I have been in the old farm tractor business for many years and now restoreing old trucks. I agree, when a 12 volt battery is replacing a six volt system the six volt coil still can be used with the use of a resistor. this is done to reduce the voltage down to four to six volts. Even the twelve volt systems including the coils marked 12 volt almost always need the voltage reduced to four to six volts. Many 12 volt coils have a resister included in them. some do not and thats when you run into trouble. The advantage is that with a twelve volt battery the spark or current to the coil is always at it's consistant level and not reduced when turning the engine over with a six volt battery. If the weather is cold ot the battery is not up to parr then the ing. system is robbed of the proper level of current to the coil causing a weak spark to the spark plugs. Weak spark results in starting trouble. I as a pratice almost always use a resistor to any twelve volt upgrade.
