If this is an Aluminum Cylinder Head it is common to break the Hex Head off of the Glow Plugs when trying to remove them.

There is some suggestion to reduce the chances of that happening:
Repair Links
Fast navigation http://www.peachparts.com/shopforum/diy-links-parts-category/146034-fast-navigation-do-yourself-links.html

http://www.peachparts.com/shopforum/forumdisplay.php?f=82
http://www.peachparts.com/Wikka/DoItYourSelf

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84 300D, 82 Volvo 244Gl Diesel