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?)
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:
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).
