I think a lot of you lot are a bit anal,, i've fitted successfully dozens of helicoils to cars and bikes over the years
and never had any problems with them. If you are careful and stake the top they work fine.
Just my 2c of course.
I agree inserts are nicer but i've not got any.
Helicoils are by nature not oil or pressure tight so my general rule is solid inserts when oil tightness or pressure tightness is essential, wire inserts everywhere else.
Having said that I do fit Recoils to the plug holes on mowers & chainsaws but a leaky plug on your lawnmower aint going to leave you stranded in outer whoop whoop.
All of the head bolts on the M20 are helicoiled as are most of the gearbox holes.
However the sump gets solid inserts.
No parallel thread is guaranteed to be either gas or pressure tight, that's why sealing washers exist. I've never seen a tapered threaded spark plug but itv wouldn't surprise me if
the idea was tried at some stage. Most plug threads are stripped due to over-tightening, for some reason people think they are required to be 100 ftlb.
The last time i had to personally use a helicoil was at a race meeting, on checking plugs after practice my passenger was a bit heavy handed when replacing one.
I used plenty of grease on the helicoil tap and did the job in-situ. We then went out and won both our races for the day. The head still had that heli-coil in when the bike
was sold many races and years later.
I suppose you blokes would have abandoned the race meeting, driven home, taken the head off, taken it to a machine shop, etc. etc. (No bloody way .)