by Barb and Eric
Please do not use a helicoil. They are not a good idea and I had many bad encounters with them in the USAF in aircraft maintance. The only places they are good in are areas that are virtually never taken apart again. Due to their tendancy to back out with the stud/bolt/sparkplug. On top of that they require two more special tools to install compared to just a tap for the solid inserts. Also the solid inserts and tap can be bought from "Time-Sert" and these inserts have a system to lock them in place just like the 36HP and earlier head steel inserts.
The main thing we would advise is to remove the heads, then you can make sure to tap the holes straight with the old ones. Also, you can ping the lock on the inside of the head so it will never come out if you use either "Never sieze" or graphite on the threads of the plugs.