Heat dissipation and heat flow
Nearly 60 % of the heat dissipation takes place through the spark plug case and thread. The seal ring conducts somewhat less than 40 % to the cylinder head. The small remaining percentage flows out through the middle electrode.
The insulator absorbs the heat in the combustion chamber and conducts it into the interior of the spark plug. Anywhere that it comes into contact with the case, heat is conducted.
By increasing or decreasing the size of this contact surface area, it can be determined whether the spark plug conducts more or less heat through the case.
The contact surface area is larger for spark plugs with higher thermal endurance. For spark plugs with lower thermal endurance it is smaller.