In selecting spark plugs bear in mind that the plug, when screwed home tight, must come flush with the inner wall of the ignition chamber.
Testing spark plugs for ignition
A plug that is too long will overheat, while one that is too short will not fire efficiently. An undue amount of carbon on the shell electrode is liable to cause pre-ignition.
The points of the electrode must always be kept clean and bright.
The distance between the sparking points should be known and the points kept at- that gauge.
In coil ignition a well-worn dime will answer the purpose of gauge, while for magnetic ignition a piece of cardboard of the kind usually used for business cards, will answer the purpose.
Related Article – How to test Spark plugin
In ‘testing spark plugs for ignition it is not necessary to remove the plug.
Instead take any tool with a wooden handle and make connection with the metal part of the tool with the spark plug and any adjacent part of the engine, with the end of the tool not quite touching the engine.
plug is getting the current the spark will be seen jumping from the end of the tool. If there is no spark trace back the wiring for trouble.
If a spark is obtained and the plug is not firing then the trouble is with the plug itself and it should be examined carefully, the width of the gap noted and the sparking points cleaned
How to Test Spark Plug Article Source : The auto data book