After letting the engine cool, I re-examined everything. The insert thing is in fact threaded and I was able to screw it partly back into the hole by hand. Unfortunately, my 13/16 spark plug socket is at work, so I won't be wrenching on my car until Tuesday at the earliest (class on Mon and Wed until late). I also couldn't pull any of the other spark plugs to see if they have inserts or are threaded directly into the head.

I also took the liberty of contacting Pick N Pull for a quote, and I talked to a guy I know last night who wants to buy the car and fix it up for his wife. Of course, he was pretty drunk, so we'll see if he still wants the car when he's sober.

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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.