1. While not perceivable to your eye, when tightening down a bolt there is a stretching that occurs. A certain amount of stretch causes tension on the fastener. This is good. However, you know what they say about too much of a good thing? Over stretching the bolt will cause fatigue and eventual failure. A torque wrench, when used with a correct specification, keeps the bolt within operating range.|
2. Sometimes a torque specification is meant to prevent damage to a component. For example, the torque load for the steel threaded spark plugs going into the aluminum head of the engine is usually around 15 ft lbs. Over-tightening of the spark plugs will eventually fatigue the aluminum threads and possibly strip them ? an expensive repair.
3. Seals! This is one that can cause a simple leak repair to turn into a cranium splitting nightmare. A seal is meant to have a specific compression. This allows the seal to (technical jargon coming up) properly squish and prevent a fluid or gas from getting past. Too little pressure and no squish. Too much pressure and the seal will crush.
4. Speaking of crush, aluminum is an interesting material. In some respects it can be incredibly strong, and yet it can be so brittle. Over-tightening aluminum with a steel bolt can cause it to break or crack. Anyone over-torque a transfercase bolt and break off a tab or even crack the case? Yup, it happens.