Found this on another forum:

Len

New Info on Glowplugs Seizing in the Head
As a follow-up to my $2600 glowplug adventure in December (http://mbca.cartama.net/showthread.php?s=&threadid=1528), here's some new info.

(This is especially interesting as both the Service Manager and the regional MBNA service rep told me then that this was NOT a common problem.)

Quote:
"Unfortunately this is a very common issue. Common enough that MBNA recently issued a Service Bulletin ( document no. P-B-8.20/134 dated January 2004) announcing that a repair kit for damaged glow plug threads and/or broken off glow plugs in the cylinder head has been developed. As a result, the replacement of the cylinder head in almost all cases is no longer required.
The repair kit can be ordered as Special Tool, p/n 611 589 00 99 00 and this kit is applicable to diesel engines 601, 602, 603, 604, 605, 606, 611, 612, 613, 628, and 646.

Seizing of glow plugs (also spark plugs) is inevitable when istalled in cylinder head for a long period of time. One technique I know that so far works all the time is by removing the glow plugs when the engine is at operating temperature and using 3/8-inch impact wrench set at very low torque."