There has been only a few members with 617 Engines that use the pencil glow plugs that have ever had stuck ones.

Your's sounds worse then theirs. Rusted Threads and or carbon got past the sealing area and filled the cavity with carbon and the carbon later hardens.

If it breaks off the extraction will be similar to extracting a broken bolt but the good news is the Glow plugs are already drilled down the center. One of the keys to successfully using an easy out is getting proper sized hole drilled down the center and using the proper sized easy out.

The thread size for the glow plugs is M12-1.25

Note some have gotten the plugs loose but found the tip of the Glow Plug was swollen.

In the attaché picture is a rounded bolt head extractor. It has to be small enough inside so that it pounds onto the glow plug and grips it when you turn it.

No one I have read of someone has done this but one member suggested heating the Glow plug part sticking out till it is red hot and letting it cool some before trying to turn it out. The thought is that the expansion and later contraction make it looser in the bore. Your head is Iron so it is possible.

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84 300D, 82 Volvo 244Gl Diesel

Note use some never-size type compound on the threads when you re-install the glow plugs.

Note that the glow plugs seal on the angled shoulder on the body of the glow plug. What the means is if there if there is sufficient threads to tighten them enough even if the threads got dinged a little it can still seal.

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84 300D, 82 Volvo 244Gl Diesel

Got it!

I bought a bolt extractor set from Home Depot. I was skeptical that it would work, but it worked immediately.
Once I got the glow plug out, there was so much carbon I had to clean the reamer off 3 times.
Thanks everyone! This job is finally done.

Wow, glad it worked. I never had those bolt extractors when I was a Mechanic. Not that I have them I used one time and it worked. Sometimes the one time you use a tool it pays for it self by getting the job done and over with and you can relax.

Story see attached picture: I bought the pictured wrench at some cheapie tool store for $2-$4 and never used it got shuffled from place to place in my several tool boxes.
A water pipe down and inside of a wall rusted through and was leaking badly. I had to cut the hole through a calamite as the shower was on the other side and I did not want to tear up the shower side.

So there was tight and limited space and none of the pipe wrenches including a Chain Type pipe wrench nor Vicegrips worked.

The pictured wrench was the only pipe wrench I had not tried and the only one that there was enough space for it to grip onto the pipe and turn it. That is the only thing I have ever used that wrench for but without it working I could not have turned the Water back on in my House. I would have to have torn up more of the wall to get the other wrenches onto the pipe.

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84 300D, 82 Volvo 244Gl Diesel

If the stuck plug works leave it and soak it for a loooong time...for a month everyday when the engine is warm. Let it cool and wick in. PB is good but I've had better results IMO with KROIL.

If you can get an impact on it you may increase the chances on getting it out if it's coming out; however, the impact may break it off. Give it some light impact tightening too.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer