Long distance discharge gap type spark plug

In a long distance discharge gap type spark plug comprising a center electrode, a porcelain insulator surrounding said center electrode and provided at its lower side with a firing portion composed of a reduced diameter trunk wall made integral with said porcelain insulator, said firing portion extending up to near the lower end of said center electrode and exposing said lower end, and a grounded electrode opposed through said trunk wall to said center electrode, the improvement comprising a surface gap L formed between the front end of said center electrode and said grounded electrode and given by

• 1.0 mm .ltoreq. L .ltoreq. 4.5 mm,
• a minor air gap l formed between the outer peripheral surface of said firing portion and said grounded electrode and given by
• 0 .ltoreq. l .ltoreq. 0.6 mm, and
• a thickness t of the trunk wall of said firing portion given by
• 0.5 mm .ltoreq. t .ltoreq. 0.9 mm,
• said surface gap L being determined within a range that satisfies the following conditions, i.e.
• L .ltoreq. 5 t + 2.0 mm when 0.4 mm <
• l .ltoreq. 0.6 mm,
• L .ltoreq. 5 t when 0.2 mm <
• l .ltoreq. 0.4 mm,
• L .ltoreq. 5 t - 1.5 mm when 0 <
• l .ltoreq. 0.2 mm, and
• L .ltoreq. 5 t - 2.5 mm when l = 0.