How to Buy an Impact Torque Wrench for the DIY Car Mechanic

Most impact wrenches are used for loosening and tightening lug nuts on the wheels of a car or truck. Vehicles and farm implements have many other large, stubborn nuts and bolts that are troublesome for the home shade-tree mechanic, handyman or do-it-yourselfer. But for non-commercial use, 90% of the time it’s going to be automobile lug nuts.

Before buying one, some people try using a cordless drill. For a couple of years I used a ¼” variable-speed drill with adapters up to my ½”-drive socket. I’d break the lug nuts loose with a breaker bar and then spin them off with the drill. To reverse, I’d spin them on with the drill and then torque them down manually with a torque wrench. It saved a lot of time, especially since one day each month I change four tires twice out in the hot sun at an autocross event.

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Many people will appreciate the portability and convenience of a cordless impact wrench similar to their cordless drills and other tools. The least expensive type is also the least common—it connects to a 12-volt outlet or directly to the vehicle’s battery. Some impact wrenches plug into a wall outlet. Professionals use pneumatic impact wrenches, but it is doubtful that many pros will be reading this, so that will be the last type covered.

This article deals with 3/8” and ½” drive impact wrenches, although you can buy anything from ¼” to at least 1”, but those are more for commercial use. Either 3/8” or ½” will work just fine on lug nuts, with ½” being a bit more expensive and powerful. Since you might want to fit other sockets to it occasionally (besides what fits your car's lug nuts), it is good to consider the size of other sockets you already own. For example, I currently have only half a dozen ½” sockets, but a full set of 3/8”. So if I’d read this article before I wrote it, I’d have bought a 3/8” wrench rather than a ½”.

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Cordless battery-powered impact torque wrenches

Many people have cordless drills and other tools—they will appreciate the portability and convenience of a cordless impact wrench. There is a wide variety, but most don't have as much torque as the other types in this article. Probably the best choice is a model that fits the same battery pack used with your other cordless tool(s). That provides economy through uniformity. Beyond that, the more expensive ones tend to have adjustable torque settings, which eliminates the need to use a manual torque wrench. (Personally, I wouldn’t trust it—at least not the first dozen times on various torque settings—I’d still give a manual tug on each lug nut just to make sure.)

While portability might sound great, remember that when your wife’s SUV has a flat tire on the interstate, the tool battery is probably back home on the charger or attached to the cordless drill you’re using on that honey-do project.

12 volts: Powered from your car

The least expensive type is also the least common—it connects to a 12-volt outlet or directly to the vehicle’s battery. I recently bought one of these (1.) and love it. There is one model (3.) that has adjustable torque ranges. With the non-adjustable types, you simply let it deliver 2-3 (or more) impacts and then torque it down manually. Between each impact, the wrench takes a second or two to spool up until a centrifugal clutch engages and delivers a single impact. That's about 50-100 times slower than the other types in this article. Mine will max at about 270 lbs/ft (supposedly), but I suppose that would take a whole lot of impacts and a lot of time. I wouldn’t have to lift a muscle to do it, though, and that’s the purpose of an impact wrench.

Since 90% of an impact wrench’s use will be on one of your cars, having to be within 12 feet of a 12-volt power outlet is no hardship. And since this type can’t be used away from a car, it is more likely to actually be in the car when it has a flat tire. You can even put one in the sports compact your daughter drives back and forth to college. Consider it cheap car insurance.

Electric-corded impact torque wrenches

Some impact wrenches plug into a wall outlet. They have more power than the lower-voltage types described so far, though the length of an extension cord will reduce the amperage getting to the tool. These are probably most useful for stationary use at a workbench. They can occasionally be plugged into an outlet beyond the reach of an air hose, but the cordless type would be handier there.

Dragging the cord around to all four corners of the car is a disadvantage this type shares with the pneumatic type. Electric cords tend to tangle and snag more easily on jacks and jack stands than do pneumatic hoses.

Professionals use pneumatic impact wrenches because they are more powerful and faster. These are generally more durable, as they are made for daily commercial use. The homeowner who already has a small compressor in his garage might want to consider one of the less expensive pneumatic impact wrenches. They get the job done quickly. This might be just the ticket if you need to switch between off-road and street tires every weekend.

Revised November 2, 2011

© Howard S.