XO-1 spur-to-pinion adapter that includes the "Cush Drive"?

[RC-Monster Mike]

You definitely need a tap specifically for hardened materials, but the hardened cast parts generally don't respond well to re-machining operations. You may have some luck if you cheat a little on the drill size(stuff is hard, so it won't likely pull the thread even if you drill a little bigger). Carbide tools and lots of lube!