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Has anyone here attempted to use a two piece lapping technique to mate gas gun main valve sealing faces? If the process can be done well enough to produce an adequate seal, it would appear to have excellent potential for improving the state of the art in hybrid gun design - the main limitation on more energy-dense piston-valved hybrids is failure of sealing face materials, and on the face of it, it doesn't seem as though it would be terribly difficult to remove them altogether.

In particular, I'm curious as to the proper technique. This is a fairly low pressure cutting operation, and pressing the parts together evenly while spinning (thus allowing for conical sealing faces) appears to be a major concern. Your thoughts?

You mean like one seal on the barrel, and one on the piston? Sounds like it would work. I don't see why it wouldn't.

You mean like one seal on the barrel, and one on the piston? Sounds like it would work. I don't see why it wouldn't.

no he means no seals at all

Oh, well it has been done before right? JSR has done it I think.

the idea is to take the piston and the barrel and rub together for so long that there would be no need for a seal - as they would perfectly fit each other

so there would be almost no leakage

The closest to this I have done was fitting the piston in the Mouse musket. Even though the valve seat had an o ring, the piston face was first sanded flat, then lapped against the rubber o-ring. This was to provide a very flat seal so it did not require a spring to close the gap to add initial pressure. The non rubber part that was also hand lapped was the piston OD against the cylinder. This part sealed so well with a thin layer of grease, that I had to add an EQ port in the piston as it sealed too well even without a rubber seal.

Don't attempt to lap by just spinning the part. This will give it wear grooves. Lap the parts in a back and forth motion and separate the parts often to re-distribute the abrasive.

Lapped piston. The sharpie marker marks show the area worn in the lapping process on the piston OD. This part was lapped into the cylinder until it entered without sticking. This lapped both the piston and cylinder ID to a precision fit and removed all out of round from the parts. This left the piston and cylinder round with a close tolerance fit.


Metal on metal lapped seals are often used in water and air check valves, butterfly valves, gate valves, etc. i would expect some slow leakage, but it could provide a high degree of sealing.

Utron used this in their poppet seal valves for cryogenic fluid injection... I plan on using a similar idea for the proposed semi-auto hybrid with gaseous injection method.

I've lapped automotive valves before and the before and after compression test results were amazing regarding pressure bleed off. It's not too hard - just read up on poppet valve mating surfaces - there's a whole science to the initial machined degree of the mating surfaces - often the poppet will have a 2 or 3 step angle to it so that only a small portion is actually sealing. This aides in the lapping process so that your not removing a lot of material.

MC wrote:often the poppet will have a 2 or 3 step angle to it so that only a small portion is actually sealing. This aides in the lapping process so that your not removing a lot of material.

In automotive machining this in known as "interference angle". It also allows for over all maring/peaning of the mating surfaces to remain a "crush fit".

The thing about auto valves is they have strong springs holding and aiding in the closing of the valve. In a Hybrid the pressure has to open the piston not a mechanical set up like in automotive use. Poppet valves are mechanically sealed as well.

After the obvious being said I think the way the Hybrid piston works it would have to be tapered fit. The face of the piston would be a point or dome top depending on how much sealing surface you want.

I would consider using a synthetic material like nylon or delrin for one of the parts, as used in HPA valves: http://www.spudfiles.com/forums/viewtop ... tml#338041

I would consider using a synthetic material like nylon or delrin for one of the parts, as used in HPA valves

Acctually I expected you'd suggest this... not a bad idea

However I am not sure if it's really lapping in this case (though it would probably work better)... the material is more like rubber than steel so the two surfaces do not have to perfectly fit

Worked well face-to-face at 150 psi, see the cartridge thread

Incomplete combustion could leave carbon residue on the sealing face, which could be enough to cause leaks if it builds up. In an IC engine it's not something that would be noticeable, given the rate at which the system operates, plus all the moving parts and oil circulation tend to be self-cleaning. I have noticed carbon buildup in my past hybrids, and I know others have as well.
It might be possible to minimize buildup by recessing the lapped seal back into the piston. It would become a low-turbulence zone, so most of the grit would not have a chance stick there. It would also help retain grease or oil between shots.

what is a metal like lead (soft metal) as a sealing face? would it be better for seal metal to metal?
what forces would i need to "dip in" the barrel/transferport in a sealing face out oft lead? i thought about cutted seals out of rubber trouh the shar barrel.