An impact wrench works by storing energy in a rotating mass called a hammer, which can be suddenly engaged to an output shaft called an anvil. This delivers a burst of torque to the bolt. For this reason, impact wrenches are useful for loosening extremely tight bolts or for quickly installing fasteners. However, an inexperienced user can easily overtorque or cross-thread a bolt. Furthermore, impact wrenches are difficult to use on spring-mounted screws. The spring can absorb all of the energy of the impact before any torque is applied. For these applications, special socket extensions called torque limiters are available. Impact wrenches are commonly powered by compressed air, although electric or hydraulic types are available in small sizes. There are even very simple impact wrenches which can be tapped with a mallet. Impact wrench sockets are specially designed to withstand high forces; do not use regular ratchet wrench sockets with an impact wrench or they could shatter. Impact wrench sockets come in a variety of sizes for many different applications. In general, as you try to turn larger bolts, you will need a larger socket and a more powerful wrench. To save money, choose the smallest type of impact wrench that supports the bolt sizes you will be working with.
Common Bolt Sizes
The most common sizes for wheel lug nuts are 7/16-inch, 1/2-inch, 9/16-inch, 5/8-inch, 12 mm, and 14 mm. In addition to these sizes, automotive engines may also have 1/4-inch, 5/16-inch, and 3/8-inch bolts.
1/4-Inch Impact Wrench
This is the smallest type of impact wrench, making it especially useful for fastening bolts in tight places that a larger wrench would be unable to reach. These wrenches can deliver 50-100 ft/lbs. of torque. Sockets are available for metric and standard bolt sizes. Metric sockets start as small as 4 mm and increase in 2 mm increments, up to 14 mm. Standard sockets start as small as 3/16-inches, and step up by 1/16-inch increments, up to 9/16-inches.
1/2-Inch Impact Wrench
A 1/2-inch impact wrench is suitable for automotive work on small passenger vehicles. These wrenches can deliver 200-400 ft/lbs. of torque. Metric bolt sizes range from 10 mm to 24 mm. Odd diameters such as 19 mm and 21 mm become common in this range. Standard bolt sizes range from 5/16-inches to 1 inch, again increasing in increments of 1/16-inch.
1 inch Impact Wrench
A 1 inch impact wrench socket is suitable for working on sport utility vehicles and small commercial vehicles. These wrenches can deliver over 1000 ft. /lbs. of torque. It becomes difficult to find electric powered models at this size or larger. Socket sizes range from 1 inch to 2 inches, increasing in 1/8-inch increments.
2+ Inches Impact Wrench
Impact wrenches are available in sizes ranging from 2 inches to 8 inches. The smaller versions can still be operated with two hands and are used for repairing tractor trailers and heavy equipment. The largest versions are used in construction and have to be mounted on cranes due to their weight and force.