Cutting the Slit
I am of the opinion, as I have said elsewhere, that the stem is the most neglected of aspects of pipe making. I believe that many, MANY, pipes smoke poorly because of the stem, and in particular, the last inch or so of the stem. This final inch is essentially what is entailed in the cutting of the slit of the stem.
Too often this area of the pipe is completely neglected, either an untouched slit in a pre-formed stem, or a slit that is nothing more than a decorational cut. While much attention is paid on the diameter of the draught hole in the stummel, much of that attention to the openness of the draw is nullified with a poorly done final inch of the bit.
Enough ranting, let me get on to the how-to:
After drilling, you will have a 1/16" hole in the end of the rod for about 1/2". This is the area that needs to be openned up, continuing back some into the tapered area of the hole through the rod. First, I cut a slit into the end of the rod on this 1/16" hole using a Dremel with bit 199. link This slit is really not much more than a guide. It is not sufficient for an open air flow. It is, however, where many stop in the slit cutting process, much to the demise of their pipes.
After the guide slit is cut, I use a diamond burr for the Dremel to smear away the sharp corners left in the bottom of the slit guide. I don't know the actual model number of the burr I use, but it is the smallest diameter I could find. I would estimate that it is about 1/32" in diameter. This burr is not of the Dremel brand, but a burr that I found in an inexpensive set of Chinese made burrs. It is similar to this set, and looks like the bit on the far left:
Next, I chuck a standard drill 1/16" bit in the headstock of my lathe, and, using the guide slit that I have cut I "smear" open the slit, extending about an inch into the bit. During this process I am constantly checking for symmetry in the slit. I am also constantly testing the stem for an appropriately open draw. Once done with this part of the process, a sufficient sllit has been cut, and needs only be cleaned up.
Minor cosmetic shaping needs be done with small needle files, as well as so cleaning up of any rough spots caused by the cutting of the slit. This can be done with the fine needle files, as well as sandpaper strips. Then, the interior of the the slit can be polished by heating some buffing compound and impregnating a pipe cleaner with it. Then that cleaner can be used to opolsih the new slit in the stem.
You now have an slit in the stem that will provide an open draw. If done well, this slit will be no taller than the 1/16" hole initially drilled in the rod. This will allow for a thin enough stem for maximum comfort.