It was a special tap for Gun Smiths used for super hard steel. I don’t remember the specifics. They used the tap to thread the hole and then gave it back to me.
I also made some cradle mounts for the transmission. I bought and didn’t like the propeller mount so I made the cradle. I have one sitting in the garage and will attempt to earn a little cruiser by sending it to you for the actual price of shipping if you continue the swap. For what it’s worth...