If you get the old Glow Plugs out and use a Never-seize type compound on the threads of the new Glow Plugs you should not have any of the corrosion issues that cause the originals to stick.

Also I believe People have said the originals that came with the Engine were Beru Glow Plugs. If that is true they are the ones that are snapping off.

The other 2 issues that cause the Glow Plugs to get stuck are Carbon Buildup around the tip likely caused by worn Injector Nozzles or low Compression and Carbon getting past the sealing Shoulder on the Glow Plugs.

No one has said what causes the Carbon to get past the sealing shoulder area. Guessing Plugs not tightened properly or their was some warping in the Cylinder head that causes it. Not unlike the warping that causes precombustion chamber sealing area leaks.

None of the above problems are actually caued by the Glow Plugs themselves.

You might try UK eBay for the Berus.

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84 300D, 82 Volvo 244Gl Diesel