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Here's the plug hole in question , have you an idea of why they failed Jim?
Since comnoz looked in, I must ask, do heads ever get so bad they can not be recovered in plug area?
LOL comnoz if that ain't a back peddling cover your ass remark! I was alerted on other forums 15+ yr ago, on cascades of head issues and life, to seek out a decent extra head, just in case, which saved Ms Peel, so hope we will all learn what it will take in this instant case. If there's not enough meat left to put another thread repair insert in, what would you do? Have you not encountered too damaged heads, porous, fracture, repeated weld weakened etc, it was un-recoverable? Melt down to re-cast is not a recovery, but total loss failure. Are my unusual suggestions un-workable?
Could an aluminum or bronze insert be made as Comnoz has said he has made for a smaller plug in the past.
The O.D. of the insert could be made as a 3/8" National Pipe taper.
I think this will allow limited removal of the pulled threads in the spark plug hole.
The striped hole would need to be cleaned up with a tapered reamer to allow the tapered tap to work.
This would give more of a wedged in (Morse taper ,mechanical type fit) engagement.
If the insert was made of hex stock, it would give a place for a socket to tighten it into the hole.
What stands proud of the head in the combustion chamber could be ground off with a dremel type tool.
What is left of the hex stock could also be ground/machined back or a plug with a longer reach used.
Just food for thought.
Anything can be repaired, it is not always equitable or wise,
but with enough money or time you could probably build anything with a welder and a file.....