SpudFiles

I've kind of run into a dead end with my 3" porting piston cannon because with the current piston, the valve destroys itself on each shot. So we need a much lighter piston. Does anyone have any tips on making lightweight pistons with minimal machining? I'm definitely looking at the two-part bolted piston style like the one on the Supah, but I have no idea where to get the discs. Also, what size all-thread or bolt is sufficient for the center bolt?

Current Piston:

(okay I lied this isn't the exact piston that one is now in pieces but the other one is built more or less the same, i.e. too heavy)

What kind of piston is that? For my 3" porting piston valve, I use two endcaps solvent welded together, with an air bumper. I have not had any problems before. My theory is, while a heavy piston certainly is not good for a cannon, a good enough bumper can neutralize these negative effects.

And by the way, if you are using a cleanout cap for the back of the piston valve, replace it; with a valve of this size, it will not be able to survive more than a few shots. Drop $20.50 and get a SCH 80 threaded plug; you will be glad that you did.

It's a 3" coupler with a drilled out 3" to 1.5" bushing on one side with a grind plate to keep the sealing surface from buckling. And thanks for the tip on SCH 80 plugs, that will definitely help. We might just have to go with a SCH 80 4" Wye, although a) that's a lot of money and b) it might mess up the geometry. Is the inside of a Sch 80 wye the same diameter as sch 80 PVC pipe? Because the piston extends partway into the wye.

Could you possibly provide a close up shot of your valve?

99% of the time, wyes are not pressure rated; they do exist, but they are more like unions (rated low; you can get them on plumbing supply.com, but they are super expensive. $61.87 for a 4" wye, which is only good up to 150 PSI).

I have never seen, nor heard of, a SCH 80 wye.

If I were you, here is what I would do

Buy the following:
(1) 4" SCH 80 plug ($20.50 each)
(2) 4" SCH 40 female adapters ($6.78 each)
(1) 4" Tee ($17.69 each)
(1) 4" Elbow ($11.92)

That comes to a total of $63.67 off of mcmaster.com. Expensive, yes, but how much will it cost to keep replacing those cleanout caps and wyes, time and time again?

Is there a drastic difference between the tees you get at Home Depot and a Sch 40 one? I'm actually thinking I might go for a Sch 80 4x3x4 tee (the input is 3"), but I think we will still definitely need a lighter piston.

It all depends on what your Home Depot sells...

I wouldn't go for the 4"x4"x3" tee... they are harder to find, and also slightly decrease flow, for the 3" elbow which leads into the tee decreases flow more than a 4" elbow leading into a tee. Besides the 4" tees look much nicer and usually cost less as well.

cast it out of quality epoxy... though at that scale, I think it would get pretty expensive.