Mercury - Cap and Rotor Kit For GM V8 MerCruiser Engines with Thunderbolt IV & V Systems 805759T3. Overview. A strong, reliable spark is the backbone of every good running engine. The Mercury Marine 805759T3 Cap and Rotor Kit restores smooth ignition performance to your GM-based V8 MerCruiser equipped with Thunderbolt IV or V HEI ignition systems. Built with durable marine-grade materials, this kit is designed to handle the heat, vibration, and humidity that come with boating. With precision-mol