The carbon rotor button in a stock HEI distributor cap has very high resistance. When the high voltage of an MSD 6, 7, or 8 Series ignition is added, this resistance builds up heat and can actually melt the distributor cap. To solve this problem, install one of these MSD low-resistance HEI bushings, which will pass the secondary voltage from the distributor cap to the rotor without the excessive heat buildup.