Mercury - Quicksilver 815407Q5 Distributor Cap Kit V6 GM MerCruiser Engines Thunderbolt IV & V HEI Ignition 815407Q5. Overview. A strong, even spark is what brings your MerCruiser engine to life - and keeps it running smooth all day long. The Quicksilver 815407Q5 Distributor Cap Kit makes sure that spark stays clean, steady, and reliable. Designed for V6 GM-based MerCruiser engines with Thunderbolt IV or V HEI ignition systems, this OEM kit includes both the cap and rotor built to factory spec..