This probably won't be too helpful but are you sure that you have the correct spark plugs?

Do you have a Dremel or tiny file that you could use to clean the edge of the threads up by hand?

The 16mm nut is the best way to go if you can do it.

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1987 300SDL 167k
1992 Volvo 740 140k
1990 Volvo 740 250k
1989 Volvo 240 269k

Anyone want to trade an old Volvo for an '87 300sdl?