Ok, I've been to Sears twice and to Home Depot and I'm not convinced I have the right tap yet. The Edge directions call for drilling a 5/16" or 21/64" hole and using 1/8" NPT tap. I know nothing about taps, but this 1/8" seems like a typo considering Sears sells a 5/16" drill bit packaged with a 3/8" tap. So what is the right size tap?

NPT apparently stands for National Pipe Thread, but NPT is not used on any of Sears' tap packages. Instead, they note the threads per inch, such as 16, 20, 28, etc. depending on the tap size. So what does NPT translate to in threads per inch?

NPT is pipe thread, and its all together different than standard thread.
Pipe thread has a taper to it, its how it seals up. It basiclly wedges itself in the hole.
Find a tap that says 1/8 NPT and you will see what I mean.
And I believe that 5/16 is the right size for an 1/8 pipe tap.
I think it gets its weird sizing because its based on what the inside diameter of the pipe would be.

npt is tapered thread. 1/8" npt is correct size. got my tap at ace hardware

Good call on Ace...they had it. It's marked exactly as Edge has it. Thanks much!

Now I just have to get off my *** and go do it.