Spark plugs that haven't been changed for a long time can become one with the cylinder head. Fortunately, it's possible to repair damaged threads instead of scrapping the head.

Let's say you've just removed an old spark plug. Does it have any life left in it? Can it be reused if it is cleaned properly? Let's take a closer look.

Heat Range: A spark plug's heat range classifies how insulated the center electrode is from the spark plug threads, and thus, the cylinder head. The heat of combustion warms the spark plug tip ...