I haven't read Greg's posting in the Diesel section but, I didn't have a problem with opening the block drain. It is located on the passenger side of the block. I don't remember what size it is. Removing this plug allows for all the coolant to be drained. With this plug removed and the radiator plug closed, I ran some clear water through the expansion tank and watched the water coming out of the block drain. Once it ran clear, I put the drain plug back.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member