I got an old Briggs & Stratton stationary engine. In order to check the spark plug I was removing it from the head and I noticed the thread of the spark plug is surprisingly wide (16mm). Since the spark plug was not reliably firing I was buying a replacement (correct part) which is when I realized it is not using the standard 14mm thread since the newly arrived sparkplug was way smaller (14mm thread).
After working a bit with the old spark plug I realized that there is something like an adapter on the spark plug, however it is not the usual kind of spark plug adapter to change threads. It is literally just a "tube" with an inner and outer thread and I see no way how it could possibly be mounted either on the spark plug or the head since there is literally no point one could get leverage to tighten it and no "stop" that attaches to the head.
The parts list does not show any adapter or anything with the spark plug:
The 337 part (spark plug) is a regular 14mm spark plug which will not fit into the head. At first I was assuming I was simply stripping/destroying the thread in the head and pulled it out, but what is left in the cylinder head is still a perfect 16mm thread where the "adapter" was threaded into.
Does anyone have any idea what this could be? I could not find any reference in any diagram and I am really at a loss...