Originally Posted by
Andrew R Stewart
One problem with threaded fittings is the galling of the threads. Especially with a soft metal like AL. Grease will help reduce this possibility. Additionally I always felt that a greased bolt steel less twisting stress and therefore less chance to fracture. Andy
Yes, and the wrenching force goes into stretching the bolt rather than overcoming the friction of the threads; the stretch is what keeps the joint from loosening.
Many high-performance cars have their connecting rod bolts tightened by measuring the stretch rather than the torque, because of frictional effect uncertainty.
Galling is not so much of a problem with dissimilar metals like steel vs aluminum, but is a big problem with stainless bolts vs stainless threads (e.g. nuts or Helicoils) so greasing or other anti-galling treatment is a must.