Given an automotive job like rear-suspension replacement of shocks, what are the dangers of over-torquing and how would I know from the failure mode that I cause?
If the bolt and nut survive over-torquing is it safe to assume no problems will result?
The main danger is that you permanently deform something, commonly the bolt. This means that the bolt no longer can handle the rated load, as it's thinner. You want to work with the bolt in the elastic regime, where it's not permanently deformed.
Over-torquing can lead to failures down the road, either in the bolt or associated components.
Note that there is some error margin; torque wrenches are nowhere near exact, as you measure the torque, not the tension in the bolt. Tension in the bolt is what's interesting, and torque is a proxy for that, but the number may be off by quite a lot due to varying friction. Torquing should be performed with lightly oiled threads.
I see two possible scenarios here: if the over-torqued fastener is part of a static assembly, for instance valves cover, etc. it would be somehow "safe" since there wouldn't much "pull" force on the fastener. Of course, you are still in risk of damaging the threads, break the bolt, etc. Second scenario would be the fastener as part of a movable assembly or assemblies that need to hold strong forces, like shock abs, head bolts, steering elements, brake calipers to mention a few. What happens is that in these assemblies, over-torqueing is already adding more stress to the fastener keeping them more near to their break point, or failure point. Normally, fasteners absorb some stress and can stretch up or deform to certain points, before breaking. Any violent or lightly excessive force on these would effectively make the assembly fail, either by fastener break, ripped off threads or worse breaking part of the assembly, when in the contrary, if they are correctly torqued could absorb the stress.
I have seen head bolts rip off the threads in the aluminum block because it got so overtighten it weaken the threads so cylinder compression did its job.
