Steve,

Do the plugs you got look like the one in the photo below?



If they do all I can say is I put those in my 1987 420 SEL and the car after a short time seemed down on power. I pulled the plugs to check their condition. Well one of the plugs had one of the four electrodes broken or burned off. It was jambed between the insulator and the inside wall of the plulg. Take about luck. If that metal had hit the top of the piston I think that would have the potential for being very bad and very expensive.

Moral of the story. I always use the plug with the part number on it that is recommened by the manufacturer.