Why no helicoil? You say that with such authority I have to ask. I never had a heli coil pull out or fail to torque to spec when properly inserted, the bolt actually forces the coil thread tighter.
This particular application does not require countersink of a time sert, in fact it may not be desirable as the nut is behind thin sheet metal.
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Our frank advice based on experience of about 40 years is that there is not a stronger thread at original size than a wire thread insert of correct length and fitted properly. However, there are some uses for thin walled thread inserts where we would not recommend helicoil type inserts but Time-Serts. If a thread is "static" i.e. not likely to be done up and undone more than a few times in its lifetime, we would always say a helicoil type thread is the strongest option. However, for threads that are used over and over again like sparkplug or sump plug threads a helicoil type insert tends to work its way out of the parent hole over time and a Time-Sert is the more permanent repair.
https://www.crosstools.com.au/helicoil-type-inserts-technical-study-of-strength-and-pullout.html
This mirrors exactly what I posted. The kit I linked to contains the drill and tap and inserts and is plenty good and strong and will not pull out.
Originally Posted by trav
On oil pans and other bolts that are frequently removed in an open hole a Time Sert is better but for this one the coil is more than good enough.
Edit: OP you will need a tap wrench with any of these tools, post a picture of the hole and its surroundings, there are different types, one may be better suited to the job than others.