Here are a few notes and photos of three fire pistons I made to get an initial feel for design considerations for these interesting throwbacks from antiquity. Many fine fire pistons have been made from wood, plastic, and animal horn, but metal is my personally preferred medium, so that's the direction I took.
This first model was quickly thrown together to provide my first experience with a fire piston. The plunger (piston) is a 1/4 inch stainless steel rod with a small (1/8 inch diameter X 3/16 deep) axial hole in the end to hold the tinder. I equipped the little tinder hole with a small lip on the end to help keep the tinder from falling out. The "O-ring" gasket is really just a small rubber ring punched out of some sheet rubber with the sharpened ends of some stainless tubing (1/4" for the OD and 3/16" for the ID). It is seated in a groove turned into the rod with a small plunge cutter.
The cylinder is made from a length of 3/8 inch stainless rod that was drilled and reamed to about 0.252". Unfortunately, the bore's interior has a poor surface finish (oops) that extensive application of abrasive paper did little to improve. This means that the device requires lubrication on a regular basis. I just use a slight dab of machine oil on the gasket.
Model A works OK when initially lubricated. It will generally ignite char cloth with one or two strokes but it is limited in that the resultant coal is very small which allows little time for transfer to the kindling. Also, while the coal is contained inside the tiny hole it must be frequently blown on to keep it lit since it has little access to air. It is difficult to dig out too.
A couple of small notches were filed into the end of the piston to allow better access to the coal for extraction with a toothpick. It was my impression that this also improved ignition, possibly as a result of better access of the compressed air to the tinder.
I found it very inconvenient to load this piston. It is tedious to tear off a small piece of char cloth and fold it into a tiny bundle to fit in the hole. It would be extremely difficult if not impossible to do with cold fingers in low light conditions.
The cylinder tube of this model was made of 1/2 inch seamless stainless steel tubing (for a smooth bore), and capped with a machined stainless steel plug silver soldered into the end.
The plunger is made from 3/8 inch stainless tubing that is likewise capped with a machined stainless plug. It features dual brass piston rings (bearing surfaces) silver soldered in place and machined to be a sliding fit in the bore. A silicone rubber 0-ring is seated in a groove adjacent to the end of the shaft. Silicone vacuum grease lubricates the bore and finishes the seal which is excellent; this thing makes a fine gas spring. When at rest, the distance the plunger rod sticks out can be altered with temperature.
Parenthetically, this implementation turned out to be overkill. I'm not sure I have ever managed to get the piston to bottom out in the cylinder as it takes a lot of horsepower to compress. I think I gave myself a case of "tennis elbow" when playing with it.
As was the case with Model A, the tinder holder at first consisted of a 1/4 inch hole bored into the end of the piston about 3/16 inch deep with ridges machined into the wall of the hole to help retain the folded wad of char cloth. I was surprised to find that ignition was very difficult to achieve. I believe that is because it took so much horsepower to compress the large volume of air that the piston never fully reached the bottom of the cylinder so the volume of the tinder was too small in relation to the compessed volume of air to create the heat necessary for ignition.
I subsequently epoxied a small piece of hardwood dowel in the end and drilled a small hole through it diametrically to hold the tinder. Not only did this significantly increase the ignition rate but also made the tinder much more secure and easy to manipulate. The end of the dowel was accidentally broken off and replaced with a piece of wooden toothpick glued in place as seen in the photograph. As it is here, the small hole still restricts the size of the tinder too much. I am confident that replacement of the old wooden tip with one of larger diameter and a larger diametrically drilled tinder hole would significantly increase performance. The elbow would still suffer.
Problem 1: Char cloth is by far the best tinder found so far, but it is inconvenient to tear off and fold into little wads to charge the piston with.
Solution 1: Use char chord, which is thicker and more substantial.
Problem 2: Axial tinder holes do not hold the tinder securely, do not allow convenient access to the coal, and do not allow enough air to reach the tinder to keep it lit without frequent blowing.
Solution 2: Use a diametrically drilled tinder hole large enough to hold a substantial tinder wad that is a large percentage of total compressed volume. This also effectively doubles the air accessible to the tinder.
Problem 3: Though they can compress and heat a lot of air, large bore diameters (1/2 inch) with long strokes require a lot of horsepower to fully compress, while smaller bore diameters (1/4 inch) restrict the size of the tinder to an inconveniently small and hard to manipulate coal.
Solution 3: Compromise bore diameter (make it 5/16 or 3/8 inch maximum).
Problem 4: O-rings wear out and are inconvenient and expensive to acquire and replace. Your design may be restricted to what is available.
Solution 4: Make your own gaskets with materials on hand. A couple of pieces of steel or brass tubing with the ends sharpened make convenient punches to produce gaskets from rubber insulation, tire inner tubes, leather, etc. The diameter of the punched hole can be manipulated by whether you sharpen the inner or the outer edge of the tubing. Note that punching the OD of the ring first allows easier centering of the ID.
Problem 5: Lubrication oil is inconvenient at best.
Solution 5: Silicone vacuum grease is forever. A tiny dab is all it takes.
This is the third prototype of my experiments with fire pistons. Model C incorporates some improvements suggested by the experiences with the first two models.
I designed Model C with a few more features intended to make it fully self-contained as a diesel-effect fire starting system. Simple fire pistons consisting of only a plunger and cylinder require that one carry a tinder box, lubricant, perhaps a small metal can for making charcloth, and maybe some spare O-rings to complete the kit. Why not incorporate all these items into one convenient package?
The photo below shows Model C buttoned up for stowage. You will note the brass thumbscrew at the top of the plunger "handle", the purpose of which you will learn below. At first I was concerned that this would hurt my hand when stroking the plunger, after my experience with Model B; but I have learned that a sufficient stroke is accomplished by simply placing the thumb and forefinger on top of the handle. This thumbscrew can be inverted and threaded in from the underside if it were necessary.
The cylinder tube is made of a length of 3/8 inch seamless stainless steel tubing with a machined stainless plug silver-soldered into the end. This plug is bored and threaded to allow convenient storage for a half dozen spare piston gaskets, pre-lubricated with silicone vacuum grease.
The brass cap for this end is further threaded for a thumbscrew which allows access to a small amount of the same vacuum grease without disturbing the spare gaskets.
The other end of the cylinder tube has a 1/2 inch diameter ring of brass silver soldered to it. This provides a bearing surface for a knurled and threaded retainer which engages corresponding threads on the bottom of the stainless steel plunger "handle". This keeps the two parts secured together and keeps dust and moisture out of the bore. At left below is the plunger in its stowed position; at right it is unscrewed and ready for use.
The plunger is a piece of brass rod with a 1/4 inch hex milled on the end (long story). This hex has a 0.180" hole drilled through the diameter, providing a convenient size for a tinder wad that makes a large percentage of the total compressed volume with plenty of air access. The coal can be pushed out with any convenient pointy item.
As long as I was turning two bearing surfaces on the brass plunger rod, I decided to also use two gaskets. Why not? I figured if one failed at an inconvenient time the other, in combination with the sealing action of the remaining close fitting bearing surface, might suffice to do the job...
The "handle" end of the plunger can be unscrewed from the piston rod and serves not only as a container for storing char cord, but also as the oven for baking it.
Buttoned up with the brass thumbscrew loosened to allow the smoke to get out while baking.
When the smoke stops, just give the thumbscrew a quick twist to seal out the air then remove from heat.
Char cord ready to use; just pinch off about 1/2 inch, fold it in half and insert it into the piston's tinder hole.
It might good to mention that not all cotton cordage is all cotton. Some is made with a core of artificial fibers. You will need to remove these. It is easy to bunch and slide the outter sheath from one end then pull the core out.
One stroke is usually sufficient to give the desired result. At first I was concerned that the brass would suck heat away from the coal and tend to extinguish it, but I have been pleasantly surprised to find that a doubled-over piece of char cord will smolder comfortably in this hole for over a minute without the need to blow on it.
I am fairly happy with this Model C, though it is far too heavy to be considered for backpacking (I have assigned it as a fireplace accessory). There are a few further improvements I have considered for a future Model D for both compactness and light weight but let's face it - matches and Bic lighters are hard to beat! ;)
- Sustinemus,
05 February, 2012