This is a bit of a mystery which has popped up on a few US forums...
Cirrus (turbo) owners have been reporting a high failure rate (due to cracked insulators) of fine wire plugs, especially. One example is
here.
Somehow (I cannot find the reference) people have decided the resistance should be about 3k. Support for that may be e.g.
this. The traditional manufacturer of these plugs, Champion, does not publish a value.
A new mfg, Tempest, does, and is making the best of Champion's apparent misfortune.
My fine wire plugs (RHM38S) are shorter thread than the Cirrus ones and I have not had any failures, but I have just measured about a dozen of some 700hr ones and all but 1 read open circuit (>20Mohms) with a normal DVM, with 1 reading a few hundred k. These plugs were installed in 2003 and removed in 2008.
I also had one heavy electrode plug which read a few k.
But I've just re-measured all the fine wire plugs with a 1000V insulation tester and except one (which remains open) they read completely differently. None were over 1M, and this was true at 250V, 500V and 1000V, though the three voltages produced slightly different resistance values.
So clearly the "resistor" is not a resistor as we know them. Whether this is intentional, who knows? I suspect it's a cockup. Maybe there is a tiny air gap at the ends, of the order of 0.01mm, which flashes over easily - the result of expansion and contraction of the plug assembly. But this is clearly not intended.
It could explain all kinds of weird engine behaviour in the field, because all these plugs test OK in a spark plug tester.
The plugs I tested were c. 10 years old. So unless there is some good explanation for this, it appears that Champion were making rubbish spark plugs for perhaps a decade or more, getting away with it only because a magneto puts out enough kilovolts to flash over the resistor, or flash over any air gap at the ends of it. And fine wire plugs are ~ $100 each.
I should add that the engine worked apparently fine, even with the plug which was open circuit at 1000V. No high altitude issues either.
Many years ago I used to work in high voltage (up to 100kV) and one quickly learns that any air gap leads to a rapid degradation of the component. Voltage distributes itself inversely proportionally to the relative permittivity of the material and any air pockets end up with a very high potential gradient across them. Having a "loose" resistor inside a spark plug is a stupid idea.
I need to find some unused Champion fine wire plugs plugs and test those... any volunteers?