When I need a pipe tap to go deeper than usual into a hole I grind a few threads off the end of the tap. Start the hole with the normal full-thread tap, then switch to the shortened tap to make the hole bigger where the reducer fits. We used that trick frequently when tapping the back of small block V8 oil passages for flow restrictors when running solid lifter camshafts.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!