I went to the parts store today and bought a helicoil. It was way thinner than the thing that popped out with the spark plug. I showed the original to the guys at NAPA and they'd never seen one. They suggested a local shop which isn't convenient for me. I might try the parts manager where I work. As an experiment, I threaded the old insert into the old hole. It made it in about two threads worth and still had a little wiggle after I ran the engine for 30-60 seconds. It didn't completely seal the cylinder. I don't like the car enough to pull the head, in fact I've been trying to get rid of it for some time. I checked with PNP to see what they'd pay, and while I was pleasantly surprised with the answer, I decided to try craigslist one more time for a little more money.

The JB Weld thing is worth considering (I had actually thought of it and dismissed it), but I don't know how long it would last. I JB Welded a cracked exhaust manifold once, and got a little short of 500 miles out of it. BTW, the speedometer cable broke about a week before this, adding to the list of things I'd need to fix in order to keep this car as my winter beater.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.