What we call a gasoline engine is a species within the internal-combustion engine genus that doesn’t actually have to use gasoline (see: propane, alcohol, nitromethane, etc.). What really sets it apart is its ignition system, using electrical spark gaps to light off the fuel-air mixture. Various parts of the spark-ignition engine have changed over time, but one constant, since before the American Civil War, is the spark plug.

That’s not to say spark plugs haven’t changed since 1860. Like most things automotive, they reached their most familiar form in the 1930s and have been continually refined in detail ever since. Because of their long history, there’s a lot of information floating around out there about spark plugs that may or may not be relevant to your classic car.

Plug selection should be tailored to the conditions under which your engine will be run. A car that putters through the occasional parade will demand a much different plug than a similar car called on to haul a load of passengers to a scenic overlook. Our hope with this piece is to arm you with the knowledge so you can do your own research when choosing your next set of spark plugs.

Read more about parts of a plug, size and sealing, cooking and cleaning, deciphering plug color and plug life.