I am rebuilding a car engine. With some of the fasteners I am unscrewing, I notice that a lot of torque is required to undo the fastener. It feels like a snap when undoing a fastener, rather than a smooth twist. If I tighten the fastener to a set torque again and loosen it, it requires the same torque as it took to fasten it.
Why would this be? My guess would be something to do with heat cycling.
Note: There is no threadlocker on these bolts.
Oxidation, dried lubricant, contaminants, rust - all that acts as a layer of glue binding the elements. As you apply a firm torque, you snap that bond. It takes a considerable time to re-form.
Under high pressure and high heat metal bonds by a process called diffusion. Where the atoms of the two parts intersperse over time.
This and oxidation, slow chemical burning of unintentional matrial and debris between the two parts, cause a strong bond that many times cuase the fastener to break before it can be removed.
A standard bolt will stretch when tightened. This stretching increases force on the threads of both the bolt and hole and thus increases the force required to overcome static friction and loosen the bolt. If we exclude other materials and chemical changes to the surfaces then that snap that you experience is the moment you overcome that static friction between the two parts.
