DIY Engine Repair Part 2 - Spark Plug Thread Repair

It was a beautiful Spring day! Perfect for pulling the bikes out of storage and taking them for a quick shakedown run. And of course, when everything is going well, something must go wrong! :-P

While riding the Bantam, it suddenly lost power (and then had to push it 12 blocks home). I pulled the plug to check for carbon buildup, found nothing, and re-installed it. And then, the dreaded feeling of a threaded part tightening, tightening, and suddenly loosening >_<. I pulled the plug out and found this:

Look closely...

Closeup of the threads... no longer in the head of course

Mangled

I purchased a thread repair kit from my local auto parts store and got to work. First, I used the provided tap to chase the original thread and use it as a guide for the larger thread.

Once the plug hole had been retapped to the correct oversize, I applied the required silicone to the outside of the insert, screwed it onto an old plug, and placed it in the hole.

The final step is to permanently set the insert into the head. The kit comes with a steel punch that spreads the top three threads (which are serrated on the outside). By hammering the punch into the top of the insert, the serrated heads permanently lock into the head alloy material.