Wayne...careful, you may get more questions about "why would you want to change to a 12v system when a 6v works just fine" rather than answers on how to do it.
But....kenparker's information is pretty much right on target. You may want to just substitute a 12v coil for a 6v one. Most 6v coils I found available locally had an internal resistor that allowed only 6v to get to the primary windings. Like the externally mounted resistors, these cause a lot of heat. Normally, not an excessive amount of heat, but enough...I don't like 'em.
The only thing kenparker left out was replacing the oriignal Bendix drive with a modern one...that alleviates the broken spring problem which can occur with the original Bendix. They're easy to install and pretty much a one-time thing. The original springs have a nasty habit of breaking when the starter is hit with a 12v battery...and the broken spring parts usually fall down inside the flywheel housing and jamb up the flywheel and starter, which pretty much requires major surgery to remove. You can try to fish the parts out with a magnet...or just save all the drama and headaches by replacing the oriignal Bendix with a modern one in the first place.
Also...Les Andrew's book,
Model A Mechanics, Vol II, has a complete step-by-step process to convert to a 12v system. That's what I used and had no problems whatsoever. You can buy both volumes through MAFCA's website, or most of the parts houses carry them, too.
Hope this helps, Wayne...
