What a household usually asks of one
For most homes, an impact wrench is a garage tool. The typical jobs are seasonal wheel changes, tightening or loosening fasteners on a trailer, mower, wheelbarrow or play set, assembling heavy outdoor gear, and the occasional stubborn bolt on a fence or deck. A mid-size tool covers nearly all of that. Buying the largest, most powerful tool available is unnecessary for these tasks and makes the tool heavier, more expensive and harder to use in tight wheel wells.
Because wheels are the most common reason people buy one, this article uses them as the main example.
Drive size: 1/2 inch is the usual answer
The most practical size for home use is a 1/2-inch square drive. It fits the sockets used on most passenger-vehicle lug nuts and many other common fasteners, and sockets in that size are widely available. A 3/8-inch tool is lighter and better in cramped spaces, but it may lack the force for some lug nuts and larger bolts. Larger 3/4-inch tools are too big and heavy for most households. Socket drive sizes come in steps of 1/4, 3/8, 1/2, 3/4 and 1 inch, and sockets must match the tool.
Some compact 1/2-inch models are small enough to rival a 3/8-inch tool in access, which is worth checking when comparing.
Torque: enough, not maximum
Published torque figures vary by manufacturer and are measured differently, so treat them as a rough guide. Most mid-size cordless 1/2-inch tools are intended to loosen typical lug nuts, but lug nuts can be tightened differently by tire shops, rusted in place, or seized by years of road grime. A tool that has loosening torque near the high end of what you expect is more reliable than one that is marginal.
An important point: an impact wrench is not a precision tightening tool. The correct tightening value for lug nuts is given in the vehicle’s owner’s manual, and the final tightening should be done with a torque wrench. Many people use the impact wrench to run the nuts on lightly or remove them, and then finish by hand. Overtightening wheels with an impact wrench can stretch studs and warp brake rotors in some cases, and it makes the nuts very difficult to remove later. Tool settings with a lower power mode can help for running nuts on, but they should not be relied on for final torque.
Where a nut will not turn, a breaker bar gives steady leverage without the shock of impacts, and penetrating oil can help on corroded threads, given time to soak.
Impact-rated sockets
Using ordinary chrome sockets on an impact wrench is a common and risky mistake. Standard sockets are harder and more brittle, and they can crack under repeated hammering. Impact sockets have thicker walls and are made from a tougher alloy, and they are usually dark in finish. For wheels in particular, look for deep impact sockets that fit your lug nuts, because some lug nuts are recessed or have a thin-walled design that needs a thin-wall socket. Using the wrong size can scar the finish of alloy wheels, so many people use a protective plastic sleeve (nylon-sleeved impact socket) made for the purpose.
Check the socket sizes in your vehicle’s manual or by testing with a hand socket, because lug nuts commonly come in several sizes, and a set of the most common sizes is a good start. Six-point sockets are generally a better fit for lug nuts than twelve-point ones. Replace sockets that crack, round out or have loose fits.
Battery and platform
Capacity in amp-hours determines how many wheel nuts you can remove before recharging. For a household doing four wheels a few times a year, a mid-size pack is usually plenty. A weak or cold battery reduces performance, so charge it before the job and avoid leaving it in a freezing garage. If you already own other cordless tools, buying into the same battery platform often saves money.
Other useful features
- Drive control: a selector with low, medium and high settings for different fasteners.
- Reverse and forward switch: within reach with gloves on.
- Anvil style: detent pin anvils hold sockets more securely than friction rings, which is good for working above your head.
- LED light: helpful for wheel wells.
- Weight and balance: try it with the battery fitted.
- Brushless motor design: typically gives more run time and less heat.
Safety at home
Never get under a vehicle supported only by a jack. Use jack stands on firm, level ground and do not rely on the jack alone. Loosen lug nuts slightly while the wheel is on the ground, if your procedure allows it, then raise the vehicle and finish. Follow the vehicle manual, including its jacking points. Wear safety glasses, gloves and hearing protection, since impact wrenches are loud. Remove the battery before changing sockets. If a bolt does not move, stop instead of forcing it, and if the job involves brakes, steering or suspension, consider a qualified mechanic.
This is general information and not professional advice.