SpudFiles

If you don't use o rings, it is important to keep the leakage to a very small space. A common problem with piston valves is the fit is too loose so the amount of air leaking from the chamber into the pilot is enough to keep the pilot pressure high enough to prevent the piston from opening. Below is a photo of a piston in a cylinder that does not use o rings. Note the gap is very small. If o rings are used, a small hole needs drilled from the chamber area to the pilot area. The hole in this piston is not an EQ hole. It is a bolt hole that was used to chuck the plastic in a drill press to machine the piston.


Piston is machined on a drill press. The piston is screwed onto a bolt that is chucked in the drill press.

we have finished building our gun! Yay! however, there is a leak. Behind the piston.

So our questions are, is 10 psi to low to test to see if the piston is working? the leak seems to let out all the air even the chamber air.

Another is, is our piston to small and it is letting too much air back so there is no pressure drop?
reasons for this question is because when we release our piston valve, everything just seems to release out and it goes on for a while.
our piston was 3in coupling and 3in cap that was fitted together and sanded down just so it would fit inside the 4 inch pipe and sanded to just where it had a smooth motion.

Could it be also that our pilot valve is too big? It is nearly 1' 6" long. This was due to all the connectors and etc. Is it because the pilot valve isnt dumping fast enough and the air from the chamber is leaking in faster that it is dumping out?

I think the assessment of your difficulty is correct. These valves require extremely tight tolerances to function. Any sideways rattle room is too much, especially in larger diameters. Work on getting a very smooth piston surface that mates very closely to the cylinder it rides inside.

Another mistake some builders face is in the choice of materials for the piston. If the piston can change shape under pressure, this can create leaks. Is your piston able to hold the chamber pressure without bending?

I hand lap all my larger cylinders until they are very smooth, then make the piston to just fit with as little gap as possible. As others have stated, adding tape, paint, o ring, or such to fill in the gap is often the fix for a piston with too much blow by into the pilot area.

Another fix for leaky pistons is to use a huge pilot valve to blast vent the pilot. Large ball valves work.

If the piston moves freely ( not stuck ) 10 PSI is plenty of pressure to test them in the larger sizes. 20-30 PSI can be impressive. Wear earplugs and safety glasses for over 60 PSI if testing indoors. My o ring video on youtube shows a 2 inch valve blast at about 12 psi. This can provide a sample of what to expect when a 2 inch valve works.

The piston seems very sturdy and seems as if there is barely any air leak around it. When we drop the piston and gravity pulls it down you can feel the air coming out of the barrel. Im not sure if that means anything but it feels as if it is fairly tight. It cannot wobble around. we are suspecting that it is either the leak or the massive size of the pilot as we have seen that most pilot valves are very small, about >3" ours is at least 1'. I am afraid that it letting the air out slower that the air is being pumped back in from the chamber.