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That's the coil that is faulty, it just should not track HT current to anything.

Once it establishes a "track" for the voltage to travel you won't be able to correct it.

There was either a pre-existing fault in the coil insulation or it has been caused by mechanical fracture.

Resistance in the following HT system wouldn't help it but also would not cause it.
 

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It's certainly strange that so many coils are giving you the same observed fault.

Basic check, measure the depth of HT post internal on the coil, then compare it to how far you are locating the lead end into there during installation. They have to be right to the bottom to get the HT metal well away from the outermost tip of the insulation tower, which is why the tower extends that far. I pull the rubber cover away, insert the lead all the way in first and then place the cover using silicone grease to push as far down as possible. Make sure your HT king lead has the coil specific end placed there as they can differ end to end from the distributor mount on some manufacturers. I prefer the straight lead approach for the leads i use as opposed to the 90 degree type for coil end.

My experience has been that once a carbon track is clearly observable, then it's unusual to recover from that status. I never routinely replace these components unless I can prove failure so not prone to swapping out usually but will admit defeat if this condition is clear.

I run with resisted carbon silicone leads, plus resisted plugs (NGK BPR6ES) reliably with zero external tracking of HT power. Have done on these engines for more that 25 years, confident it's not resistance in the leads that's the origins of this problem.

Plug gaps, I run them on the absolute minimum of their specification and never open them wider.

The problem you describe is unusual because when the coil is fired to produce HT output it's the low tension "negative" coil terminal that's being switched down (so no earth present) to force the coil to spark. This is done by having the positive 12v supply to coil from ignition switch (this is how you turn the ignition on and off) with the ignition amplifier controlling the earth line via a a heavy duty transistor. The earth line switched down forces the accumulation of volts in the high tension side of coil to exit via nearest earth but this post has no earth during the HT fire event, making the exit route easiest through the leads.
It may appear to go to the earth terminal but I'd suspect it may "daisy chain" out to the coil's casing which is earthed.

Anyway, worth exploring your HT lead fitment to appreciate if there's anything to be gained there, then let us know what you've got.
 

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Certainly a bit of a puzzler ;)

Immediate question springs to mind though, you've not got the coil wired in it's opposite orientation by any chance? To check, you should have 12v positive feed to the + terminal with ignition on and not running.

The amplifier switches the - negative line to make the coil function. This often confuses initial overview of the system (in other words, how can switching a negative do anything) to any positive effect.

I think it will work when wired backwards, but it's not designed to do so. Worthwhile checking with voltmeter to establish what you have.
 
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