Spark plugs are constructed by using two different manners of sealing in cylinder head.
Methods of spark plugs sealing
Spark plug is sealed in cylinder head with a conical seat. In this case no sealing washer is used. The assembly of spark plugs with this kind of sealing requires an especially sensitive approach. If the torque is exceeded, the spark plug shell can be stretched, thermal characteristics can be lost and there may even occur a rupture of the spark plug during its assembly or dismantling in the engine.
Spark plugs can never be mounted into cylinder head without a sealing washer. In such a case the combustion area is not sealed sufficiently and heat removal into cylinder head does not work well enough.
Tightening of spark plugs without using a torque wrench
Conical (tapered) Seat
Gasket (washer) Style
|Spark Plug||Head of Engine|
|plug sizes||seals||cast - iron||aluminum|
|M 10 x 1||sealing ring||10 - 15 Nm||10 - 15 Nm|
|M 12 x 1,25||sealing ring||15 - 20 Nm||15 - 25 Nm|
|M 14 x 1,25||sealing ring||20 - 40 Nm||20 - 30 Nm|
|M 14 x 1,25||conical seat||10 - 20 Nm||10 - 20 Nm|
|M 18 x 1,5||conical seat||20 - 30 Nm||20 - 30 Nm|
Selection of proper spark plugs
Vehicles produced in series (which are not additionally modified for engine power output enhancement), whose engines are properly adjusted and in good technical condition, can be equipped according to the current application tables.
Any comparison charts of spark plugs are always for informative purposes only and they do not substitute the current application tables in full.
In the case of engine power output enhancement by means of additional modifications it is suitable to contact a producer's representative. There always applies a principle that in the case of medium modifications of the power output it is suitable to use the spark plugs "colder" by 2 degrees than those forming its original equipment (e.g. a change from 15 to 12). After driving several miles and subsequent assessment of the insulator tip appearance it is possible to decide about the most suitable equipment. This operation requires enough experience.
Proper length of the threaded shell