I had an adapter ring on both of mine.(that I remember, one definately did)...I do have two different ones on the two cars....did you take a rough measurement of what you do have yet? if I can find my old block plug I can measure it tonight....see if yours needs the adapter or not.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche