No no no, don't lock tight it, you really shouldn't lock tight a helicoil. As far as drilling too far just get some jb weld and a bit of grease on the end of the bolt, drive the bolt almost all the way in, stop where it normally stops, use another bolt in another hole to figure that out. You will need to split the case for this part. Put jb weld into the hole from the back, and try to avoid letting air bubbles form. The grease keeps the epoxy from grabbing the bolt.
Option 2: if you don't care about being able to remove the bolt in the future, put strong tape like gorilla tape on the underside of the hole inside the crankcase, this is way easier without a crank in the way but do as you see fit, put a blob of epoxy the size of a quarter into the hole and torque the bolt in but not farther than it would normally go. Once it is set its set forever but won't leak.
Final option is put a tiny bit of tissue or toilet paper into the bottom of the hole, then fill only the bottom of it with a milliliter or 2 of epoxy. To do it accurately get one of those cheap baby medicine or pet medicine dispensers, looks like a syringe. They even have milliliter marks on them how great! So put a drop of water, A SINGLE DROP, down into the hole to wet the tissue, pack it down with a smaller bolt, put epoxy in and wait for cure, afterwards you can use the helicoil as normal and remove the bolt if needed, leave the tissue it will disintegrate in time harmlessly.