MG Car Repair: 1976 mg midget ignition, ohm coil, external resistor

Question
Hello Howard, I have a 1976 mg midget 1500 that had a 45DE4 Distributor with the Lucas/Opus electronic ignition which at some point was changed and now has an external Piranha module mounted on the fender well with the stock coil. My question is I would like to install a Pertronix Ignitor system which looks to be entirely in the Distributor with only two wires going to the coil. I know the stock coil is bad. Can I use a standard 12v coil and wire it off a hot ignition fuse at the box? The 12v coil says "use external ballast". Is the stock ballast alright to use? I'm trying to be careful and not burn up the new ignition system. Thanks

Answer
Hi Greg,
The ignition system determines what coil you use. If your new Pertronix ignitor was for your car then you need to use a original coil which I am sure was a 6v coil that used the resistor wire that was on the car. There was an additional resistor used to power the Lucas Opus ignition system but that would not be used.
12v coils don't use external resistors, there is no reason to. This may be a terminology problem because both the 6v (as they are called) and the 12v coils are used on a 12v car. The "Low Ohm" coil called a 6v coil must use a external resistor because if you connect straight 12 volts to it then it will over heat and burn up.
Use an ohm meter and measure the resistance across the primary of the coil. You should have 1.3 to 1.45 ohms. If it measures around 3 ohms than it is a 12v coil and not what you need. The resistor wire on your car should measure 1.3 to 1.5 ohms.
Howard