This is a bit of a mystery which has popped up on a few US forums... I hope I am not missing a recent thread here. 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 insulator 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. The erosion status of them all is as-new which is typical for fine wire plugs of that life; they wear extremely slowly. I also have one conventional heavy electrode plug which reads 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 at the higher voltage. 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 expected result of repeated expansion and contraction of the plug assembly if there is no spring pushing against the resistor. 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, and looking at the US data they still are, 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 so this is outrageous. I should add that the engine worked apparently fine, even with the plug which was totally 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. Applied across a stack of differing nonconductive materials, voltage distributes itself inversely proportionally to the relative permittivity of the material and any air pockets thus end up with a very high potential gradient across them, causing them to flash over and gradually carbonise. 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? My feeling is that the reported cracked insulators in turbo SR22s are a separate issue, perhaps connected with the longer reach of these plugs in combination with a high EGT. Regarding the resistors, I fly peak-EGT all the time (non-turbo) and I found all of my plugs to have a massively elevated resistance too.