How would you remove an old helicoil from a sparkplug hole

[GreenV8S]

Johnny G Pipe said:

my theory is that the helicoil is just sitting in wrong with a badly spaced thread somewhere, installed badly by the PO and the plug just screwed in hard and fingers crossed.

I suppose that's plausible. If so, you might be able to detect it by putting a strip of plastic or card down the hole and pushing it hard against the threads to leave a witness.