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Thanks in advance for reading. Have some heavily rusted bolts on my hubs, trying to do as much of a needed hub replacement as I can myself. Got a mixed bag of auto repair tools from someone a while back, among them a 1/2” air wrench and some assorted bits. I like to do my own repairs, and have run into this problem a couple of times so far. So thinking of buying an air compressor to run the air wrench.

Question is, though, a lot of random literature says I need about 6cfm to run them, but compressors that have that kind of output are at least $700. What happens if I use one with an output of 4 or 5cfm?

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  • Welcome to Motor Vehicle Maintenance & Repair! Something to consider, it's not only the amount of air the compressor can put out, but also how much can the lines/connections you are using can pass through them as well. Ensure your hoses and couplings can support the needs of your air tools or you won't be getting the full use of the tool.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 19, 2019 at 16:07

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4cfm is the usual rating for most air tools - including intermittent work with an impact wrench.

Also consider buying an Electric Impact Wrench (not a battery powered one - they're usually too low in power). And breaker bars with long (4ft) extensions can persuade most bolts..

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  • You haven't tried a Snap-On CT8850. I've ripped heads off fasteners that my Nitro Aircat 1/2" wouldn't budge. I've found the new (not Chinese junk) battery units have far more grunt than the electric ones. The battery units can provide hundreds of amps of current for a short while, while the electric ones are limited by the line cord.
    – SteveRacer
    Jan 19, 2019 at 18:18
  • Agree with @SteveRacer on this one ... newer 1/2" drive battery operated impacts can produce in excess of 1,100 lb-ft of torque ... that's a lot of grunt. Milwaukee's is good to 1,400 lb-ft of break away torque. I would bet you'd not find too many bolts which could stand up to that much torque.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 20, 2019 at 3:24
  • Good information on Battery Impact Wrenches. I was only basing it on my knowledge of DeWalt etc.
    – PeteCon
    Jan 20, 2019 at 14:07
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If the air wrench does not get enough air then it does not develop the full power it is capable of, which means it won’t always undo the nut. (I don’t have that problem - even by hand I can undo , break or shear nuts I have to undo...)

Either buy a large enough compressor or consider using a secondary tank to have a larger reservoir of air...

If you go the second reservoir route, then use rated tanks - truck brake tanks are a possible option - but consider that the small compressor will have to run much longer to fill a larger reservoir and that may lead to overheating.

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If the air wrench is the only tool you intend to drive from the compressor, consider and electric impact gun as, for the same money as a cheap and cheerful compressor, you’ll get a really nice electric or even better impact gun.

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Get a corded impact wrench, all that torque on tap and cheap. My 78 280z have a 35mm nut holding together the rear hubs. 40 years of rust makes for a tough customer. I got a $30 impact from menards and after about 4 minutes of hammering with it they came loose! That corded impact is unbelievably strong

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The size and pressure in the tank determine what tools it can supply, not the output CFM of the compressor.

However, a small and/or low pressure tank might only give you a few seconds of runtime on a large impact wrench, and then the small compressor would have to struggle to refill the tank.

Keep in mind there's a huge difference between 10 gallons on an oil-less compressor running 120psi and 10 gallons of compressed air from the oil bath "industrial" models at 175psi. If your tank is small and you regulate to 90psi (typical) at the tool, that 120psi will drop below 90psi in a hurry.

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