The coil sounds abnormally hot to me, but it's your call. Add a ballast resistor now for $10 - $15, or keep running it and see when and if the points or the coil fail, and then add a ballast resistor or a true 12 Volt coil, and replace the points. As a side note to the hot coil, several years ago someone brought an old Chevy truck over to our shop. They had converted it to 12 Volts without a ballast resistor or a 12 Volt coil. A few minutes after they had dropped the truck off, we heard a BANG outside the shop, almost like a shotgun blast. We could not figure out what the noise was until a day or two later when we went to start the truck to drive it into the shop, and it would not start. Upon further examination, the top of the coil was blown out of the metal can, and there was oil splattered all over. Apparently, when they dropped off the truck, they stalled it and left the key on. They coil overheated, and boiled the oil inside, blowing the top off. I have seen this one other time, on an IH truck with the 404 motor. Apparently, the key was left on, and the electronic ignition box shorted out, causing high current flow through the coil, until it also blew it's lid! The ammeter does not care if the system is 6 Volt or 12 Volt, but a determining factor in choosing an ammeter would be your charging system... Do you have a 60 or 70 amp 12 Volt alternator, or a 30 amp 12 Volt generator? You need to select an ammeter with the full scale reading compatible with the charging amps. Of course, the ammeter will seldom read the full charging rate unless the battery is unusually drained upon startup.
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