Happened to me last fall on my 1979 240d. After I flushed and went to tighten I broke off the "head" Ordered a few new drain plug. Was able to drive around a while until the new one came in. I thought it would be tough to remove but leave it to how ever designed it I could get a small head srewdriver in there and remove the old one. Made the swap quick and only lost about a half of pint of coolant./Tom