If you do not exceed the yield strength of the bolt it will return to it's orignal length when the torque is released.
A properly designed and tightened threaded fastener will apply more clamping force than there is force to seperate the joint (combustion pressure, thermal expansion, dynamic forces such as the piston stopping and changing direction at BDC and TDC).
The question is has anything happened to the fastener in it's life time?