I just had to do my plug holes .. I had 2 that were almost gone, so i did em all while I was at it anyway.
I used a product called Sav-A-Thread, was really easy to use, took about 30 minutes a hole with the head on.
I used a vacuum tube attached to a shop vac to clean up inside the cylinder.
I searched and saw several videos on how to do it to gain the confidence.
I would definitely do this again without any fear.
End result was way cheaper then head removal.