Hell I don't remember Captain. It is something I learned many years ago building high performance engines.

It is common knowledge among race engine builders that you don't want anything impeding the heat flow from the spark plug to the surrounding cooling surfaces, whether they are air cooled or liquid cooled. It can really mess up your heat range selection of spark plug, and in extreme cases can cause engine failure because the spark plug can't cool itself and overheats. Usually before it fails completely it starts acting like a glow plug and causes preignition and engine failure.

When we set the spark plug depth, and look to ensure that no threads are exposed into the combustion chamber either of the plug or the threads in the head, we are advised by the spark plug companies not to even use two gaskets if we need to space the spark plug out a bit. The proper procedure is to use a special solid gasket that is thick enough to provide the clearance you need, and this is something that Champion and other companies offer for that purpose. Even the practice of using two regular spark plug gaskets if needed to obtain proper thread depth is not recommended, as it can also impede heat flow.

AV8TOR