One purpose of the bellypan is to catch drips of oil from all the usual sources, including the oil pan. The dirty pan is proof that it works and the diaper layer was all rotted out so I removed it. The 124 diesel uses the thermo flap on the drivers side to regulate the air temp in the engine compartment, maybe this has to more usefulness in really cold weather?

I believe another function of the pan is to improve aerodynamics
I use a Topsider but once a year I take off the bellypan when changing antifreeze and at that time do an inspection. I've not had trouble w/ lining up screws in their holes but the wrong screw can spoil your morning. I finally bought some of the proper screws, they are an MB special part.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!