Cold Type Iridium Long-Life Spark Plug (3403) by Denso®. Iridium Long Life Plug – Patented Iridium Alloy Plug (90% Ir) with Platinum Ground Strap. Denso’s patented Iridium alloy is more durable than even Platinum, allowing the center electrode to be shrunk to only 0.7 mm in diameter. This produces a dramatic improvement in ignitability and spark plug life.