There's two things that must be correct with spark plugs. One is the thread size and the other is the heat range. There's only two common thread sizes for cars and trucks so 50 percent of the world's spark plugs will fit in that regard. That means the store you bought the new ones from only has to stock two part numbers, and you know they have a lot more than that.
The heat range refers to how quickly the center electrode dissipates the heat that builds up on it. If the plug is "too cold", carbon will form on it and not burn off. Soon the engine will start hard and it will develop misfires and poor fuel mileage. If the plugs are too hot, the electrodes will burn away. Once they melt off enough you'll again have misfires and a no-start condition.
A hotter spark plug is longer too so the heat has to travel farther to the threads, and in a lot of cases the piston will hit the outer electrode and close up the gap. If you're lucky all that will happen is the spark will be shorted out and the engine won't start. In severe cases a piston can crack, but usually the engine will just lock up first and won't crank.
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Friday, June 28th, 2013 AT 10:25 PM