SpudFiles

I am building my first piston gun an have had alot of troble trying to get the piston to seal aganst the barrel. Im using quarter inch soft rubber and a hefty spring behind the piston, any suggestins?

How square is the surface the piston seals against to the piston? You have to have a perfectly straight cut to get a good seal.

Make sure the barrel is cut perfectly square and make sure it is very smooth start with 100 grit sandpaper and work your way up to 400 or more.
EDIT:You beat me to it.

Wait are you just blowing into it? If soo it probably wont seal well because humans produce only soo much Psi.

no im not blowing into it, im filling wit a co2 tank (compressor is broken ) Ill try sanding the joint very smooth next time.
Im such an idiot, i glued the gun together without any way to work on the pison, there goes 20 bucks

Cut the barrel pipe with a pipe cutter, it leaves a smooth ridge on the end of the pipe which is perfect to seal against.

yea pipe cutter works good or a chop saw. I used a rubber pvc cap for the diaphram. I traced the inner diameter of the barrel on it and cut it slightly oversize and then sanded the edges down to fit. It the type thats has a hose clamp on it. Its plenty thick enough and very soft. It will be slightly belled out. Put that side to where it jsut touchs the barrel and use a small equalization hole. The spring only needs to be strong enough to pop it back to shape.

When I first built my piston for a 2" barrel, I had problems sealing which I attributed to the piston fetching in the bore. After judicious use of prayer words, I found it was because the rubber I used for a seal was getting sucked towards the barrel and folded over as the piston came back forward. I swapped to a firmer rubber (quick cap material like you are using) and larger diameter for strength and more allowance for slight bending.

E

Yeah same hear. I get my piston up to 5 psi and the sealing face pops threw the barrel and releases all the pressure. It was working but it stopped. The cap that you are talking about, is it a black flexible rubber type like at lowes? If so I'll try that out.

Yeah, the one with black rubber and a hose clamp around it. I used a 3" as it was going on a 3" cap/piston. I cut it just inside of where the sides met the front, then smoothed it up with a grinder. I also used a fender washer in the center for support. I think the washer was roughly 1/4" smaller than the bore of the exhaust port. I also mounted it so the curve of the plug laid against the piston (cone shaped), figuring it'd seal better by pushing against the inner edges of the exhaust port. MUCH better than the crap I was using.