Ahh yes the newbies, welcome aboard. We need a couple of things. You mentioned its a 300TD. This is a wagon? Correct?. Not a sedan turbo.
Second. We need to know what part you are talking about? is it the suspension strut? or exactly what part are we working on here.
If it is an Allen bolt head then first make sure it is completely clean and the Allen is seated correctly--- meaning deep. Make sure it is the correct METRIC size Allen tool.

Let us know what part were working on and we can give our advise.

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