Fab:

You have the M103 motor in your car - same as mine.

13mm wrench or socket gets the oil drain plug out. There may be a large plastic encapsulation panel underneath the car that will also have to come out first in order to expose the pan area. The 126 models with the M103 motor use 4 metal type screws with an 8mm head to secure this panel. I seem to recall looking at the photos of the underside of a 124 car(your model) and there were maybe 6 or 8 screws holding the panel on.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"