Touch pistion
Dirt deposits around the stretchy part
Of thread
It is difficult to
Replace spark plugs
The side wire to
Be over-heat.

Leakage of the
Combustion gas
Lose power
Poor conductibility
Over heating spark plug.
Pre-ignition.

Normal ignition
Without
abnormality

Waste gas deposits round the ignition end and affect the ignition feature.
Dirt aeposits around the lower part of the instauation hole

Waste gas heavily deposits around the ignition end and probably cause no fire. Dirt heavily deposits around the stretchy part of thread. It is difficult to replace.