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what will happen if i put 2245 sparks from my m151 to my m38????

That would depend on the condition of your M38's engine and ignition system. It is simply a move to a slightly hotter plug which is often done just to reduce fouling of the 2243's caused by high oil consumption. The hotter plug will do a better job of burning off the oil deposits from it's tip.

Here are two sites that will explain this in detail:
http://en.wikipedia.org/wiki/Spark_plug
http://www.gsparkplug.com/shop/spark-plug-heat-range/











Using too cold of a plug results in fouling of the plug and using too hot of a plug can result in spark knock or pre-ignition. That is why the engine and ignition system's condition will determine how the engine performs with the hotter plug.