Rick:

The story I got from the manual on the 1987 300D oil filter change is that the center bolt is designed to come off and release the center post. The cover and center post together are too long to clear the firewall.

However, the center bolt on my filter cover was stuck tight. Rather than take it apart, I found out that the top of the firewall (actually, its a plastic divider) is easily disassembled. You pull off the two long rubber gaskets that fit against the hood, and then the top firewall piece is held on with four clips. Remove the clips, pick up the firewall top piece, and the oil filter housing top clears.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles