Tightening Torque

Torque is the most critical part of installation. A spark plug can only transfer heat effectively when it is fully seated on the cylinder head. Under-torqued spark plugs will not seal and transfer heat effectively. Over-torquing can distort a spark plugs internal seals and cause poor heat transfer. aplug with poor heat transfer will allowcylinder temperatures to rise dramatically - eventually leading to detonation and plug or engine damage.

To ensure proper torque when installing spark plugs, clean the threads in the head and on the plug, use anti-seize sparingly, and don't guess - use a torque wrench.
Sealing to the cylinder head:

Most spark plugs seal to the cylinder head with a hollow metal washer which is crushed slightly between the flat surface of the head and that of the plug, just above the threads. If the torque used to install the plugs is not excessive, the washer can be reused when the plug is removed and reinserted, although this practice is, strictly speaking, not recommended and replacement washers are available.

Always tighten spark plugs to the engine manufacturers tightening torque. If this information is not available, screw the spark plug in finger tight and then tighten a further half to three quarters of a turn for plugs with a gasket and 1/16 of a turn for conical/taper seat plugs without a gasket. This is applicable for 10mm, 12mm and 14mm spark plugs only. Almost all cars and light commercial vehicles use 14mm spark plugs.