SpudFiles

I've built a double o-ringed piston that will be housed in a 2" galv. iron barrel nipple, I need a 100% seal. The barrel nipple I have had a seam, as you would expect, so I removed the seam with a dremel. To cut a long story short, when the piston is inside the barrel nipple it is a pretty tight fit except for where I have removed the seam. At the seam I can see a very small amount of light coming through if I hold it up to a light in a dark-ish room.

One fix would be to find a barrel nipple without a seam. Two problems here: I can't find a barrel nipple without a seam and I would probably need to rebuild the piston since the inside diameter of the fitting could be different.

So if under pressure, will the o-ring deform enough to seal the gap where the seam was? I don't think getting a thicker o-ring would work since this piston is pretty hard to push through the barrel nipple as it is without any lube.

edit: I could change the o-ring seal from a static seal (fixed o-ring) to a floating o-ring seal if it would help.

If the piston is a floating o-ring then it might deform over the gap but if it isn't then I don't think it will work.

Sand the entire seam area smooth so there are no narrow deep features.

So if the seam area is tenths of a millimeter deeper than the rest of the surface, if it is smooth the o-ring will push out and fill the gap?

MrCrowley wrote:So if the seam area is tenths of a millimeter deeper than the rest of the surface, if it is smooth the o-ring will push out and fill the gap?

Yes it will under higher pressure. One of the reasons my valves have ports cut into the pipe instead of just a gap in the chamber is because an earlier version simply removed the floating ring off the end of the piston each time it was shot. It as no fun looking for the o ring after each shot. Floating o rings tend to lift under air pressure enough to clear the groove at about 30 -50PSI.

I had a piston with a nose with a ring like the Mouse Musket. The nose extended into the barrel to plug it. Lost o rings was the norm.

If you look at the bottom edge of the photo below inside the pipe, you will see the groove where the pipe seam is located. It is just right of the 6 o clock position. It is now a smooth wide valley and very shallow. It seals fine. This camera angle tends to hide it. I was showing off the polished pipe and piston clearance, not the seam of the pipe.

Tenths of a milimeter? I'm tempted to say yes, simply based on the flexibility of most o-rings and that minuscule distance.

I'll go with Technician on this one. If the dent/groove thingy isn't to abrupt, the o-ring has a chance of sealing it up when it is pushed against the groove by the pressure. If it's too abrupt, the flexibility of the o-ring won't be enough to fill the gap.

I say just keep trying to sand down the seam area, using high-grit sanding material.

I don't have photos of the early piston that lost the o ring all the time. This is a photo of the generation following it. It did not come out of the barrel. The barrel was ported so the o ring simply moved back past the ports. The ports cause a huge restriction, so the performance sucked, but it kept the o ring. The Mouse Musket and ABS cannon followed this early valve. This valve worked, but barely. The black is Graphite. I tried to use it as a lube. FAIL

Okay, I'll give sanding a go this afernoon. I wont know if it works for another few weeks probably.

Attached is a photo of the barrel nipple. The red-orange area is where the seam was but as far as I can tell, is raised and not pitted. The yellow-orange area is where the seam was but appears to be pitted, i.e. has a surface deeper than the original inside diameter.

The red-orange area is where I will work on smoothing out to match the depth of the rest of the surface. The pitted area I will try and smooth out, though I am not too worried that it is pitted because this will be near the back of the valve with the o-rings being in front of this area. Once the o-rings are back behind this area, the valve will be open and the 100% seal wont matter anymore.

Mr C,
If the seal is designed to travel within the cylinder, it's a dynamic seal. A dynamic o-ring needs a smooth surface in order to seal and survive. You could have a machinist do a cleanup bore on the nipple or....... try using a cylinder hone (from an auto parts store) coupled with a drill motor.

Ah, dynamic! That was the word I was looking for. Thanks for the tips, I'll look in to it if I can't get it to work.

A sanding flap wheel works wonders for smoothing the valley to the honed surface. To hone the surface, use a sheet of stick on sand paper and stick it on a piece of 1.5 inch ABS pipe. It makes a wonderful sanding block. A saw cut can make it expand and stuffed with paper towels to adjust the tension, it does a nice job. It is how I am doing my pipe.

Start with 80 grit and then 120, 300 & 600 to finish.

Thanks for all the help. Tech, I just tried the sanding flap wheel on my dremel (although, after I had already done all the hard work) and it worked pretty well. The sanding made it really clear where the pits are (seems to be about 3 or 4 near one end) and the seam is virtually gone and I can't feel it when running my finger over it.

The piston fits bloody great with the help of some vaseline. Can't see any light coming through (tested before I used the vaseline) at all. Here are some pictures. In the last picture, the seam is at the 6 o'clock position. You can see the pitting at the back of the nipple in this photo. In the other two photos the seam is at the 9 o'clock position.

Glad to help. You can either sand down everything to the bottom of the pits, or use epoxy to paint it and then sand it smooth. Make sure it is oil free before using epoxy.