Son and I machined up a scrap of seasoned osage orange into working fire piston today. It works!

But - with a 0.300" diameter bore and a 3" nominal stroke, it's a PITA to use and the coal is VERY small and short-lived. So... when we have another free evening, we'll make another, larger bore one.

It was a fun project to do together. I might also take a crack at a PVC pipe one as a Scout project - see http://www.trackertrail.com/survival/fire/firepiston/firepiston2Ron.html and watch the video at the bottom of the page.

Tom

Good one Tom. That does seem a bit of a narrow bore, but the fact that it works is great. What was your tinder?

Char cloth. The bore diameter is OK, but that small a bore x stroke means virtually zero clearance at full compression in order to get into the 20 - 30:1 compression ratio range. With a larger swept volume I would have considerably more headroom for a larger bit of char cloth. I was constrained on this one by the obscene way the "blank" split off from the log - by the time I got it turned, I was not comfortable going to a larger bore.

I read one person's report that osage orange fire pistons have a tendancy to "blow apart" after some use, but I am skeptical - maybe. Or maybe it was not fully seasoned and developed drying stresses -> cracks. We'll see - I'll make another, larger bore and longer stroke one next week as I have a little time. So far this one works well, but it takes practice.

Tom

I know it's totally not the point, but I keep wanting to modernize the fire piston.

Make one out of metal, with a crossbow-like assembly for powering the piston. Twist off end cap to quickly access the coal, rather than pulling the piston out, etc. Or maybe a crank operated version. Insert tinder, turn crank one full cycle, out falls the glowing coal. Making one out of wood and leather seems so... primitive. <img src="/images/graemlins/wink.gif" alt="" />

No, I don't think that's wierd. I've thought about that general sort of thing as well. I thought I'd start with something more resembling a "traditional" fire piston first. I actually have something else entirely different in mind at the end. I learned several useful things with this first "primative" one.

And I still think the PVC pipe one would be a fun scout project. Sneak a little science in when I can...

Actually I think they already started making them in brass in Britain years ago. Cant remember where I read this though.

I just realised after my last post that there is a quite modern version. It called Diesel engine <img src="/images/graemlins/smile.gif" alt="" />