I've seen this problem before on #8 spark plug. Some time in past #8 plug wasn't properly tighten. Over time it started backing out. The exposed threads filled with hard carbon buildup to the point the plug will no longer screw all the way in. This can easily fixed by running spark plug thread chaser down the hole to clear out the carbon buildup from the lower threads. Pack the chaser flutes with dielectric tuneup grease to catch the debris from falling into the cylinder. Install a new set of spark plugs and you should be all set. Be sure thread chaser you get will fit down the spark plug hole along with a socket and extension.
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