I'm currently doing some tune up work for my father-in-law on his MF35(1963 model). It has the Continental Z134 engine. I pulled a spark plug out today(not sure if they have ever been replaced). On the end of the plug was an outer sleeve that was threaded into the cyl head. It had an approximately 1/4" hole in the end of it. I could see the end of the plug up in the hole. A second plug I removed came out without the other part. Anyone have an idea what these sleeves are? I've pulled lots of plugs in my time and this is the first time I've seen these. I wasn't sure if they were some type adapter or what.
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