It’s taken me a little longer than expected but I have finally got round to getting some photos and dimensions. After a little more experimentation I have now got a piston that is 100% reliable and is effortless to use. I am glad I have persevered with this as it has been frustrating me for a couple of years now. I know it is a really simple design, but trying to find a bore length/diameter that worked well and was easy to use has been a PITA. A couple of other simple mods have, I think, improved it a little. Anyway, pics and dimensions below
THE FIRE PISTON
PLUNGER
PLUNGER GRIP
Dimensions are as follows. I have not included the exterior dimensions of the piston tube as they are not relevant, all that matters is you incorporate whatever pattern or design you like into it.
The plunger is 170mm in length and 12.7mm in diameter. The hole to hold the char cloth is 8.2mm in diameter and its centre is 5mm from the end. The slot is 4mm wide. I know 8.2mm is a strange size but I had just finished drilling for an M10 thread and was too lazy to change for an 8mm. The groove for the O ring starts 10mm from the end, is 1.8mm wide and 1mm deep. Thanks to Fergus Omore I was able to make the tool for the groove with little effort. The O ring is 13mm OD and 11mm ID, which obviously gives a thickness of 1mm. I also put some grooves on the end to make it easier to pull out of the tube. When finished with it you just reverse it and pop it back into the tube for storage. If you make the plunger 1mm shorter at the grip end the O ring will get enough grip to hold it all together when not in use.
The tube has an ID of 12.9mm and is 170mm deep. I didn’t have any long reamers or drills and started off drilling one end, then the other. This worked ok but I was lucky enough to find some cut offs of aluminium air pipe with a thin wall, I later found some aluminium tube with a thick wall that was left over from a previous project, it was lovely and smooth inside and didn’t need any polishing.
I turned down and press fitted an end cap to give a nice tight seal that won’t leak air or come apart, I also rounded of the edges to make it more comfortable to push down on.
And finally.......................
WE HAVE FIRE