The return torque required to remove a spark plug is sometimes abnormally high. Forcibly removing the plug in these circumstances can cause tearing of the metal shell beneath the hexagonal portion. See Photo 1.
- The reason for an abnormally high return torque is that the spark plug was fitted with excessive torque. This may also cause seizure of the plug thread and thread in the engine head
- If the spark plug cannot be removed because the torque required is too great, it is sometimes effective to warm up the engine and then spray the plug thread section liberally with a penetrant, then wait a short while before attempting removal again.
