There are thousands of bolts in your vehicle that hold together critical components in your automobile. These components must be fastened together correctly to ensure that your vehicle can perform as safely and flawlessly as it was designed for. Many mechanics and do it yourself repairmen do not correctly follow torque specifications or, sometimes, completely ignore them altogether. Yes, there are some bolts that do not require torquing and can simply be 'snugged'; however, the majority of bolts in your car should be torqued to the design engineer's original standards.
The contact surfaces between any components that have been thoroughly analyzed for stress, fatigue, strain, or thermal expansion need to be fitted together within a certain tightness tolerance in order to successfully meet the design requirements. The tightness between two components will affect many parameters including, but not limited to, vibration, stress, fatigue life, strength, thermal cracking, and durability.
One common example of overlooked bolt torquing is the brake caliper guide bolts and mounting bracket bolts. The brake caliper guide bolts are generally torqued to around 25 ft-lbs and the bracket to wheel housing bolts are around 100 ft-lbs. First of all, there is the obvious problem with under torquing (insufficient bolt tightening) because the normal thermal and vibration cycles can loosen the bolts overtime and lead to catastrophic brake failure. Loose bolts will also cause the braking system to chatter and put more stress on the actual fasteners. When the components are firmly together, a good amount of stress is evenly transferred through the actual mating surfaces, the bolts are there to simply hold them together by tension. If the bolts are loose, the bolts may be subjected to abnormally large amounts of shearing force (an adjacent force to the direction the bolt points to) and can easily break if it was not designed to handle the excessive shear.
Over tightening is also a huge problem, sometimes even worse than under tightening. The bolts need to be able to stretch to compensate for the increased thickness of the parts due to thermal expansion. If those same caliper bracket bolts are over torqued then they lose that stretching ability and can fail as the brake parts expand at temperatures exceeding 500 degrees Fahrenheit.
One last important tip is to make sure you know if the threads of the bolt were dry, oiled, or moly lubed for the torque procedure. Moly lube reduced the amount of friction between the threads significantly, so less torque is needed to obtain the same clamping load. When in doubt, install the bolts with clean, dry threads.
