Off with it's head!
Well, my 1998 E300 is at the shop for a glow plug-ectomy. The #4 plug sheared just below the hex flats when attempting to remove it late last year. The #6 plug was tight enough that I just left it in place.
The #4 plug has about the best access of any of the six - it's rearward of the injection pump and in front of the oil filter. The mechanic has drilled it out until there's essentially nothing left of the plug, but he still can't get it removed. It just spins in place and cannot be pulled out. He was able to successfully remove the #6 plug, but it required his full vocabulary of mechanic words.
Both he and I have the same theory. The glow plugs on the 606 engine are about five inches long - twice as long as those on the 617, or 601/602/603 engines. They are the same diameter - all use a 12mm socket - and appear to be made of the same material. The extra length of the 606 plug gives a lot more surface area for carbon to pack in and hold the plug in place. Given that the plug is held in place more tightly and doesn't appear to be any stronger, the force required to remove it occasionally exceeds the shear strength.
Both the mechanic and I had the same experience - he with #6, me with #3. We completely unthreaded the plug, but were unable to remove it. It was held so tightly in place that it couldn't be pulled, pried, or wiggled out. In both cases 24 hours of soaking with penetrating oil was required to loosen the carbon enough to permit removal of the plug.
Anyways, #4 won't come out, so he's pulling the head on my 52K mile car and sending it to the machine shop. When I get it back it's going up for sale - I've had enough.
More 606 trivia for y'all.