Finished the Fire Piston Monday (out of town yesterday so a little late posting it). I copied this from the one Clickspring made on the YouTube video with a couple of my own modifications. Instead of a compass in the end cap (because you can't find a really good one and I want things to be truly useful) I put a small pick in there to help pull the char cloth out of the end of the piston (got that idea from the video as Clockspring was using a scribe to remove the cloth). I also put a slightly curved bottom on the striking end of the piston as I thought this would be easier on the hand when using it. Unfortunately the 3/8 reamer I have is a very old and dull 2 flute that did not cut a really smooth bore. When the piston is driven through the bore you can feel "waves" for a lack of a better term so it is not real smooth sliding through. The piston rod is hollowed out to store extra char cloth wrapped in foil. I don't know if it will actually work as I haven't made any char cloth yet but will try try to get to that today. This was fun to make and the type of thing I enjoy doing with the little South Bend.
The piston/O ring to bore fit is pretty good but the inconsistency of the bore only gives me real compression about halfway down. I may try reboring to 7/16" with a smoother, more consistent bore (as in better reamer) and make a new piston. Plus the larger bore should give me more "horsepower". 
