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. (Photograph by Chris ...

The gap between the center and side electrodes needs to be an exact distance across; otherwise, your plugs won't fire efficiently. Remember: Gap new as well as old spark plugs, even if the package ...

For hot rodders, spark plugs are often nothing more than inspection tools that reveal whether an engine is running too rich, too lean, or just right. Street guys look for a nice tannish-brown tint on ...