Spark Plug 411

William C. "Bill" Anderson offers some advice on cleaning and caring for spark plugs and oil for old cars.
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Spark Plug Low Down: Useful Maintenance Tips

  • When cleaning the spark plug recess prior to plug removal, a vacuum cleaner can be used if you don’t have access to compressed air.
  • Plastic food wrapper tabs can be used to mark plug wires if you don’t have access to a cable number set. Write the cylinder number on the tab and attach it to the plug wire.
  • Always change spark plugs in aluminum cylinder heads when the engine is cold. Removing plugs from a hot aluminum head runs the risk of damaging the threads.
  • Use die-electric grease for spark plug boots.
  • *Some say cleaning the plugs is questionable as it may wear the insulator and alter the heat range. The solution is replacing dirty spark plugs with new ones.

*While the practice of cleaning a spark plug might not apply to high-energy ignition systems introduced in the mid-1970s, it remains good practice for cars with six-volt systems.

The typical voltage at the spark plug in six-volt systems is not sufficient to efficiently clean the plug, unless the car was driven under high-speed conditions to bring the plug to the upper part of its temperature range. Cleaning with a spark plug cleaner (a small media blaster) was the standard practice recommended by plug manufacturers in the days before high plug voltage eliminated its need. The limited exposure to the cleaning media will not alter the heat range.

In addition to having a clean plug, the condition of the center electrode is key. It should have sharp edges. Typically, this condition can be obtained by a few passes with the point file. Sharp electrode edges, along with the proper point gap, encourage good spark propagation in low-energy ignition systems.

William C. “Bill” Anderson, P.E., has been involved with the automotive hobby for more than 30 years with experience ranging from hot rods, to sports cars, to sports car racing, and to restoration of vehicles from the 1930s through the `80s. He is an author, magazine editor, car show judge, and professional engineer. A member of several car clubs and a leader in some, through Anderson Automotive Enterprises he restores and appraises cars.

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