Spark Plug Helicoil Thread Insert Failed. Now what?

Nov 12, 2023
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I have an 08 outback 2.5L standard. Initially a spark plug blew, stripping the threads in the cylinder with it. I was out of town on the road and shops/auto parts stores were closed, so I wasnt able to get a proper thread tap/helicoil kit. A couple of nice locals graciously lended their hand and time and took me to their shop at home and put in a new thread insert and got me back in the road at 2am. We drilled it out then put the thread insert directly in. Fast forward now a couple weeks later, the thread insert and the spark plug blew upon start up running errands. I had a mechanic friend come over and help put in a helicoil kit but didnt think it through all the way till the time came. When we re-tapped with the tap from the kit, it seems the hole diameter is to big? Put the thread insert in, let the loctite sit for an hour and when I was putting the plug in, i guess the insert didnt grab, and the plug just continues to spin never tightening in place. Whats the next step here? I read something abt “big-serts” and “keen-serts” but those examples I found werent specific to Subaru. Thanks for any help!

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