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When you don't know the correct torque for a fastener does the torque required to loosen it equal the correct torque to tighten it to? Presuming it's not rusted on, that is. Or, if you can't use it directly is there a rule of thumb?

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No, absolutely not. During tightening the torque you’re putting in gets “used” in three ways:

  1. Overcoming friction between the head of the bolt and the joint surface

  2. Overcoming friction between the threads

  3. Stretching the bolt/compressing the joint via the “ramp” action of the threads. This is the actual “tightening” action, though in typical bolted joints only about 10% of the torque you’re putting in goes to this factor; the rest is used up in the friction described above.

During loosening, you still have the two friction components to overcome, but obviously you’re not working to stretch the bolt—in fact the ramp effect kind of helps to loosen the joint.

You can sometimes establish a relationship between tightening torque and loosening torque for a given joint, but it’s not very precise.

Sometimes people talk about “restarting torque”, ie the torque required to tighten a bolt just a little more than it has previously been tightened. In this case, the physics is more similar to the initial tightening, but restarting torque is still not equal to tightening torque, because typically “static” friction (what you’re working against to get the bolt moving from a stop) is greater than “dynamic” friction (keeping the bolt moving during tightening).

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  • How do you “love” a bolt?
    – Solar Mike
    Dec 2, 2017 at 18:19
  • So, if the engine for example with cast iron block and aluminium head, the change in temperature will cause the head to expand more than the block and change the load on the fixing which means item 3 on your list will still be relevant.
    – Solar Mike
    Dec 2, 2017 at 18:26
  • Relevant to your #3, you couldn't use the untightening method to try and gain a torque value on a torque to yield bolt, either. Since they've already been stretched to the yield point, it's kinda moot to even try.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 2, 2017 at 18:44
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In theory yes, but there are other factors apart from corrosion that you mentioned : such as temperature : different metals expand at different rates so if the part was torqued at 12 Deg C then the ambient T rose to 25 Deg C it could increase the clamping force...

Edit : "in theory" means that there is no dirt, corrosion (mentioned by the OP) and the correct lubricant (if any) is used as specified.

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  • I would argue that "in theory" would be an absolute no simply due to debris, galling, and galvanic corrosion. I believe it's AvE who has a video demonstrating that torquing two stainless fasteners (a nut and bolt) into each other without lubricant become a single piece of metal due to some kind of metal wizardry.
    – soolus
    Dec 2, 2017 at 17:07
  • @soolus don't you clean threads before putting things together? Oh, since you mention stainless, what is the correct lubricant?
    – Solar Mike
    Dec 2, 2017 at 17:08
  • In an ideal world, yes, I would chase threads, but nothing is ever truly clean in my shop, hah. Figure two brand-new bolts from a hardware store, you'll still experience some level of galling and thread deformation when you're torquing it. As far as stainless, geez I don't know, I've always just used your every-day copper antisieze with relative success.
    – soolus
    Dec 2, 2017 at 17:18
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Temperature, mechanical welding, corrosion, solvents or lubricants involved, as well as many other variables will change the force required to loosen a fastener.

There are many torque charts such as this one, one here, and even for SAE here, as well as calculators out there that will give you a general idea of the proper torque specifications for tightening, though keep in mind these assume like materials. If you have a steel fastener going into a brass rivnut or aluminum casting, you're going to want to use substantially lower values. Most non-critical fasteners can live with "good and tight", whereas most large fasteners are fine with one or two ugga-duggas.

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  • Are these torque charts you reference for tightening?
    – Solar Mike
    Dec 2, 2017 at 17:06
  • Yes, I'll update that for clarity.
    – soolus
    Dec 2, 2017 at 17:12
  • Those are great references! I'd like to point out on the chart, it does state these are the "max torque value assuming clean dry threads". If lubricated (it says oil, but would throw anti-seize into this mix as well), decrease values by 10%. Also, coarse/fine pitch makes no difference. I also wonder if there's something similar chart for SAE? The world does not live by Metric alone!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 2, 2017 at 18:37
  • Sorry, was being clever with the linking. There are three linked charts in that string, the last of which being SAE. There's a little engineering pocketbook every toolbox should have that has all this information in it as well, I forget what it's called..
    – soolus
    Dec 2, 2017 at 20:54
  • Zeus tables spring to mind : drill sizes etc
    – Solar Mike
    Dec 2, 2017 at 22:40

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