Cordless Impact Wrench Basics: Torque, Lug Nuts and the Mistake That Warps Brake Rotors

An impact wrench is the tool that turns a twenty-minute fight with a seized bolt into about four seconds. It's also the tool most likely to be used wrong, in a way that quietly damages the thing you were working on.

Here's what the specs mean and how to use one properly.

What an impact wrench actually does

A regular drill applies steady rotational force. An impact wrench does something different: an internal hammer mechanism spins up and slams into an anvil, delivering hundreds of short, sharp rotational blows per minute.

That matters because a seized fastener responds to shock far better than to steady pressure. Corrosion and thread-lock give way under a sudden hammer blow when they'd happily resist you leaning on a breaker bar for a minute. It's also why the reaction force doesn't twist your wrist — each blow is over before it can transfer back into your arm. A one-second impact burst does what a long bar and your body weight can't.

Reading the torque number honestly

Torque is quoted in newton-metres (N·m) or foot-pounds (ft-lb); one ft-lb is about 1.36 N·m. A tool rated at 550 N·m is putting out roughly 406 ft-lb.

The number to pay attention to is breakaway torque — the peak force available to crack a stuck fastener loose. It is not the force the tool applies smoothly and continuously, and it's not a number you should be aiming to hit when tightening something.

For scale: a typical passenger-car lug nut is specified somewhere around 100–140 N·m (roughly 75–100 ft-lb). So a 550 N·m tool has three or four times the muscle needed to remove one, which is exactly what you want, because the nut you're trying to remove was very likely put on by a tyre shop's air gun at far above spec and has been rusting there for two years.

Why speed modes exist

Full power is right for maybe a third of the jobs you'll do. Selectable speed and torque modes give you the rest.

Low is for small fasteners, softer metals, and anything where stripping a thread or snapping a bolt head is the real risk — alloy components, interior fixings, trailer hardware.

Medium covers most general work: running nuts down, assembly, deck bolts, mower blade bolts.

High is for the genuinely stuck: rusted lug nuts, suspension bolts, anything that's been outside for a decade.

The TorquePro™ 3-in-1 Power Tool gives you all three plus forward and reverse, so the same tool that cracks a seized suspension bolt loose can also run down a fastener without shearing it. There's a built-in LED in the head as well, which sounds like a small thing right up until you're under a car in a garage with one overhead bulb behind you.

Brushless, and why it's worth caring about

A brushed motor uses carbon brushes physically rubbing on a spinning contact to switch the current. They wear out, they generate heat, and they waste energy as friction. A brushless motor does the switching electronically instead — nothing rubs. In practice that means more of the battery's charge becomes torque instead of heat, the tool runs cooler under sustained load, and there are no brushes to eventually replace. On a tool that spends its life doing short, very high-load bursts, that's not marketing; it's the difference between a tool that lasts and one that fades.

Sockets: use impact-rated ones

This one catches people out. Chrome hand-tool sockets — the shiny ones — are hardened to resist wear, which also makes them brittle. Hit one with hundreds of hammer blows a minute and it can crack and throw fragments. Impact sockets are the black ones, made from a softer, more ductile steel that deforms rather than shatters. They cost a little more and they are not optional. Same goes for extensions and adapters.

Beyond that: safety glasses every time, seat the socket fully and squarely on the fastener before pulling the trigger, and use short bursts rather than holding the trigger down. If a bolt hasn't moved after three or four seconds of hammering, more hammering usually isn't the answer — penetrating oil and ten minutes of patience is.

What you'll actually use it for

Most people buy an impact wrench for wheels and then find it lives on the bench. Seasonal tyre swaps and roadside wheel changes, obviously. But also: mower blade bolts at the end of the season, trailer and hitch hardware, deck and fence construction, lawn and garden equipment assembly, rusted bolts on anything that's spent a winter outdoors, and the general category of "bolt that a socket wrench has decided it isn't going to win against."

It's the tool that comes out when the yard is finished and the garage starts — which is why it sits alongside the trimmer, the hedge trimmer and the pole chainsaw in our range rather than in a separate catalogue.

The short version

Buy for breakaway torque, because removal is the hard job. Use low and medium modes more than you expect. Only ever use impact-rated sockets. And whatever else you take from this — never do the final tightening of a wheel with an impact wrench. Snug it up, then finish with a torque wrench to the manufacturer's figure.