The Stovebolt Site Forums The Shop Area Engine Shop helicoil on oil galley?

1.Make sure tap drill size is correct. Run a 9/16" drill in the hole. Likely problem is undersize hole.
2.If there are no obstructions in the bottom of the hole use the tap you've got and it needs to go in at least 2/3 of the length of the teeth on the tap. Then snug a pipe plug in there and back it out counting turns. If it goes 3 turns and then falls out it's perfect. If not go in with the tap again. Back up and use lubricant each turn as npt threads rip easily.
3. There is no such thing as a bottoming npt tap but as Stuart pointed out they are available if you ask for a short projection pipe tap. This is the same thing as Hotrod Lincoln is talking about but if you are unfamiliar with reconditioning drills/taps buy one as no clearance can result in a broken tap in there.
4. If it's completly thrashed go up to the next size and use a reducer. Even if a npt helicoil exists it will likely leak.

Good luck.

Curt B.