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Join Date: Apr 2013
Location: Somerset UK
Vehicle: SLK230 '99 R170
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Tightening any bolt correctly.
To complement (and compliment) the comments above:
If you use a long extensions on the torque wrench side of the swivel, it allows you to hold the extension like a shaft and to have a "pure torque" that goes through the swivel into the target item.
The torque value is intended to give the correct longitudinal/axial force for the thread, and includes pitch of thread, contact areas, etc in the calculation, and also has assumptions about the cleanliness and lubrication of the thread. More dirt or less lube will mean the effort is spent on the thread, and not on the axial force, so the same torque will not provide enough holding force. And for the example mentioned, adding lube will reduce the thread friction, so the torque will spend less effort on the thread friction, and more will go into the axial force, so the correct torque will provide a higher fixing force, which may exceed the strength of the threads.
Spark plugs have a much simpler system; the washer at the base of the thicker section is a crush washer, and will provide a good axial force over quite a range of crush. Hence the plug should be done to (for example) finger tight, and then 90°, then 45° more. (See the plug manufacturers instructions for their crush washer). For a spark plug, if the plug has been removed and refitted too many times, then the crush zone may have been exceeded, and this method will result in over-torque, and hence strip the threads and result in a need for a heli-coil re-threading. Therefore, the use of a torque wrench is advised; this is set to the maximum value, and if you hear the click, the next time you will need new spark plugs (or at least the crush washers, which don't tend to be available separately).
In place of a torque wrench (or to compliment its ultimate safety) it is possible to get an protractor to measure the turn of the bolts; as long as this is downstream of any ratchets, it can show the angle of rotation, so the wrench can provide the torque to turn the bolt, and the protractor provides the control.
(Note this philosophy is very similar to that of stretch bolts; use the bolts, tighten to angles, so the bolt has entered the plastic deformation range, and then all the forces will remain at the maximum of the plastic deformation, and not exceed the thread loading. However, once installed, the bolts have started to stretch and work-harden, so they cannot be used again.)
You had all the answers from (almost) all of the big hitters above; just a bit of detail that might help you understand the different methods of ensuring a bolt is fitted correctly.