Step 2: Materials and tools
The piston is a rod that fits snugly inside the bore of the cylinder using a gasket or seal of some type to make it air tight. The bore and the piston must be perfectly straight and smooth to achieve and to maintain an air tight seal while it is being operated to produce an ember.
Natives might collect woods and cutting tools and keep at least one firepistons under construction in various stages of completion to be able to go from start to finish within minutes, hours or days.
To speed up some of the process required for construction requires use of a 1/4" by 6" brass nipple with an end cap filled with JB Weld and an aluminum or oak dowel to construct two test pistons. The pistons require a 1/4" OD O-Ring (or slightly larger diameter that can be filed down to fit) to act as an air tight seal. Natives would use fibers of some sort and perhaps anything from animal fat to earwax as a sealing lubricant. We can use lip balm or Petroleum Jelly.
I used a 1/4" power drill and a piece of dowel wrapped with an inch long sheet of tissue paper soaked with a solution of water and Bar Keeper's Friend (Oxalate acid) to polish the inside of the brass nipple to a mirror finish. (On the larger 3/8" nipple I'm working with now the inside of the nipple is much less smooth and straight requiring various grades of sandpaper to hone it smooth before polishing.)
I used a hack saw blade, a glass file and the threads on a bicycle wheel spoke to fashion the seat for the O-ring in both the aluminum and in the oak dowel. A 1mm bit was used to drill the hole in the char twine box. (Commercial units have a much deeper cylindrical hollow to hold more char cloth better.)
Originally I used a temporary piston stop made by using a pair of vice grips clamped around the piston shaft where the handle would be attached to the piston shaft to keep the char twine from striking the bottom of the bore. This worked but it appears that commercial makers of firepistons hollow out the end of the piston much deeper to provide greater room for more char cloth and allow the end of the piston around the hollow to serve as the piston stop, rather than using a shaft pin or the handle as the piston stop. BTW - commercial versions can run upwards of $50, but some come with all of the amenities such as char cloth and lubricant.