The correct torque
For the professional installation of a glow plug, the use of a torque wrench is recommended. Even for professionals the estimation of the tightening torque is nearly impossible.
That is because a torque can be calculated from two sizes which are multiplied by one another: the force which is applied to the respective centre of rotation and the length of the handle.
If the incorrect torque is applied to a glow plug, a compression loss can result. Based on the vibrations, it is also possible in the case of ceramic glow plugs that the ceramic will break.
The application of a tightening torque which is too great, in particular, can lead to the contraction of the annular gap between the metal case and glow tube. In this case the glow plug overheats and fails.