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I've been working on old cars for a while and have twisted off a few bolts in inconvenient places.

Just wondering, all things being equal, which is more likely to result in twisting off a rusted bolt- breaker bar or impact gun?

I did a particularly nasty one in the frame of a vehicle the other day using a breaker bar and was wondering if I had reached for the impact gun first instead, if that would have made a difference.

I already know about using penetrating oil like pb blaster.

Thanks!

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In most cases, using an impact gun will be less likely to cause the fastener to fail. Here is my reasoning:

Impact guns provide high torque for very short periods of time. This jarring action or "impact" is going to be more likely to free the fastener. A breaker bar has a continuous torque applied to it. This will be less likely to break it free and will be more likely to shear off the fastener at a stress riser due to the constant twisting force which is applied.

Mind you, this is not a hard and fast rule. Things happen all the time. It also depends on the amount of torque your impact gun can provide. Obviously, if you are applying 1000 lb-ft of torque through a massive 1" drive impact gun, while supplying it with 200 psi of air pressure, and doing this to a 1/8" fastener, all bets are off. Using a little common sense with whatever type of fastener and whatever type of tool, you should be in good shape. No tool usage is fool proof; no fastener is immune to failure.

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  • My gut was telling me the same thing- that a sharp whack could cause the threads to act in a stick-slip mode by allowing 2 rusted surfaces to jump across rather than have constant drag, but thats just a guess... I'll reach for the impact gun first next time and keep it on a low setting. Thanks
    – 111
    Jul 23, 2015 at 15:55
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    Agreed. If I 'snap' the breaker bar and give it a strong pull quickly, its more likely to break it loose. If I hang on the bar and pull, pull, pull, its more likely to strip or break the bolt. Impact guns are all hard hits, not a constant force. In addition to using PB, applying some heat to the nut can help sometimes.
    – rpmerf
    Jul 23, 2015 at 15:57
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    When using an impact gun, the thing to remember is to pay attention to what the fastener is doing. For instance, on rusted exhaust parts, lots of times it will back off a nut for a while, then it will stop. You have to pay attention or you at this point will destroy the fastener. The best bet here is to run the nut all the way back to tight and hit the area with some more penetrating spray, like the PB Blaster, then try and bring it back off again. This may take several iterations and may ultimately fail, but you are at least paying attention.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 23, 2015 at 15:58

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