For effective retention, a certain amount of tension (tensile preload or tensile stress) is necessary - it's what provides the clamping force on the join, and what provides the enhanced friction that keeps the screw threads from backing out.
Tension is measured directly in rare cases (by measuring the stretch of the bolt directly) or indirectly (interpolation of the tensile stress by measuring the force needed to turn the screw.)
Thus, the torque wrench.
Torque specifications given in the service manual are for "clean, dry" threads unless specifically noted otherwise (for instance, the crankshaft nose screw is meant to be lubricated with clean engine oil then torqued to 80 pound-feet.)
Modifications to the "clean, dry" values are as follows:
- Threadlocker uses full torque - it doesn't change the friction of the threads.
- RTV sealant uses full torque value.
- PTFE paste uses 90% of torque value.
- Graphite uses 85-90% of torque value.
- Engine oil or chassis grease uses 75-80% of torque value
- Never-seez uses 50% of torque value.
The reductions in torque are due to the reduction in friction between the mating thread surfaces - reducing the friction translates to increased tension for a given torque application. Reducing the torque value helps prevent over-tensioning the screw and/or material failure of the mating part.
Interestingly, you also get more consistent application of tensile force - which is why I usually lubricated screw threads for cylinder heads, for instance, which allowed for more consistent clamping force across the deck (variation in preload for "clean, dry" threads runs 20-25% from nominal, while lubricants reduce this potential variation direct proportion to lubricity - the more "slippery" the lubricant used, the more consistent the preload application. Using "torque indicating" washers (like from SPS/Unbrako) or direct measurement of screw length is the most reliable method of measuring preload application in a screw.)