As engines and their electronics become more complex, one of the few things left to hobbyists and auto enthusiasts who like a little grease under their fingernails is the ability to change their spark plugs. Although just about every other car repair out there takes a code reader and a college degree to diagnose and fix, spark plugs remain accessible and easy to understand. The first reliable spark plug was invented in 1903 by Oliver Lodge. They're aptly named as well; spark plugs are simply insulated plugs that are screwed into an internal combustion engine's cylinder head to deliver the spark that ignites the mixture of air and fuel in the combustion chamber. Spark plugs also transfer heat away from the combustion chamber. Basically, this is what happens: The spark plug sits at the top of the cylinder head. The piston first travels down the cylinder, drawing in a mixture of fuel and air. The piston then goes back up toward the spark plug, compressing the mixture. At t...