Mercury - 815407T5 Ignition Rotor and Cap Kit V6 GM MerCruiser Engines Thunderbolt IV & V HEI Ignition 815407T5. Overview. When your engine lights up instantly and runs smooth from idle to plane, you're hearing the sound of a healthy ignition system. The Mercury 815407T5 Ignition Rotor and Cap Kit helps keep it that way. Built for marinized GM-based V6 MerCruiser engines with Thunderbolt IV and V HEI ignition systems, this kit restores strong, consistent spark delivery and smooth throttle respon