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A friend damaged the spark-plug threads on his cylinder head when he was REMOVING the top #4 plug.  It was so badly damaged that it took us 1.5 hrs with a thread chaser to completely repair it.  Have any of you experienced such a situation?  We’re struggling to come up with a plausible explanation.  These are Rotax plugs and the engine has less than 200 hrs on it.  He last removed them during his CI last September, so not like they had never been removed.  We’re stumped. 

  • Re: Damaging Spark-Plug Threads on Removal

    by » one month ago


    We always use an anti-seize paste on the threads of the spark plugs.

    This prevents any problems like what you are describing. 


  • Re: Damaging Spark-Plug Threads on Removal

    by » one month ago


    The damage may have been done when the plug was installed last time. The aluminum threads in the head are sensitive to over tightening.  Rotax is quite specific about torquing to “16 NM on a cold engine”.  Also I have seen the threads damaged when screwing in a compression test tool adapter.  Sometimes the threads on the tool are not protected when stored and can get dings in the threads, which then damage the threads in the head when used.  

    You have to be very careful with these tools. There should be essentially no effort when threading in the compression test tool adapter, if there is, stop. The adapters with a built in t-handle are the most dangerous, and it’s easy to damage the threads with them.  Also, adapters are often made of steel so the threads are harder than the threads in the head.  I have purchased at least two threaded adapters that were damaged right out of the box and not suitable for use.  


  • Re: Damaging Spark-Plug Threads on Removal

    by » one month ago


    Hi all

    For sure never use anti-seize on a Rotax head, it is expressly called out to use silicone heat transfer paste.  The heat paste acts as a lubricant as well as improves the heat dissipation,  The most common issue is someone torquing the spark plug into a hot cylinder head.  If it was installed without the heat paste then it may be that is the problem.

    Cheers


  • Re: Damaging Spark-Plug Threads on Removal

    by » one month ago


    Thanks for the responses, guys.  He definitely uses heat paste and a torque wrench and installs them in a cold engine.

    Jeff, you probably saved my butt with your response.  We both have the T-handle adapters.  I went to the hangar to take a look at my threads, as I’ll be starting my 400~hr inspection next week.  Guess what I did as soon as I picked it up?  Yup, I dropped it on the hangar floor.  Had you not made your comment, I doubt I would have thought to look at the threads before screwing it into the cylinder head.  And sure enough, the thread is damaged!

    Back to my friend, he’ll take a look at his adapter when he can, but didn’t recall having any difficulty screwing it into the cylinder, but at this point can’t rule out any possibilities. 


  • Re: Damaging Spark-Plug Threads on Removal

    by » one month ago


    Bob,

    You are not alone.  I almost damaged my threads with a new one right out of the box. I started it and thought it’s a bit rough going in (hey, I’ll just try one more turn), fortunately I stopped. Then I ordered a brass one thinking that would be safer, and it was also damaged.  I finally got a thread chaser for spark plugs and used it on my adapters, then finished by polishing the threads with 1000 grit sand paper.  They go in very smooth now and I put caps over the threads when not in use.  

    I really like this particular thread chaser for the heads, it cleans when backing out only, so you don’t drive debris into the cylinder.

    https://www.amazon.com/Powerbuilt-641148-12mm-Thread-Repair/dp/B001SOHXLQ?th=1

     


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