Repairing Spark Plug Threads

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The Story

I bought a used 2007 accord from the local dealership a little less than a year ago. So far it's been a pretty good car, getting almost 30 mpg and not really giving me any problems until this week. This week on the way to work it decided to blow a spark plug completely out of the engine. So I got new spark plugs, but when I went to put in the one that had blown out I found there were no threads for the plug to screw into. Now a mechanic or the dealership would probably charge $800-$1000 to take the head off, rethread the whole and put it back together. I don't know about you, but to me, that's a good chunk-o-change. So I started researching how I could repair this myself.

NOTE Trying this is DANGEROUS. Any metal shavings left in the cylinder can cause catastrophic engine failure. That would turn a $1000 mechanic repair into a $5000+ engine replacement or worse. I do NOT recommend you try this at home.

The Fix

After a good amount of research I discovered a product made by HeliCoil made specifically for repairing stripped spark plug holes. The M14 x 1.25 kit for my accord ran me just over $30 at Oreilly Auto Parts. It comes with a tap to cut new threads in the stripped out hole. Once the hole is tapped you use the provided insert matched to the length of your spark plug, screw the insert in, and use the included expander tool to hammer little spikes out at the top of the insert to keep it in place. This type of repair is intended to be done with the header taken off the engine so that you can clean out any metal shavings from tapping before running the engine. However taking the header off requires more work than I'm comfortable doing in my home garage. Here's a few tips I used to complete this repair without taking my header off.

  1. When tapping the hole apply thick grease to the tap to catch the shavings. I used White Lithium grease and applied it several times. I applied grease, turned the tap 4 turns, backed it out, removed the shavings, and repeated until the hole was tapped.
  2. After tapping the hole I taped a piece of 1/4 inch hose to the end of a shop vac and stuck it inside the cylinder to try and suck up any loos shavings that may have escaped the grease.
  3. My spark plug hole is recessed down into the engine so I had to thread the insert onto a spark plug in order to screw it into the newly threaded hole. I had to be careful not to over tighten the spark plug and insert into place, which would cause the insert to come out when trying to remove the spark plug. I also loosened the spark plug by turning the ratchet counterclockwise very quickly.

I've driven the car over 500 miles after completing the repair. The check engine light went off after a couple cycles of turning the car on and off and everything appears to be running normally for the moment. I'll update after more time has passed.

Once again, attempting this repair in this manner is very dangerous and can completely ruin your engine or worse. I don't recommend anyone try it.

[UPDATE] Over a year and several thousand miles later and I still have not had any real issues. I did get an engine error code related to the catalytic converters, but I think that was more from pumping unburned fuel through that cylinder. I replace my converters and haven't seen an engine code since. I don't go easy on the car either.

Enjoy and feel free to report any problems or suggestions to