Generally speaking with post-1940s coils, there is no such thing as a "6 volt system" or "12 volt system" coil. That because many systems, regardless if 6 or 12 volt, feed the same voltage to the coil under normal running.
What you need to determine is IF that coil needs an external resistor hooked to it when used on a 12 volt system.
You also need to know if it was designed for a system WITH ignition points, or for a more modern high-energy breakerless system. Such coils will have very low resistance across the primaries; often only 6/10ths of an ohm.
A typical OEM coil on an H that had 6 volts fed directly to it is checked as thus with an ohm-meter.
2.6 ohms across the primaries and 7000 ohms across one primary to the secondary.
Same coil was also used in 12 volt systems along with an external 1.8 to 2.2 ohm resistor.
A coil that has a built-in resistor is only good for use in a 12 volt system. It will measure around 4 ohms between the primaries.
Then you've also got the many "hot spark" coils made for older tractors. Often 1.4 ohms on the primaries and 10,000 ohms between one primary and the secondary. Can make 40,000 volts instead of the standard 20,000, but also can be hard on ignition points.
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Today's Featured Article - An Old-Time Tractor Demonstration - by Kim Pratt. Sam was born in rural Kansas in 1926. His dad was a hard-working farmer and the children worked hard everyday to help ends meet. In the rural area he grew up in, the highlight of the week was Saturday when many people took a break from their work to go to town. It was on one such Saturday in the early 1940's when Sam was 16 years old that he ended up in Dennison, Kansas to watch a demonstration of a new tractor being put on by a local dealer. It was an Allis-Chalmers tractor dealership,
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