In the end, hammering the tap and turning it did not work to produce a viable hole that would allow the tap to bite. I spent literally hours doing this.
The spark plug hole is 14mm. But the threaded hole for the helicoil would be 18mm.
The well shaft leading the the spark plug hole on the Hyundai is about 6-7" deep and just under 1 inch wide.
The Drill Bit:
I did an online search for a chart that listed hole sizes for various sizes of taps. For the 18mm insert, a traditional tap would require a 16mm (0.63 inches) hole. Since the main part of the Lisle tool is 16mm, I purchased a long 16mm metal drilling bit.
Given that hole was 0.37 inches wider than the drill bit, it required me to fabricate or find a bushing to keep the drill bit from going at an angle. To do this, I took a one inch section of metal pipe, slit it down the long way, and compressed it in a vice until it overlapped in the slit. Then I wrapped tape around it until it could slide into the spark plug whole without wiggling.
Preparing the drill bit:
The flutes of the drillbit were well greased in preparation for the procedure. This will produce a lot of shavings. Better to trap them in grease.
How to drill with a drill bit that big:
The drill bit is bigger than most chucks will accommodate. The bit I purchased also was a smooth circle at the top instead of a step down shank or a shank with flat sides. I did not have a chuck large enough, so I used a hammer to get the the drill bit to bite. Then I used a pipe wrench (very important you put a bushing in the hole before doing this) to turn the drill bit. After a while, I bored out the entire hole.
Be careful doing this. At the end of the procedure the drill bit will want to fall into the cylinder. Be prepared for that and have an extra set of hands be ready to grab the bit before it does so.
After boring out the hole. I was able to use the Lisle tool like a normal 18mm tap without any issue. I simply hammered it a couple taps to get it started, then threaded the hole (making sure to back up and go forward throughout the process to prevent binding).