What is the difference between a 6 volt coil and a 12 volt ignition coil? I am building a model gasoline engine and it calls for a standard 6 volt coil but 6 volt is hard to find. The guy in the parts store told me that there was a magnetic switch in the coil. That is news to me. Any reason why a12 volt coil would not work?

The simplest answer is that a 12v coil is made for 12v systems only and a 6v is made for 6v systems only. If you're having trouble locating a 6v coil possibly look and see what you can find for ignition coils for riding lawn mowers as they usually run on 6v systems.

I believe that the Model A was 6volt....

I searched froogle and this is the first thing that came back: http://store.ford8ntractorparts.com/igncoil6v.html

most fords up to the mid 50's were 6 volt

The traditional 12 volt coil ignition system does not use a full 12 volts to the ignition coil. There is almost always a ballast resistor or a resistor wire built into the coil wire to drop the voltage to approx. 9 volts. Often, using a full 12 volts on the coil may damage distributor components.

A 12 volt coil will work fine in a 6 volt system; I have done just that on several engines.