This plug has rust around the shoulders and can not get a socket on it. The center blew out when we attempted to start the motor. We have sprayed on blaster and can not break it free of the rust. We need help. Thanks.
You know what I’d do in that case? As a DIY’er I mean. I wouldn’t even try to remove it myself. I’d take the truck – tow it if you have to – to an auto machine shop. They’ll be able remove it no problem, and much less likely to damage something else in the process. I think you’ll be many $$$ ahead by letting a machine shop solve this for you. It won’t cost much.
Do you have access to an oxy/acetylene torch?
If so, heat the spark plug up until red. Then let everything cool back down.
The head will absorb most of the heat while plug cools down. And this thermal shock will allow removal of the spark plug.
If you don’t have access to an oxy/acetylene torch, then use a propane torch to heat the plug.
Only, it’'ll take forever.
That’s why an oxy/acetylene outfit is called the Blue Wrench.
Tester
Since the center has blown out, a properly-sized screw extractor may do the trick.
A 21 mm socket is a little bigger than a 13/16 so I might try that. Who am I kidding, I am a cheapskate, I would take a 13/16 deep socket and hammer it over the nut.
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Yeah, I’ve done that before too. You do what you gotta do 
If you can manage to get something to bite the stuck sparkplug housing, an impact wrench might help free it. It’s much better at freeing stuck stuff than just a socket.
Something like the attached tool might help bite the housing.
