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Which bolts and other fasteners should not be loosened using an impact wrench? For example, I've heard that it's not generally recommended to loosen spark plugs with an impact. Nor is it recommended to loosen head bolts using an impact.

What's the reasoning behind not using an impact to loosen these fasteners? What other fasteners should not be loosened using an impact wrench (e.g. cam sprocket bolts? crank bolts?)

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As the name suggests, impact wrenches have the ability to impart high amounts of momentum change.

The delivery of this momentum change is hard to meter, so it is quite easy to damage:

  • soft metals and alloys
  • thin threads (both male or female)

This applies to both loosening and tightening.

Take the example of spark plugs and head bolts, which screw into cylinder heads and engine blocks respectively. Most modern longblocks are made of aluminum alloys, so using an impact wrench here would very likely result in damage to the block. (Sometimes even tightening by hand results in stripped threads, so go figure).

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    Anything that must tightened with a torque wrench, should NEVER be touched with an impact driver. – Tonny Mar 8 '18 at 12:28
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    @Tonny, that's basically every single bolt on a modern vehicle. Lug bolts, brake caliper bolts, engine mounts, suspension bolts, etc etc. They all have to be torqued down to spec. I can''t think of a single bolt without a torque spec. – Ives Mar 8 '18 at 12:35
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    The mechanics I go to always use a torque wrench to do the nuts up to the correct specifications - perhaps it is a question of knowing "good" mechanics as well... Also comes down to how much you want to pay... – Solar Mike Mar 8 '18 at 15:56
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    @Tonny so the big 32mm nuts holding the driveshafts in place do have a torque specification (and its not FT !!) and are regularly undone with an impact wrench... – Solar Mike Mar 8 '18 at 15:59
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    In my experience loosening bolts with an impact tool is often less likely to damage things than using a regular wrench. When metal surfaces have bonded due to corrosion or simply prolonged contact it is common for the bond area to have much less resistance to sudden impact than the surrounding metal, even if the bond is strong enough against steady pressure to cause the bolt to fail first. Many times have I noticed a bolt starting to twist as I attempted to remove the nut with a wrench and had applying an impact tool break it loose without twisting the bolt off. – Perkins Mar 8 '18 at 23:02

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