What am I doing wrong here?

[maverik94]

Ok, my latest project is building an in-line (non-coaxial) piston valve. I have it completely built, except for one problem...it won't seal. It's essentially this design.
Here is a picture of the sealing face of the piston:

(Yes, i used the macro setting for this picture, but it still didn't turn out too well, sorry.
And here is a picture of the area the piston is sealing against:

My question is, why isn't it sealing? Any ideas?
Thanks!
If you need any more info or pics, please don't hesitate to ask!

[Gun Freak]

Well the rubber looks like it has something on it, and maybe the seat isn't cut evenly?

[maverik94]

Well the rubber looks like it has something on it, and maybe the seat isn't cut evenly?

That's just Vaseline on the rubber, and the seat is prefabricated. It's a brakeline fitting actually. i mean, i suppose can take a look at that seat and see it's nice and clean.

[Gun Freak]

yeah try cleaning averything off.

[reedfe]

Well the rubber looks like it has something on it, and maybe the seat isn't cut evenly?

yes it looks like glue but then again you said Vaseline and i recommend the same thing. also your sealing face looks almost as if it would slide into that silver busing in there. but i can't tell from the pictures.

[jrrdw]

The seal looks a little off center, witch can allow the piston to slip in a little to the side thus loosing the sealing area. That's my guess. If the center bolt where big enough around to keep the seal centered it might seal...

[Technician1002]

Any distortion of the rubber may wrinkle the surface preventing an initial seal. I found wetting the rubber with a water based marker (coloring marker) and quickly assembling it will show spots that are not touching. Proper use of a valve lapping compound can often fix some surface flaws. Low speed on an electric drill and time can wear the surfaces together in a perfect seal. I hand lapped the valves on my Mouse Musket and Dragon cannons to get them to seal.

[mark.f]

How close does the piece holding the rubber seal on fit inside of that sealing face? Can't really tell what it is (by the way, your camera was working fine, it just focused on the top of the bolt. )

If it's a close fit the piston can turn slightly askew in it's track and the washer/nut/whatever holding the rubber on can bind on the sealing face.

[Crna Legija]

maybe the air is going between the bolt and rubber
how did you attach the nut to the barss fitting mabe going that way

[Technician1002]

Just a quick note on Macro photography with autofocus digital cameras, use the half press to hold the focus (locks) and then re-adjust the distance to focus on the portion you want sharp and finish taking the picture. I took pictures of my valve inside pipe that way.

I moved back and let it focus on the end of the pipe, then locked focus (shutter 1/2 press) and moved down to focus the seat and took the picture.

[maverik94]

also your sealing face looks almost as if it would slide into that silver busing in there.

No it doesn't, it sits on that..it's just not to scale

The seal looks a little off center, witch can allow the piston to slip in a little to the side thus loosing the sealing area. That's my guess. If the center bolt where big enough around to keep the seal centered it might seal...

Nope, it's on center, and it's tightly on there

Any distortion of the rubber may wrinkle the surface preventing an initial seal. I found wetting the rubber with a water based marker (coloring marker) and quickly assembling it will show spots that are not touching. Proper use of a valve lapping compound can often fix some surface flaws. Low speed on an electric drill and time can wear the surfaces together in a perfect seal. I hand lapped the valves on my Mouse Musket and Dragon cannons to get them to seal.

Ok, I'll give that a try.

How close does the piece holding the rubber seal on fit inside of that sealing face? Can't really tell what it is (by the way, your camera was working fine, it just focused on the top of the bolt. )

The rubber sits on the sealing face, not inside it.

maybe the air is going between the bolt and rubber
how did you attach the nut to the barss fitting mabe going that way

It's pretty tight, and it is attached by a ground-down nut.
Just a quick note on Macro photography with autofocus digital cameras, use the half press to hold the focus (locks) and then re-adjust the distance to focus on the portion you want sharp and finish taking the picture.

Yeah...I did that, but it was still giving me crappy results. oh well.

[maverik94]

Edit: Dammit. Sorry, didn't mean to double post. Sorry

[mark.f]

The rubber sits on the sealing face, not inside it.

I know that, I was asking about the part that holds the rubber on your piston. How close of a fit is it to your sealing face/barrel port?

[maverik94]

Its a ground- down nut. It fits well..not tightly, but not overly loose.

[mark.f]

And how is the fit of the piston in the pipe it slides inside of? A loose fit or short piston (the part that contacts the pipe walls) could cause the piston to "wobble" and that ground-down nut to bind on the sealing face.

If this seems to be the case, you could improve the fit of the piston or increase the length that contacts the pipe it slides inside of. Another potential solution it to grind the nut into a cone shape so that it won't bind on the sealing face and will instead center the piston on it.