Harvey,

Where do you live? If there is a Sears nearby they have 13mm Allen wrenches, as well as every other size up to about 22mm. I still have my original Snap-on 14mm unit from 1976 when they really were hard to find.

I second the recommendation to verify the size after cleaning out the hex hole in the plug. I have yet to see anything but a 14 mm hex socket on these or the transmission. But I have not tackled an E300D yet either. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)