Model "T" Fire Piston
Article and photographs by Rob Bicevskis

This is the trickiest part of the whole project - the plunger itself.  We are going to use an O-ring to provide an air-tight seal to the cylinder.

Don't cut the wooden dowel to length at this point.   It might take a try or two to get the O-ring fit just right.  If you make a mistake, it is a simple matter to cut off the end of the dowel and you can try again. 

Step one is to check whether the dowel fits inside of the 1/2" CPVC pipe.

In theory, the inside diameter of the CPVC pipe is about 1/2" and the outside diameter of the dowel is 1/2" so there is a chance that the dowel might fit.

(We all know that: "In theory, there is no difference between theory and practice, but in practice there is. "  Source unknown.)

We don't want a tight fit.  The O-ring will provide us with an air-tight seal, not the wood.  Usually I have had to reduce the diameter of the dowel slightly.

A knife can be used as a scraper to remove material quickly from the dowel.

Try to keep the dowel round during the scraping process.

After scraping, or if the diameter is almost correct, smooth things off with some sand paper.

Remember, the dowel needs to slide freely.

Now for the gasket.

This is our goal.  An O-ring seated in the dowel a short distance from the end.

In order for the O-ring to seal properly, we need to cut a groove into the dowel.

For this, we are going to make a jig.

Another view of what we need to achieve.

This is what the jig looks like.

It is an assembly of CPVC pieces that allows us to cut a very controlled slot near the end of the dowel.  The knife/saw is held stationary, and the dowel is rotated.  This forms the slot.

The width of the saw should be equal to, or slightly less than the thickness of the O-ring.

Here is an exploded view of the jig.

Use the parts list above for dimensions of the pieces.

Here is a partially assembled version of the jig.  Note the slot that has been sawn into the T-fitting.  The trick is to cut the slot just deep enough that the knife/saw blade won't cut too deeply into the end of the dowel.

A top view of the jig.

There is no need to glue the pieces together. A press-fit should be good enough.

We are now ready to make the cut.

Cut a ways into the dowel, then remove it from the jig and test for a fit into the cylinder.

This is an iterative process.  Test, cut a bit, and test again.  At this point we are going for a tight fit.

A dental pick is a useful tool for removing the O-ring from the slot.

When dowel with O-ring can be forced in the cylinder, it's time to smooth out the slot with some sandpaper.  One technique to to fold some sandpaper over a piece of string  It is important not to remove too much material during the sanding process.

A folded piece of sandpaper can also be used.

Sand and retry the fit in the cylinder.  Keep sanding until the dowel + O-ring starts to slide freely in the cylinder.  You should be able to generate a distinctive "pop" sound as you remove the plunger from the cylinder.

Almost done....

Mix up some 5 minute epoxy and slather it all over the end of the dowel.  This will seal the wood.  Now roll the O-ring into its slot and wipe off the all of the exposed, excess epoxy.

Slide the plunger into the cylinder and let the epoxy set.

To get a feel for how much friction there should be on the plunger, watch this video: Bounce Video.

Notice how easy it is to depress the plunger, and how far back it bounces from the air pressure.  If there is too much friction, one won't be able to generate enough speed to get a high enough pressure to light the tinder.  This is the biggest factor in the success of the fire piston.

Now drill a hole into the end of the plunger.  Using a 1/4" bit and going about 3/8" deep is a good starting point.  Since this design gives some much volume and compression, the hole isn't all that critical.

Note that this hole already has some tinder fungus loaded into it.

Here are some ideas for non-traditional holes.  A hole that goes across the dowel makes for easier removal of the tinder.  This particular plunger has a hole in both dimensions.  The thought here was that the hole in the end would better expose the tinder to the high pressure/temperature air, and the hole in the side would make it easy to pop out the tinder.  This worked out well.

Now we cut the dowel to size, and insert it into the T handle.  The length of the dowel should be such that it "just" touches the bottom of the cylinder.  Any air-space left over is a bad thing since it reduces the possible maximum pressure.  If the dowel is too long, it will bottom out - which is OK, but puts greater stresses on the whole assembly.  A perfect fit isn't all that hard to do.

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