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.
Talking about spark plugs must, of necessity, be done in generalities. There are a lot of ignition products on the market and many of them are excellent. Nomenclature especially varies from brand to brand. Thus, one maker’s heat ranges may run from hot to cold with the numbers going down as a plug gets colder, while another may do the opposite.
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.
1. Parts of a plug
Viewed externally, the spark plug is a pretty simple device, made of metal and porcelainized ceramic. The ceramic part, called the insulator, is formed from sintered aluminum oxide. The insulator keeps the ignition spark from shorting against the cylinder head. At the upstream end of the insulator, nearest the coil, is the terminal. This is where the spark-plug wire terminates. Various attachment configurations exist. Most modern cars use snap-on wires with rubber boots, but earlier cars used a nut to hold a bare eyelet onto the terminal. The terminal connects through the center of the insulator to the central electrode. Once at the central electrode, current from the coil jumps to the side (aka lateral) electrode. The side electrode is attached to the jacket (aka the case or shell), which is the metal part that screws into the cylinder head.