Top 5 Impact Wrench Mistakes

1.   Over tightening fixings

This is the number one problem for inexperienced users who underestimate the torque being applied by an impact wrench. In an effort to ensure that the nut or bolt is adequately tightened, they stretch or strip the threads. Strictly speaking, an impact wrench is designed to loosen fixings rather than tighten them. It is also useful for speeding up the fitting or removal of fixings but, as you will see later on, this function has its own problems.

Note: Over tightening fixings can be problematic even if no damage occurs. Most of us will have experienced the scenario of struggling to remove a wheel nut (lug nut) to change a tyre that’s been fitted by a mechanic with an impact wrench. If tightened to the manufacturers specifications, the nuts can usually be removed by hand, using the wrench provided.

Some of the latest, high end models however, have a built in torque limiter, which is designed to prevent the fixing being over tightened. This is acceptable for tightening general fixings that have a broad torque setting but is not suitable for fixings that have a more precise torque setting. Non-safety critical fixings, such as facia bolts for example, have a broad torque setting and can be tightened with a torque-limited impact wrench. A safety critical or engineering fixing, such as a wheel nut (lug nut) or cylinder head bolt on the other hand, has a critical torque setting and should only be tightened using a torque wrench.

If any fixing is tightened beyond the manufacturers specifications, there is a risk that the threads will stretch causing the fixing to lose tension, which could be dangerous. A more common occurrence (something I’m sure most people will have experienced at some point) is a stripped thread, which means having to repair the fixing; often an expensive and time consuming job. It is important to note that most manufacturers quote torque figures for clean, dry threads. If your threads are not in this condition when tightening them and it is a safety critical application, seek advice from an expert.

In conclusion, the best course of action is to use an impact wrench for loosening and removing/refitting nuts and bolts, and a torque wrench for tightening them to the manufacturer’s specified torque.


2.   Damaging threads

Damaging threads is usually caused by cross-threading the nut or bolt prior to tightening it with an impact wrench. The best way to avoid this is to ensure the nut or bolt it correctly threaded by hand before using the impact wrench. Your hands are much more sensitive than a torque-limited impact wrench so use them first.

This is particularly important for aluminium or other soft metal fixings as they are easily damaged by cross threading. If a fixing cannot be threaded by hand, it is likely too tight and may require modification, or cleaning with a tap or die.

Tip: Consider using helicoils for threads (particularly in softer metals) that are frequently removed and refitted, or subjected to high loads.


3.   Using the wrong sockets and damaging fixings

It is not uncommon to see people using ordinary sockets with an impact wrench, only to find that they begin to slip. Ordinary sockets are not designed to be used with a cyclic load so tend to wear quickly and become oversized, causing them to slip on the fixing. Impact wrench sockets are heavy duty and grip across the flats (for hex fixings) to prevent them wearing, damaging the fixing and slipping.


4.   Buying the wrong size impact wrench for the job

Another common mistake is buying the wrong size impact wrench. It can be difficult to determine the size of impact wrench you need; too small and it won’t do the job, too large and it may be awkward and tiring to use. As discussed in my “Beginners Guide to the Impact Wrench”, a good rule of thumb is to think about the size of fittings you would use with a hand socket wrench. For example, you wouldn’t use a 1-Inch drive hand wrench to release a 3/8-Inch nut, and the same applies to an impact wrench.

Next, consider where you will be using the impact wrench and how long for. An air impact wrench for example, is smaller and has a greater power to weight ratio than a cordless one, making it more suitable for use in restricted spaces and for longer periods. This makes the air the perfect choice for professional mechanics, but the cordless (or electric) impact wrench remains the best option for infrequent and home use, because there is no additional infrastructure required.

From a more technical perspective, if you know the torque required to release your largest fixing, then you can check this against the torque rating of the impact wrench. As long as it has a rating greater than the required torque, you shouldn’t have any problems. Bear in mind however, that while a fixing may have been tightened to a specific torque, the torque required to release it may be greater if it has fretted or corroded. If this is the case, consider a torque rating 20%-30% greater than the specified tightening torque to assist in releasing the fixing.


5.   Buying the wrong type of impact wrench

Similarly to mistake number 4, people often buy the wrong type of impact wrench. As discussed, each type has its advantages and disadvantages, depending on the application. For mechanics and other busy professionals, air tools have the advantage of having the best power to weight and volume ratio. For occasional home use, electric tools are a good option as they have a good power to weight ratio and are generally less expensive. Cordless tools are the only portable option but tend to be larger, heavier and more expensive than air or electric units.

Additional features such as a torque limiter, battery indicator or extended warranty should also be carefully considered to ensure you get the right tool for the job.


Our impact wrench reviews are designed to walk you through the decision making process and help you to avoid making these 5 common mistakes. After reading the reviews, if you are still unsure about which impact wrench is right for you, submit your question as a comment and we, or one of our members, will try to guide you. If your question still isn’t answered, please contact us and we will do our best to assist you.