SpudFiles

A maximum 3 RPM would be nice, but I would like to comfortably and accurately shoot 1 RPM, i.e. Relaxed fueling and careful loading.

double post, but new updateish thingy...
:EDIT:
I'm going with an o-ringed piston and air spring + pop-off combo. I have decided to go this way because i figured out how to use a manifold to pressurize the chamber and the airspring simultaneously. With this design, I can achieve one touch fueling on a high-ish mix hybrid (10x)

now my only issue is to create the piston...

now my only issue is to create the piston...

For your piston, I would recommend using UHMWPE. It is impact resistant, it has a low density, it is self-lubricating with a low coefficient of friction, and it will seal against a sealing face by itself. This means that no rubber washers are needed.

However, it isn't the easiest material to work with, as it will creep when worked on with dull tools or using the wrong turning speeds. I have never tried turning it with a drill press, but it is relatively easy with a lathe.

I think the only sort of o-ringed piston I could make would have to be a cast epoxy one because i have no machining facilities, not even a drill press

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thats another option, I found 98032A489 washer to fit inside of a .957 ID pipe, now into O-rings and smaller washers. What should the OD and ID of an O-ring be. I know it has to be a bit bigger than the ID of the pipe, but by how much?

Could be as little as .005", just make sure it doesn't slide too well when not lubricated.

I have chosen a 1" OD O-ring with Tech's help, but I don't knowhow thick it should be, how hard it should be, and what the ID should be

As thick as you can get it without jeopardizing the strength of the material it's around, hardness really doesn't matter. Softer will seal better but usually will dissolve into lubricants, especially the cheap ones. The one on the Trident is sized as OD 1/2", ID 1/4", Shore A: 90. You can see on the piston I made I actually had to round off the front part of the piston to stretch the o-ring over it... so just be aware of what you're buying.