I have to respectfully disagree that a ballast resistor has anything to do with it.
A 6-volt coil gets just as hot as a 12 volt.
I have been involved with antique Harley Davidson motorcycles for years and they use a 6-volt, wasted spark system (1 coil, 1 lead for each cylinder).
The inside of the coil is full of stiff tar and the plug wires cannot be removed when the coil is cold. The recommended procedure is to turn on the ignition and allow the coil to heat for about 5 minutes or so. This moderately softens the tar inside and the wires can be pulled out. New wires are then inserted along with a new wire seal, which is held in placed by threaded packing nuts.
It is not recommended that the coil be allowed to heat for longer than 5 minutes or so because the coil will get too hot. This leads to the tar becoming too thin and when the plug wire is removed the thin tar will enter the cavity where the end of the plug wire is fitted. This then requires a homemade tool (steel tubing with teeth cut into one end) to cut out the tar after the coil has cooled a bit. JMHO anyway.