Why the spark plug in together? Not only in a?
Sometimes finish maintenance, you may conveniently let teacher to check the spark plug, if you have 1, 2, a need to change, they may let you directly in a whole, is there a need?
The answer is necessary.
The spark plug is just like the inside of the lighter small electrode, with time is long, small electrode is bad, lighter was not on fire. Spark plug is the same, it is a life, almost, can affect the normal operation of the engine.