They are mainly used in 4 cycle engines. 2 cycle engines do not get the fuel charge directly from the intake port like a 4 cycle. A 2cycle's fuel charge is sent from the intake port into the crankcase then it has transfer ports that bring the fuel charge up from the crankcase, therefore indexing the spark plugs does not do anything as the fuel charge is comming into the cubustion chamber in many directions.
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