Which feature of a spark plug design helps purge lead-containing gases from the plug?

• A. Open firing end.
• B. Narrow electrode gap.
• C. Thick shell.
• D. Low heat tolerance.

Answer: (A)

An open firing end keeps the spark plug tip exposed so combustion gases can circulate around it and sweep away lead deposits. In engines running leaded fuel, lead-containing gases tend to foul the plug tip and insulator. With an open firing end, these gases can flow around the tip and the flame can help burn off or purge the deposits, keeping the firing end clean and reducing fouling.

The other features don’t help with this purge function: a narrow electrode gap mainly affects ignition characteristics, a thick shell is about durability, and low heat tolerance affects operating temperature rather than cleaning the tip.