Chapron67,
The originals are a hard bakelite material and they do sit on plug wobbily and loose but the metal connector inside can be tightened a bit with a very small needle nose pliers to stay on the spark plug end. My opinion on this is to use modern silicone wires with rubber boots. These are better and less likely fall off or have spark jump from wire to engine block.. I also agree with Gord on checking plugs tightness after using car a couple of times. I always tighten tight and I have noticed over the years a few loosened up and you wonder "How did that happen"??? Alot of vibration and shaking going on back there in that Hemi-Amphi 43hp motor...
Cigarman