For readers who may not know this:
Common Helicoil problems.
1. Drilling too deep so that the hole goes into the water jacket.
2. Failure to use Bottoming tap in the hole. If not done the last 4 to 5 threads are tapered (smaller in diameter) which causes stud to jam in the hole before it reaches the bottom.
Also a problem on open end holes like the 5/16ths-18 Zenith mounting bolt holes on the one bolt which will break "the ear" off
if bolt is too long.
If bottoming tap is not used the bolt will jam the last 2 -3 threads and rip Helicoil out of the hole.
3.
As mentioned in post #5, use drill blocks for straight holes, guides and other ideas for angled holes can be found here on Google.
https://www.google.com/search?client...rill+guide+jig
click above link