David R. Birch wrote:

Some years ago, I drilled some holes that were to be pipe tapped in a
later operation. I got the drill size (don't remember the NPT
involved)from a standard pocket guide. The guy who was to tap them said
I drilled 'em wrong, his pocket guide called for a bigger drill. We
showed this to the foreman, who checked his own reference, which listed
a third size! With NPT, the hole is a cylinder, the thread is tapered,
so going up or down a drill size is not likely to cause a problem.

I picked up some surplus tapered reamers apparently designed to ream
holes prior to tapping for NPT. This makes the tapping much easier on
both the tap and my arm. Could the smallest of the drill sizes you
encountered be intended for use prior to reaming?

Ted