Did you happen to rotate the engine backwards at any time when you were working on it? That can slacken the timing chain tensioner. I also had an E320 and I can see how some debris could fall in but how did the debris get in to the spark plug cavity in the first place?
Did you check all of the old plugs to see if anything broke off of any of them?

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.