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 - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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