piston not sealing

jonnyboy · Sat Jun 07, 2008 3:14 pm

Why won't my piston seal?The tee is 3'' diameter. the pilot volume is largish 45inchs-cubed. The piston itself is about 5 inches long and protrudes almost to the fill nozzle. I have a bike inner tube as the sealing face bolted and epoxied on. The piston is wrapped in some tape so it is a tight fit. It won't fall out if you put it on a vertical angle. But it comes out with a little blow of air from the barrel port. I lubed it with some grease. Is it my piston being to long or what?

bigbob12345 · Sat Jun 07, 2008 3:21 pm

Well I would say get a better sealing face such as some 1/8-1/4in neophrene.
But another possible problem is that your barrel port is uneven and it wont seal.
If that is the case all you need to do is bolt some sandpaper onto the piston and then put the piston in the tee and apply some force while spinning the piston around and eventually it should even out your pipe.
That was suggested by gippeto in some thread somewhere on this forum.

jonnyboy · Sat Jun 07, 2008 3:42 pm

bigbob12345 wrote:Well I would say get a better sealing face such as some 1/8-1/4in neophrene.
But another possible problem is that your barrel port is uneven and it wont seal.
.

Nope its a flat surface right now I can't get any neoprene. Would it be a problem if my piston was 5 inches long? I plan on cutting it down to 1.5 inches to reduce dead space and have more airflow behind the piston (right now the piston is just touching the fill valve.)

Check out my YouTube SpudGun Channel! · Sat Jun 07, 2008 4:16 pm

What is your pilot valve?
What are you trying to seal the piston with? (bike pump etc)
What is your fill valve?
Does the fill valve get in the way of the piston? I.E does it protrude into the pistons path?

Try and get a thicker sealing face, doesn't have to be neoprene. I know you can get some orange rubber disks at hardware stores. The thicker the better, it might compensate for the seat (barrel) if it is uneven.

I would also recommend making your pilot volume far smaller.

Your piston length wont matter, it doesn't reduce or add dead space, not sure what you are talking about there. Dead space is the space between the valve (the seat(barrel)) and the projectile. Piston length has nothing to do with that.

The length of the piston doesn't concern the 'airflow behind the piston' either. You only want to exhaust the air behind the piston, you don't want to exhaust any air from the chamber or in front of the piston. That is if your pilot valve is directly behind the piston, not on top where the fill valve is.

Answer my questions and I should be able to help you.

Sticky_Tape · Sat Jun 07, 2008 5:27 pm

Crowley I think that he means the lenth from the back plug to the back of the piston is the lenth that the piston will be from the back of the barrel when the piston is piloted. Therefore if he shortend the piston the lenth from the end of the barrel to the front of the piston will be increased and there would be more flow. Just thinking the way I think he is thinking I know about d4 and all. Also I think that 45 cubic inches is a big pilot volume size. We need to know the type and pilot valve size.

jonnyboy · Sat Jun 07, 2008 5:30 pm

@mrcrowley
What are you trying to seal the piston with? (bike pump etc)Air compressor
What is your fill valve?quick connect
Does the fill valve get in the way of the piston? The back of the piston sits right in front of it

I have some thick shower liner about 2mm thick. Is that good enough or should I use 2 layers?

@sticky_tape
Yeah thats what i am thinking. See above for other information

Sticky_Tape · Sat Jun 07, 2008 5:41 pm

So uhhhhhhhh what's your pilot valve and size?

jonnyboy · Sat Jun 07, 2008 6:14 pm

Sticky_Tape wrote:So uhhhhhhhh what's your pilot valve and size?

Oh sorry wasn't thinking. It is a 3/4 ball valve. But right now that doesn't matter 'cause I can't get the piston to seal. ;(

Sticky_Tape · Sat Jun 07, 2008 7:09 pm

I don't build big cannons normally but for small sclae if the barrel isn't perfectly aligned I put a foam disk not styro foam behind the rubber on the sealling face so you don't have to layer rubber it makes the sealling face more squishy.

jonnyboy · Sat Jun 07, 2008 8:57 pm

Sticky_Tape wrote:I don't build big cannons normally but for small sclae if the barrel isn't perfectly aligned I put a foam disk not styro foam behind the rubber on the sealling face so you don't have to layer rubber it makes the sealling face more squishy.

Ok I'll do that with a pool noodle.

Check out my YouTube SpudGun Channel! · Sat Jun 07, 2008 9:29 pm

Well your answers should give you decent results with the piston. Can you take a few pictures?

jonnyboy · Mon Jun 09, 2008 2:17 pm

sorry crowley no pictures atm.

I got the piston working, added a couple layers more of this pvc water liner.
Now I can't get it piloted.

I'm using a 3/4 inch ball valve. The pilot volume is 45 cubic inches a little big. I've only tried it at 35psi though 'cause of leaks.

Check out my YouTube SpudGun Channel! · Mon Jun 09, 2008 3:00 pm

45 cubic inches is far too big. Just check a piece of wood in their with a hole drilled in the middle for flow. That'll eat up some of the pilot volume.

MaxuS the 2nd · Mon Jun 09, 2008 5:32 pm

I think the sealing problem was probably due to the fact that you had epoxied the sealing face aswel as bolted it to the piston. The epoxy would have made the sealing face hard and would make it difficult to seal onto the barrel.

dongfang · Mon Jun 09, 2008 5:39 pm

Hi,

Glue a piece of paper (just with paper glue) on the front of the piston Fasten some carbon paper to the paper, carbon side towards piston, with a little dot of glue. Mark "up" side of piston. Insert piston and push it against the barrel.

- Do you get a nice full circle on the paper?

- If not: Does the result change if you turn the piston "upside down" and try again?

I once built a piston valve where I cut off everything at perfectly right angles. That didn't seal at all.. now I adapt the seat to fit the piston, and then glue it in.

Regards
Soren