I hate to say anything, fearing it is likely to curse me. But, after reading about this problem, I go out of my way to give my car an "Italian Tuneup" daily for a week or more before I attempt to take mine out. I have always done it on a cold engine and so far, have replaced all of them in three separate "sessions" without breaking any off. I have applied anti-seize compound for brake components (high temperature rating) each time.

For how much they cost to replace at the dealership, there is no excuse for their short lifetime, or their tendency to break off. This is clearly the result of bad engineering - they are under the intake manifold so changing them is a PIA, or very costly if you have a dealer do it. The fact that they can break off outside the threaded area because they get stuck is just a design flaw. If they broke off inside the threaded area, the removal would be one hell of a lot easier.

I have noticed new cars have lots of relatively meaningless plastic shrouding and the like to cover the engine and make it look cool. That money and effort would be better spent making them maintainable. Most people who actually do something under the hood don't appreciate removing useless parts to gain access to items needing attention or maintenance.

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)