SpudFiles

Ok, I just built my first piston gun, which is a coaxial. It's kinda like a pengun, but a bit longer. Anyway, I am having some problems with it, but i will start by describing it so hopefully someone can diagnose the problem. The gun has a barrel of aprox 9" of 3/16" brakeline (thats 3/16" o.d.) About 4" of that is inside the chamber. In the non-muzzle end, the barrel is flared and has a tiny rubber o-ring on it to seal. The piston is a 1/2" length od 3/16" brakeline (o.d. again) and one end is flared and has a 4.5mm (.177") bb glued to that end. That is the end that seals on the barrel. The chamber is about 5" of 3/8" brakeline (o.d). On one end it has a schrader glued to it, and at the other end it is glued ot the barrel. Anyway, here are some pictures to help explain.


It is epoxied with JB Weld, so it holds plenty of pressure. Anyway, I have pumped it up to 200 psi with my shock pump, and there are no leaks and the piston seals just great. However, the problem comes when I try to shoot. I detach the shock pump, I take a 4.5mm bb and press the schrader in with the bb between my thumb and the needle. Instead of most of the air coming out of the barrel, 100% of it comes out of the schrader.....Did I miss a step when I built this? It's pretty much based on jsr's (I think) pengun. I didn't lubricate the piston, but I kinda doubt thats the problem. does anybody have any idea what's wrong here? I'm sorry if it's just a basic fix, but this is my first coaxial, and I did research it and looked at othere's work and diagrams. Anyway, help is appreciated...sorry for writing a book Thanks!

My first guess is that the air is leaking out slowly (relatively speaking) try pushing the pin on the schreader with a nail that so there's alot of area for the air to get out.

Basically I think the bb is blocking the airflow to much.

How tight is the piston in the chamber? And is there a lot of pilot volume behind the piston?

Ok, I tried using a nail to push in the pin at 300 psi, but almost all the air still came out of the schrader....so it's still not workin....anyone else have any ideas? Thanks for the suggestions johnnyboy!

EDIT: JSR, the piston is not very tight on the sides, maybe a millimeter on all sides. What do you mean pilot volume behind the piston? between the piston and the schrader? or the other side?

maverik94 wrote:JSR, the piston is not very tight on the sides, maybe a millimeter on all sides.

That's far too much and probably the cause of your problem, try wrapping it in tape until it's tight but not too much that you can't move it with one finger.

ok, I will try that, but I'm gonna have to tear off some JB weld and re-epoxy it, so I will not be able to know if it worked untill 15 hours

maverik94 wrote:ok, I will try that, but I'm gonna have to tear off some JB weld and re-epoxy it, so I will not be able to know if it worked untill 15 hours

Joys of working with epoxy you should have known beforehand though, if the area between the piston and the chamber is greater than the area of your pilot valve, the piston will never fire. In this case the schrader has a pretty low flow so the piston has to be as tight as you can afford it.

ok, I'm working on it, thanks JSR. I'll get back with you and report how it worked...should I lube the piston?

It depends what tape you're using to build up the diameter of the piston, you don't want to use lubricant that disolves the adhesive. If you use masking tape it's pretty slick stuff, shouldn't need lube.

Ok, sorry, I removed that. I'm using duct tape, does that need to be lubed?

cheers

Just make sure you don't overdo the wrapping and you should be fine, without lube.

ok, thanks!

Can't wait to see how it performs ...
This just inspired me to work on my first co-axial gun...
Where do you get thick walled small diameter tubing?

Well, just buy brakeline at autoparts stores. It isn't REAL cheap, but it's so strong It makes up for the price. It's used for hydrolic brakes in cars and motorcycles, and can stand up to a LOT of pressure (in the 1000s of psi if I'm not mistaken). A 20" length of 3/16" brakeline costs about $3.25 USD, but it comes with the ends flared and brass connection fittings on either side. If you plan to use a lot of brakeline, I suggest investing in a brakeline cutting and flaring kit. Anyway, it's made of galvinized (won't rust) steel, and is AWESOME!!! Back on topic, my gun is drying, It won't be ready till tomorrow morning, but I have high hopes!

All the best for your gun mate, If what JSR has suggested doesn't work, I suggest a pilot valve with higher flow than the schrader. I thought my piston was a perfect fit but it would not fire using a blow gun ( I had the same problem as you). I put a 3/4 inch ball valve on and it fires beautifully. The higher flow will help to counter the loose fit of your piston