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I am familiar with the concept of torque wrench. However, with a torque wrench, one can only apply a desired amount of torque. Is there a way to measure how much torque has been applied to a certain fastener without actually applying the torque as in reverse engineering a torque spec?

An example where that would be practical is if you have to take something apart, yet don't have its torque spec and trust the existing torque and want to retorque it back to the original torque.

  • One thing you could do - this is what I do when I take apart my watches - is to draw a line with a sharpie running from the center of the fastener onto the surface it's attached to (think like a Q). Then, when you retighten the fastener, you just have to line everything back up. – zhang Aug 7 '15 at 21:46
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    @zhang - What if the new gasket (like head gasket) is of different thickness than the original? Your torque would not be the same. If it was thicker, then your torque would be tighter. If it was thinner, your torque would be loose. You would just never know. Plus if there is any sort of fastener stretch, it would be off as well. – Pᴀᴜʟsᴛᴇʀ2 Aug 7 '15 at 22:05
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You have several options:

  • The internet is a vast resource ... use your Google-Fu and figure it out. You can always ask on here. There are enough of us on here we can get you the torque value for your fastener.
  • You can use the following torque chart from the Bolt Depot:

enter image description here

  • If all else fails, get a dial indicated torque wrench. Put it on the fastener and turn the handle until you just start to feel the fastener turn. Do this slowly and pay attention to where this happens. Round it to the nearest 5lb-ft and you should be right there. I know this is basically reverse engineering the torque, but it should put you in the right range of torque.

enter image description here

While torque specs are very important, what is more important is getting the torque specs even between fasteners, especially with things like head bolts.

Another thing to be aware of when torquing bolts is to figure out whether the bolt is torque-to-yield (T2Y). These fasteners are one use only and must be replaced once taken apart. These fasteners will also most likely have the final torque sequence which works from the stretch of the both and not from the torque itself. The final sequence is done by turning the fastener a certain amount of degrees. A torque sequence may look like:

  • First: 20 lb-ft
  • Second: 40 lb-ft
  • Third: 60 degrees

(NOTE: These are just numbers I've thrown out there for understanding.)

You cannot tell by the way a bolt looks as to whether it is a T2Y bolt or a regular one ... you have to look up the spec. If you don't know and don't do it right, you'll most likely blow a head gasket. This is very important.

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The short (and physics-correct) answer is: no, you cannot vicariously observe how much torque is already applied to a fastener. (By vicariously, I mean: without changing it.)

A somewhat simplified explanation:

Torque is a force. Only when the loosening force you are applying exceeds the pent-up force holding the fastener still will it begin to move; and only at that point will you know how much torque (force) was originally applied. But since by that point you have moved the fastener, you have violated the conditions of your question.

However, above I am answering on technicalities. On practicalities, @Paulster's suggestion in the accepted answer is the best you can hope for.

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