Mercury - Quicksilver Distributor Cap For Marinized GM V8 MerCruiser Engines with Thunderbolt IV & V Ignition 805759Q01. Overview. A strong, clean spark starts with a reliable connection. The Mercury Marine 805759Q01 Quicksilver Distributor Cap is built specifically for V8 MerCruiser engines made by General Motors using Thunderbolt IV and V HEI ignition systems. Made from a high-strength composite blend, it's engineered to resist heat, vibration, and corrosion - all while delivering consistent e