NGK ITR4A15 Spark Plug

NGK's laser iridium in the ITR4A15 number. The iridium tip is the reason to spend more: a fine-wire iridium electrode is roughly six times harder than platinum, so the gap stays where it was set through a long service interval instead of eroding open and asking the coil for more voltage every hour.

• NGK part ITR4A15, stock number 5599
• Iridium center electrode tip on a copper core
• 5000 ohm resistor construction, standard ground configuration
• Pure alumina ceramic insulator
• Available as a single plug or a four pack
• Replaces Mercury 33-8M2018369, 33-884019001 and 33-8M0204739

Set torque with a wrench, not by feel. Iridium plugs are expensive and an over-torqued plug in an aluminum head turns a tune-up into a helicoil job.