⅜ vs ½ Impact Wrench. What Size for Lug Nuts & Auto Work? (2024)

Confused between 3/8 and 1/2 inches? This is one of the most common questions people ask before buying an Impact Wrench.

I will help you analyze the best choice for you and what you should consider before buying one based on facts and my experience as an automotive professional.

First of all, 1/2 is bigger than 3/8. If we convert the fractions into decimals, 1/2 = 0.5 and 3/8 = 0.375.
Now that we know the 1/2-inch impact wrench is bigger let’s compare the two sizes.

⅜ vs ½ Impact Wrench

Even when a ½″ impact wrench is the most recommended for several uses, including automotive work; it’s not that simple. Before choosing one over another, it’s important to understand the characteristics of impact wrenches and choose a tool that best suits your needs.

1. Anvil Size

The size of an impact wrench is denoted by the size of its anvil. This is the thickness of the square output shaft that is driven by the wrench’s mechanism. The sockets are embedded into the tip of the shaft using different coupling mechanisms.

Anvils are usually made of strong steel alloys and can perfectly handle the machine’s torque.

I used to think that a larger size anvil has more mass which would translate into higher output. While this may be true in the case of a 1-inch model, when I compared the ⅜-inch vs ½-inch impact wrenches, the anvil size does not significantly affect the impact wrench’s torque output.

So, why is there no significant difference in output torque?

I can’t answer this question with 100% certainty. But here is my take on it.

When compared to a 3/8″ impact wrench, the ½ “ model has over 25% increase in anvil size and more than 30% in weight. However, most manufacturers are not making the other parts of the tool, such as the wane motor, correspondingly bigger.

I suspect many are just replacing the anvil on a nearly identical pneumatic motor.

2. Torque Output

This is one of the most useful figures that will help you determine which impact wrench best suits your needs.

Generally, ½″ impact wrenches will have more torque output than ⅜″. This is because ½″ impact wrenches are usually bigger and are built of stronger materials than ⅜″ wrenches.

But that’s not always the case; in the market, you will find tools that use the same casing, are built using the same components, and are almost identical except for the anvil size.

I have seen torque tests on YouTube and read reports on Reddit about tools of certain brands and models that don’t show a big difference between the ½″ and the ⅜″ versions and, in some cases, the ⅜″ even surpassed their supposedly “big brother”.

Torque Test Video

Direction of Rotation

Another thing to keep in mind is that most impact wrenches have different torque outputs depending on the direction they are turning. They usually have more power for what manufacturers and vendors call “nut-busting or breakaway torque” (the torque needed to make a stuck nut start moving) than fastening torque which, as the name suggests, is the maximum torque that the tool can apply to tighten a nut.

That’s why it’s important to check for the tool’s specs before buying, and it’s important to know how much power you need. When you are on a budget, it’s also helpful to search for as much information as you can. Sometimes, as I mentioned, you will find a ⅜″ impact wrench almost as good as or even better than the ½″ model of the same brand, and cheaper too.

In any case, the output torque is one of the most defining elements when buying an impact wrench.

3. Overall Size & Weight

½-inch impact wrenches are usually bigger and heavier than their ⅜″ counterparts. That’s because, in theory, they use stronger components. That’s why ⅜-inch impact wrenches are more suitable for working in tight spaces. Besides, air impact wrenches are lighter than electric and cordless tools.

This being said, it’s important to note that some cordless impact wrenches have the same power output no matter the anvil size; however, the battery size and weight may differ, so it’s important to consider the battery size and type of cordless tools when choosing an impact wrench.

4. Available Sockets

Even when there are plenty of sockets available for both ⅜″ and ½″ impact wrenches, and you can find adapters to snap ½″ sockets into ⅜″ anvils, it’s always best to plug sockets straight into the machine.

Consider what size of sockets you use the most and what size of nuts and bolts you usually work with. Big and strong ⅜″ sockets are expensive and hard to find; the same goes for small ½″ sockets. Besides, if you work with small pieces, you won’t need as much torque as with bigger parts. You should consider the availability of sockets for the size of the impact wrench you are buying.

It’s also important to check in your toolbox what size of sockets you already have and use the most. It’s nonsense having to spend unnecessary money to buy sockets you already have just because they don’t match the size of your new impact wrench.

What Size Impact Wrench for Automotive Work?

The ½″ impact wrench is the most commonly used size for professional automotive work. They have enough power to deal with stuck bolts, lug nuts, and nuts, but they don’t deliver excessive torque to round or damage them, which is good in terms of safety and practicality.

I used a ½″ electric impact wrench for jobs where I needed to apply a lot of force fast or to remove tires. It wasn’t one of the most expensive brands or models; it was in the range of a Hart tool. After years of heavy use, the most significant maintenance it needed was a new set of carbon brushes.

For light-duty work, a light and compact ⅜″ is more practical and comfortable, mainly when you have to use it all day.

I prefer to work on the delicate engine and transmission parts by hand. The closest thing to an impact wrench I used was an electric driver, only for disassembling. Some of the reasons why using impact wrenches in some parts is not advisable exceed the scope of this article. When it was time to put everything back together, I always torqued the cylinder heads, valve covers, and flywheels manually, using a proper torque wrench.

1/2-inch or 3/8-inch Impact Wrench for Lug Nuts

If you want to play safe, go for a ½″ impact wrench to do this job. No matter the brand and model, you will be able to remove even the most stubborn lug nuts with it.

Some ⅜″ wrenches, such as the Milwaukee M18 FUEL ⅜″, or the Hart 20V ⅜″ impact wrench, work fine for cars and pickup trucks. It depends on the brand and model; you will find most ⅜″ wrenches that will work well with lug nuts in perfect conditions, but some won’t unscrew stuck or rusty wheel nuts.

As always, use impact sockets when working with impact wrenches on tires.

It’s also important not to reinstall your tires with your impact wrench; you can over-torque or under-torque the lug nuts. The best way to fasten your lug nuts is by driving them gently with your impact wrench, just to make it quicker, and then making the final adjustment by hand using a torque wrench.

Engine/Transmission Work

When doing engine and transmission work, you will have to consider what kind of vehicles you are working with and how much room you have. It’s not the same working inside a roomy mid-truck engine bay as in a small compact car engine.

Another thing to consider is whether you work with the engine mounted in the vehicle or on a bench. If you work a lot with heavy-duty vehicles, you will have plenty of space, but you will be dealing with tough bolts, while small compact cars don’t have much space to work, but the bolts are easier to deal with.

In the engine department, under normal circ*mstances, you won’t need more than 250 lb-ft of nut-busting torque, so my advice would be a compact ½″ impact wrench (usually called mid-torque ½″ impact wrenches) like the Craftsman CMCF920 or a ⅜″ Milwaukee M18 FUEL impact wrench. They both would do a great job; the difference lies in the price.

The Milwaukee costs about twice as much as the Craftsman, so if you are on a budget, I would go for the Craftsman CMCF920 without any doubt. This would be enough to work with cars to, pickup trucks, and SUVs.

Heavy-duty vehicles like medium to full-size trucks need more powerful tools up to ¾″ or even 1″ sizes.

I don’t advise using impact wrenches on flywheels or harmonic balancers, but many mechanics do. Some engines need at least 450 lbs-ft of torque to undo those parts; in that case, a ½″ impact wrench is recommended.

In the transmission department, a good ⅜″ tool will disassemble bell housings, clutch pressure plates, automatic transmissions oil pans, and most CV axels without a hassle.

Steering and Suspension Work

Steering and suspension components are usually tougher than engines and other components. Besides, the parts are more exposed to weather, road debris, mud, and other contaminants that lead to rust, wear and tear.

So, if you want to make sure you will be able to deal with all steering and suspension bolts and nuts, I advise a good, powerful ½″ impact wrench.

Rusty Fasteners

These are always tough to deal with. Sometimes, an extremely powerful ⅜″ impact wrench like the Milwaukee M18 FUEL might work. Still, if you want to be entirely sure that you can deal with rusty fasteners, or if you work with SUVs or off-road vehicles, you should consider buying a powerful ½″ cordless impact wrench or an extra powerful pneumatic impact wrench.

In extreme cases, you will need to use an air hammer to loosen the most stubborn rusty fasteners.

Pneumatic vs. Cordless Impact Wrench

Air impact wrenches are smaller than cordless impact wrenches; they don’t have an electric motor or a battery and don’t lose power as the battery fades. Pneumatic impact wrenches can be practical, but they require additional investment in infrastructures such as air compressor and pipelines.

Unless you work in a big shop or already own a decent air compressor, a cordless impact wrench is a best and most affordable solution for all kinds of bolts and nuts. The pros and cons of air vs. electric impact wrench are discussed in detail here.

Conclusion

As you may notice, there’s not a clear winner in this matter. ½″ impact wrenches are best for some jobs, and ⅜″ works better in certain situations.

If you can afford to buy the two, my best advice would be to have one of each size in your toolbox; this is not the case when “one size fits all”. However, if your budget is not enough to buy two, then I would suggest you think about what kind of jobs you do the most. If your work involves heavy-duty applications such as removing rusted fasteners, lug nuts, etc., and if you have enough room to work comfortably, go for the ½″ impact wrench. I used one of those for years, and I never regretted not having a ⅜″.

The 1/2-inch impact wrench will make sense for most people, and that is the size I would recommend.

If you will use it for light-duty jobs, or you will be using the tool for DIY work, and you would also like to use it for other applications besides automotive work like home improvement work, scaffolding, etc., then go for a good ⅜″ tool. If you choose to go this way, try to buy a powerful cordless impact wrench, and try to get one that allows you to set the output torque. This will help you to find more uses for your tool.