SpudFiles

Bolt it on with a washer in front of it as large as it can be without touching the inside of the seat.

Do I put a nut of the back side or thread is into the piston? Also should I put a rubber seal on back to protect seal? Do I drill hole into piston so air will pass into forward portion of chamber?

If you can tap it, thread it in, but if you can't tap it, use a washer and nut on the back if the piston. And having an EQ hole depends on how tight your piston is.

I am going to have the machine shop decrease the diameter of the piston to fit the barrel so I can have them make it as tight as possible. Any suggestions. Should I have them make a notch for an O ring in piston?

Atattack wrote:I am going to have the machine shop decrease the diameter of the piston to fit the barrel so I can have them make it as tight as possible. Any suggestions. Should I have them make a notch for an O ring in piston?

It need to fit the chamber not barrel, and just tell them you want a O-ring groove and then give them the O-ring that you want to use.

Sorry Crna Legija, I meant to fit the chamber. I figured I would have 2 or 3 O-rings in the piston. Any suggestions on how proud the rings should sit from the piston? Also what are you all using for a spacer to hold barrel steady inside of the chamber. Figured it would need some structural support inside. And are most people just welding the barrel to a reducer at the end of the chamber? Appreciate it guys.

Another material to consider is Delrin, it's more durable than UHMW and similarly light and low friction.

I would have it cup shaped and simply made to close tolerance, without o-rings. That avoids the need for an equalisation hole.

Also what are you all using for a spacer to hold barrel steady inside of the chamber. Figured it would need some structural support inside.

You can thread the breech end of your barrel to fit in a couple of bolts which can be adjusted to centre the barrel and then epoxy/weld them in place.

@jsr
you can easily add a pneumatic bumper to your design... air trapper behind the piston should stop it perfectly - that's how air cushioned air cylinders work

Good point quite familiar with the design and it's a great and simple idea.

Alright guys, I think I have a pretty good idea, of how it will look.

A few things I'm unsure of are what material to make seal on end of piston out of. Just a round rubber disk?

If barrel is 2" diameter and chamber is 4" diameter should make disk 3"?

When bolting rubber on, should I seal it with epoxy or loc-tite to make sure it doesn't leak?

If piston diameter is going to be 4" how long should piston be? I have heard it should be as long as the diameter. I am going to use a cup design as JSR suggested.

I liked the idea of the bumper, on the end of chamber piston is going to hit I am going to have a 4" to 2" reducer and a 2" - 3/4" reducer so it will taper down and I am unsure how to make the bumper. I just have a rubber seal that is same diameter as the piston? Thanks guys.

To actually build POLANDs bumper you could just use a much longer bolt to hold your sealing face on. With the extra bolt threads coming out the back of the piston, you could use some washers and an o-ring to seal it off. I'm kinda bad at explaining, but I could draw it if you'd like

Atattack wrote:A few things I'm unsure of are what material to make seal on end of piston out of. Just a round rubber disk?

Yep, at least 1/8" thickness. You can buy neoprene sheet, or use the inner tube from a truck tire

If barrel is 2" diameter and chamber is 4" diameter should make disk 3"?

That sounds about right.

When bolting rubber on, should I seal it with epoxy or loc-tite to make sure it doesn't leak?

Always a good idea

If piston diameter is going to be 4" how long should piston be? I have heard it should be as long as the diameter.

If you can get the piston machined to close tolerances, you can get away with a piston length 50% the diameter.

I liked the idea of the bumper, on the end of chamber piston is going to hit I am going to have a 4" to 2" reducer and a 2" - 3/4" reducer so it will taper down and I am unsure how to make the bumper. I just have a rubber seal that is same diameter as the piston?

A 1/4" thick rubber seal should be enough. Remember that the piston doesn't need to travel back more than 0.75" inches or so.

Thanks guys, so basically use neoprene sheet 1/4" thick for both the front and back bumper. 3" diameter on the edge touching barrel and 4 " diameter on back of piston. Will have piston be 3" long to make be on safe side even though it will have tight fit.

Use loc-tite to seal bolt that runs down middle of piston. I will probably use loc-tite to seal neoprene sheets to piston unless you guys so other wise. And use a washer that is just smaller than barrel diameter to hold neoprene sheet on.

When I am having them hollow out center of piston, it will have an outer diameter of 4" and I figured will have a wall thickness of 3/4" on the sides and have the head of piston be 1.25", think that would be ideal?

Also, has anyone had a metal shop put threads on the inside of a pipe? After seeing how far back 4"-2" reducer goes I am trying to figure out how to have flush fitting on end of chamber where valves come out. Might be easier to use end cap and cut hole for 3/4" valve, but worried about 300 psi integrity.

Sounds like a plan.

How do you plan on getting to 300 psi?

Using helix 200 psi valve for majority of shots. And for higher psi get a hand or shock pump.

Think I'm gonna go with end cap and insert a 3/4 nipple for valve attachment. Should I just have machine shop cut a hole for it and thread hole or have them weld it in. What is easiest and strongest way?