Looking at this job on my 560SL (same engine manual for the 420 and 560). The manual calls for the oil pan and oil pump to be removed. I'm not sure why. All my exploded view diagrams show no interference. But the manual does call for it. On the drivers side of the timing cover is the oil pump chain tensoiner. This may be hung up on the chain preventing it from releasing.
I would drop the oil pan and see if this is where its hanging up.

Do you have the service manual for the engine? If not give me your email and I will forward the PDF files for the whole job. It will be 8 jobs so it will be a large file.

Pictures and info on this job would really be appreciated at least by me. I'm looking at replacing the upper and lower chain guides as well as the chain.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car