is applied to the wrench's bending handle, the scale or dial shows the amount of torque equivalent to the handle's deflection. If you're buying a beam- or dial-indicator-type, you'll want a large pointer or gauge face for working in dark underhood locations.

Clicker wrenches are different, and wonderfully convenient to use. They have an adjustable ring around the base of the handle that lets you set the exact torque you want to apply to a nut or bolt. When you have tightened the fastener to the set torque, a ratchet mechanism clicks loudly to alert you that the set torque has been reached and you should stop tightening. No scrutinizing hard-to-read dials while you tighten. Screwing the handle in and out moves an indicator up and down the scale, varying the point where the torque-indicating click occurs.

Digital torque wrenches are among the most expensive and the most accurate. These use an electronic strain gauge inside the handle to send a signal to an LED window. The downsides are that they must be handled carefully and require a battery.

It's important for proper measurement to keep the wrench calibrated and stored at its lowest setting and to know whether measurements are taken on dry or lubricated fasteners.

What's the Best Torque Wrench?

The best torque wrench for you is the one that meets your needs in terms of capability and price. Torque wrenches have been around for so long, there are many top-quality ones. You don't need the most expensive, and many brand-name wrenches will work well for you. You do need to decide what kind of work you need it for, but in general, a ½-inch wrench will do most of what you need. That's the size we use in the Car and Driver garage to torque the wheels on our test cars and wreak all sorts of other mechanical mayhem. Here are a few things you'll want in a good-quality torque wrench:

  • A robust lever arm for torque applications of 20 to 150 lb-ft.

  • A grippy handle for situations where grease or oil has made things slippery.

  • An ISO 6789 accuracy certificate (plus or minus 4 percent for up to 25,000 cycles).

  • A protective case to ensure long-life accuracy in a gritty shop environment.

  • For clicker-type wrenches, an easy, quick, and precise torque-set adjustment with a locking ring.

Now, get out there and start tightening stuff up!

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