Fire Piston

[Jack Russell]

Bernie, sorry tried to post earler on this but it got lost in cyber sapce somewhwere! Fear not son there's nothing wrong with your build. If you are using thread as your gasket/compression seal as opposed to rubber o-rings it needs to be fat moistened with something to prevent the thread from binding. Also, if your piston plunger was already "popping" on removal it's a tad too tight you run the risk of it binding as you describe, the plunger itself just needs to ride inside the piston chamber. Have you treated the ember cavity with anything to prevent leakage through the plunger face? Don't bin it - it will fire. Here in the sand dunes when it blows a hooley the fire piston is the weapon of choice. cheers jack.

[Bernie]

Good advice chaps - Looks like I need to work on the o-ring to get a better fit. I've put my first attempt online for a laugh.

[Proventurer]

Nice bit of turning Bernie, I've cheked a couple of mine and I think martin has knocked it on the head, without a seal the shaft should be slightly loose in the cylinder, also you need +/- 2mm end clearance when the piston is fully compressed to form a combustion chamber, once the shaft size is correct, this end clearance will be critical to enable combustion.

[Ben Casey]

Great video just a shame it did not work keep on Im sure you will get there

[basicbushcraft]

great work mate

[fish]

makes you appreciate how good the abo chaps are making them with a machette!

[Jack Russell]

just watched your video Bernie. The compression ring seal you're using won't work - you need to wind a fine thread onto the plunger and use some sort of fat or lubricant to make it seal (the phillipino's swear by dog fat so the house pooch needs to hide quickly under the table).

I think you need to skim the plunger so it rides a little easier in the chamber before you "gasket" the seat - and take a short cut, even if you want to end up with a totally natural piston set it up with rubber o-rings first then choose a natural material to make your gasket after.

From the footage it looks like you've set your seal groove a little too far back - you may not get enough temperature at the fuel face in the chamber remaining. Looking at the video there's room to cut another seal groove closer to the piston face. Again have you done anything to seal the piston face?? Many wooden plungers leak air through the ember cavity - I use five layers of cyanoacrylate (superglue) as a quick prevention of this problem - once the piston is bedded and naturally charred this is no longer necessary.

Exactly as Martin described, the plunger should "bounce back" a little on every stroke but it does need to feel that you can bottom out when you give it a strike. You are looking to compress the available air to a space not more than the region of 1.0 mm to 1.2 mm of volume plus the size of your piston face cavity (your cavity looks pretty good).

Make sure that while you're testing you clean out the piston chamber of any charcloth debris after failed strikes as the debris will stop you achieving full compression and as an aside while testing strike it the other way up (thats knob down) your charcloth will have less tendency to spill into the chamber.

Final tip for now - even if you withdraw your rod (I just love fire piston lingo) and it looks like a fail to ignite, give it a kiss with your lips - you'll get a charcloth lipstick job but you'll feel if any heat has been generated.

Now I've seen the video, if you lose your temper with it stick it in the for sales and I'll happily buy it, make it work and sell it back to you for at least double. You're close matey...... so very close. Don't quit now - Jack

[Bernie]

Thanks for your encouraging words Jack. I've not given up on the idea yet, I'm just away from home at the moment. I think I might need to make another now that I have more detail on the dimensions and tolerances. On the next I'll make the fit a tad loser, the notch for the washer a little lower, and I'll pay more attention to the size of the chamber that remains when fully plunged. I've not given up on this one yet, but I've already learnt a few lessons.

Thank you all for the advice and encouragement.

[Jack Russell]

If you're happy that the cylinder body is true and smooth bored, stick with the one you've made so far Bernie, skim a bit off the piston rod with fine emery (I shove mine in a battery drill chuck and spin 'em up but might be problematic if you've already permanently fixed the striker knob) - there's enough room to cut a new o-ring groove below your first attempt and make your seal with thread (I use fishing rod whipping silk, the stuff they put rod rings on with set in the groove with a smidge of epoxy then greased up with vaseline or a silicone base) - I've been asked to knock up some first generation pistons for a couple of local chaps - have to see if I can post a "how to" with piccies. I tend to use brass tube chambers and silver steel plungers now for quicker builds then put them into pretty cane or turned wooden bodies but the principals are the same on this type. Get this one sorted then think about making a second generation piston with a release vacuum valve so the piston rod stays captive when not in use. Fire by compression is absolutely "de rigeur" in my sand and salt air very windy environment, you can stick a tinder bundle under your armpit, strike a piston and quickly transfer the ember without the makings of your campfire wandering off into the sand dunes at a high rate of knots.

[Bernie]

Thanks Jack, I will have a go at shaving the shaft. For what it's worth, the striker knob and the shaft are one piece of wood. No time now - loads of work still to do. Been a long week and tonight I must pack and prep for Ivans bushmeet.

"release vacuum valve" Wassat?