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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?)

Ives
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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).

Zaid
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    Anything that must tightened with a torque wrench, should NEVER be touched with an impact driver. – Tonny Mar 08 '18 at 12:28
  • I’ve never done it, but on the 3v Ford Triton motors, I hear a 3/8” impact works wonders on the sparkplugs. – Ben Mar 08 '18 at 12:32
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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 08 '18 at 12:35
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    @Ives I know. In my view the "official" torque-rating for many of those bolts is a bit overkill. It seems manufacturers these days are so afraid of lawsuits they have a tendency to write everything down to exact specifications. Take wheel lugs: I've seen torque-ratings for those, but no car I have ever seen comes with a torque-wrench to do them up "properly" when you need to change a flat tire at the road-side. And I have never seen any mechanic use one either in a workshop. They just bring out the pneumatic socket-gun in the "one setting fits all" mode... – Tonny Mar 08 '18 at 12:55
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    Also lock nuts for alloy wheels shouldn't be loosened by impact wrenches or rattle guns. – mickburkejnr Mar 08 '18 at 13:32
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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 08 '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 08 '18 at 15:59
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    @Tonny they also might have been using a torque stick like this http://a.co/9LRciDi They look just like extensions but they will limit the torque applied to the fastener. If you did not know, it would look like they are just cranking them down. – Ukko Mar 08 '18 at 18:48
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    @Tonny: You need to go to better garage/tire/wheel shops then. – Ellesedil Mar 08 '18 at 19:05
  • @Ives: I would actually include wheels/lugs to this list of avoiding impact wrenches. I know tons of places use them, but those places apparently don't actually care about the condition of your wheels and lug nuts. A place that cares about not damaging anything will do it by hand. – Ellesedil Mar 08 '18 at 19:07
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    I'll just leave this here: https://www.reddit.com/r/Justrolledintotheshop/comments/7vp48h/the_results_of_a_professional_tire_service_just/dttyyi9/ – Zaid Mar 08 '18 at 19:09
  • @Ellesedil That's the first I've heard of lug bolts being damaged by loosening them with an impact. Of course you don't tighten them to spec with the impact, but Zaid's picture is a result of incorrect tightening and not related to loosening. – Ives Mar 08 '18 at 19:39
  • @Ives: I'd also be concerned with the impact contacting the wheels/lug holes and damaging the finish. I'd assume that 90% of car owners won't care too much, but I would care a lot, particularly for nice aftermarket wheels. Using tools that can impart a lot of sudden torque means that the tool can also suddenly move and contact something it's not intended to hit if not under complete control. Slow and steady is the way for me in this instance. I actually tell my dealership to skip on the free tire rotation specifically to reduce the chance of someone carelessly damaging my wheels. – Ellesedil Mar 08 '18 at 19:48
  • @Ellesedit, I have special sockets specifically to take off wheels lugs & bolts. They have a soft plastic exterior that won't damage alloys. And I run the impact on the lowest setting with only half a trigger depressed. – Ives Mar 08 '18 at 20:08
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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 08 '18 at 23:02
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    It's important to note that there is not just one "impact wrench" that applies one level of force. Impact wrenches can actually be safer than breaker bars for loosening, and if you're drastically over-tightening lug nuts with an impact, you need to read the manual. – barbecue Mar 09 '18 at 02:13