Sometimes prototyping is crude. An idea for an internal thread change came out of a meeting, but the part is die-casted. and any outsourcing at the time would not have been within budget. We decided that we would cast the proof-of-concept ourselves, something none of us had done before.
In order to get the internal tapered coil thread, I decided to use its existing mating bolt as a core for the mold. Coupled with some scrap square stock lying around, we had a functioning mold. It was by no means a typical mold with a sprue, runners, or a riser, but it was cheap and quick.
The pieces were then gravity cast out of ZA27 alloy, and machine finished on a lathe. Their preliminary results once tested were better than anything we had made to date.