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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: Head parts description
https://www.ereplacementparts.com/briggs-and-stratton-060301942123-engine-parts-c-16758_17347_237407_237480.html?page=2

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...

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    I'm wondering if it is a solid thread repair, something like a Heli-coil, only solid? Jun 15, 2022 at 19:32
  • It looks kinda similar, I thought I made some photos in the shop but forgot, I will make some tomorrow and attach them, maybe this is also helpful Jun 15, 2022 at 19:53
  • I just watched a video, I think it is actually just a helicoil, but just so old that it by now appears solid, I never heard of that before! Jun 15, 2022 at 19:59
  • @Pᴀᴜʟsᴛᴇʀ2 you can add this as a respons, I was watching some more videos and checked some products, it is defintely a helicoil, everything matches! I had no idea such a product exists, you learn something new every day :) Jun 15, 2022 at 20:44

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It very well could be a Heli-Coil type thread repair insert for the spark plug hole. If the threads become damaged or stripped, you cut a new, larger thread of the same count, put the insert in, then you're back to your normal size.

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  • Just wanted to quickly confirm that it really was that. I ordered a helicoil kit from China and the engine works perfect again :) Jun 25, 2022 at 14:24
  • @YanickSalzmann - Glad to hear it worked out for you! Thanks for the follow-up :o) Jun 25, 2022 at 14:35

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