On a lathe use a triangle file to reduce the shaft diameter to 0.175" The tapered part should be at roughly a 45 degree angle. Next file one side of the piston head flat so that you can center punch it to drill a hole. Make sure you don't do it too close to the end of the piston!!
Note: It is important that the o-ring grove is tapered at 45 degrees on the one side, as the piston is forced down in the cylinder the o-ring rides up a little and is expanded by the taper thus providing a tighter seal. If this angle is too low the o-ring will slide up the shaft completely which is no good, I found 45 degrees was about right.