A torque wrench is required for the professional installation of a spark plug. 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.
Most spark plug failures can be traced back to an incorrect tightening torque. If it is set too low, there is a risk of compression losses and overheating. A break of the insulator or middle electrode as a result of vibrations is also feasible.
If the tightening torque is set too high, the spark plug can snap off. The casing can also expand or warp. Heat dissipation zones are disrupted, overheating and melting of the electrodes or even engine damage can occur.