8mm Heli-Core Pre-Made Assy. HEI 90 Plug / 90 Dist.8mm Heli-Core Ignition Wires The MSD Heli-Core Ignition Wires are perfect to replace factory wire sets. The wires are 8mm and can deliver all the high voltage your engine needs! The Heli-Core wires feature a stainless steel conductor that is wound around a special glass core. This wire will not interfere with other electronic devices on your boat. Heli-Core Wires also prevent a phenomenon known as Inductive crossfire. When two wires are routed close to each other, happen to follow the firing order of the engine the spark from one wire could be jump into the other wire. This would cause the other cylinder to ignite far in advanced and will cause detonation. All of the MSD spark plug terminals feature “Dual Crimp" terminals, guaranteed so that every factory crimped plug wire end will not “pull off" for up to FIVE years!