You probably just rely on a regular wrench to tighten parts on your car, but any important fastener should be tightened with a torque wrench so it will be neither too tight nor too loose. Doing otherwise can cause a part to break, leak or come undone.
Torque wrenches come in three basic types that I list below, but they all do the same thing: allow you to apply a specific amount of twisting force to a nut or bolt to achieve the correct amount of clamping force. Cowboy the process and you can end up with a water pump that leaks or a cracked alloy wheel.
In the US, fastener torque is most commonly expressed in "foot-pounds" or the finer "inch-pounds," depending on the part being fastened and the wrench being used. I won't give you a physics lesson on the definition of those metrics; just know that they are the standard for tightening things.
There are three main kinds of torque wrenches, listed below in my order of preference.
These tighten to the desired tightness then click to momentarily stop tightening and to give you a tactile and audible indication. You will need to know how to use a vernier scale to set a click wrench, but that's easy, as I show in the video above. Click wrenches sit at the intersection of affordability and quality.
These wrenches have a digital interface for setting the desired torque and displaying the max torque used when loosening a fastener, which tends to be more trivia than useful information. Digital wrenches typically emit a beep and vibration when you reach the desired torque level rather than momentarily decoupling like a click wrench does. Digital wrenches may seem like the modern way to go but, as with anything electronic, the cheap stuff tends to be crap. If you're going to buy a digital wrench, spend $150 or more.
I don't like this style of torque wrench because it requires you to watch a loosey-goosey swinging needle or beam as you tighten, and there's no provision to prevent you from overtightening. Beam wrenches are mechanically simple and typically inexpensive but mostly suitable for coarse work which is anathema to the whole point of a torque wrench. I'd rather you borrow a good torque wrench than buy a cheap one.
A few finer torque wrench points
- Adding an extension between the head of the wrench and the socket it's driving should not meaningfully skew its accuracy. Use a good-quality, thicker-diameter extension to reduce any small inaccuracy.
- Do not use a longer handle on the end of a torque wrench to give yourself more leverage. That will likely damage the wrench so that it no longer reads correctly.
- Grip the wrench where you find the obvious built-in hand grip. "Choking up" on the wrench may throw off its accuracy considerably.
- Fasten parts without any unusual oils or lubrication. In most cases fasteners are designed to be tightened with a clean dry metal-to-metal interface. Some deep internal engine parts may vary from this, but if you're rebuilding an engine you don't need this article.
- Don't drop or toss a torque wrench. Most wrenches are as tough as nails, but torque wrenches are instruments first, wrenches second.
- If you want to buy just one torque wrench and get the most applicability, make it a 3/8-inch drive as opposed to a 1/4-inch or 1/2-inch drive. The drive size refers to the width of the square peg onto which sockets are attached on the wrench.