MATERIALS AND TOOLS
Available at smallparts.com or mcmaster.com
Fire Piston Assembly Drawing
Rubber O-rings, ¼"ID× 8"OD×r" wide ( 2)
Clear polycarbonate or acrylic rod, ½" diameter,
cut into 2 lengths: 12" and 1¼"
Clear polycarbonate or acrylic tube,
½"ID× 3"OD× 9½" long
Ball knob, 13" wide, with ½- 13 female thread
Tap and die set to cut a thread matching the
PVC tee fitting, ½"×½"×½" (optional) instead of
the ball knob and tap and die set
Cyanoacrylate or methylene chloride-based adhesive
Lathe, table saw, or handsaw
Plastic polish and steel wool
Drill and ¼" bit
½× 13 screw threads
1" gap between rod and
plug when fully unscrewed
Thread length =
knob thread depth
not cut screw
threads on rod
if tee is used
Making a Fire Piston
1. Cut the O-ring groove.
Referring to the assembly diagram (Figure A), cut
the groove for the O-ring about ¼" from the end
of the rod. The depth of the groove should be just
slightly less than the width of the O-ring. If the
groove is deeper, the O-ring won’t seal against the
tube properly. If the groove is too shallow, you won’t
be able to insert the rod into the tube.
The best way to cut the groove is with a lathe.
But if you don’t have one, then what? Improvise!
I used a table saw to cut the groove, and after a
bit of trial and error, it worked fine. Raise the saw
blade so the height of the blade protruding over the
table equals the diameter of your O-ring, minus 1–2
hundredths of an inch. Carefully spin the rod as it
contacts the blade to make an even slot (Figure B).
(You might not succeed on your initial tries. The
rod is long enough so you can cut off mistakes
and try again.)
2. Glue the plug.
Photography by William Gurstelle
Using cyanoacrylate or methylene chloride-based
adhesive, glue the short rod into an open end of the
tube (Figure C). It’s very important to make this end
airtight. Glue it well, and rotate the plug in the tube
to distribute the glue.
3. Install the handle.
Use a die to cut a ½- 13 thread on the other end of
the rod (Figure D). Screw the ball knob onto the
thread. Alternatively, instead of using a ball knob,
you can glue the rod into the middle hole of a ½"
PVC tee fitting for a handle. If you do this, you won’t
need a die.