VM-68 Black Magic cup seals

[wthomas333]

Wanted to give this a try since it was mentioned in another thread https://www.mcarterbrown.com/forum/b...rsal-cup-seals. I don’t have these for sale or anything, just made a few for myself and wanted to pass along the information.

I measured the valve stem and bought some stainless dowels that were supposed to be about the same size from McMaster. In a couple of my valves these worked just fine, but in another it was kinda snug. I ended up cleaning out the little brass fitting inside the valve body by hand using a drill bit and that helped remove any little burrs or whatever was causing that friction. https://www.mcmaster.com/90145A495/

Sent them off to BMC to be properly threaded. Had him trim down 2 for testing purposes.
Overall length is 1 3/8”
8-32 threaded portion is 1/4”
Two I had trimmed down are narrower from the base of the threads to .60” from the threaded end. That includes the threads. There is no reason behind this length other than it seemed to make it so the shaft was narrow all the way into the valve body when the hammer sits flush against the other side of the valve.

I had to drill out the cup seal a little bit because the hole is pretty tiny. I guess you could get the stems turned down to 6-32 and it would probably work well. Nonetheless, I got it all put together and it sealed and fired. Took a few dozen shots and all seemed fine. Once the weather warms up I’ll try and do more thorough testing between the different stems and update it here. I was hoping it might help a little with HPA, so we will see.

[Myrkul]

I'm curious to see how these do long term. Personally I've had really bad results running 950 HPA on those black magic cup seals. Then tend to try and escape the valve by any means necessary, usually taking the valve stem with them, at least at those pressures. It could just be age, or it might be the extra bit of pressure.

Anyway, I'm curious to see your results.

[wthomas333]

I am trying to get my VM up to velocity using 800-850 psi. So maybe that will prevent any extra wear on the seal. Who knows, but we will find out.
I still don’t think I will be able to reach velocity at that pressure without matching the bore to the paint though which is why I just bought a VM freak back from Doc.
With the few test shots I did yesterday with paint it was shooting in the 240s with a .690 barrel and small sub .670 paint.
I may also test with a standard Spyder style cup seal since those are already threaded for 8-32.

[Carp]

Wanted to give this a try since it was mentioned in another thread https://www.mcarterbrown.com/forum/b...rsal-cup-seals. I don’t have these for sale or anything, just made a few for myself and wanted to pass along the information.

I measured the valve stem and bought some stainless dowels that were supposed to be about the same size from McMaster. In a couple of my valves these worked just fine, but in another it was kinda snug. I ended up cleaning out the little brass fitting inside the valve body by hand using a drill bit and that helped remove any little burrs or whatever was causing that friction. https://www.mcmaster.com/90145A495/

Sent them off to BMC to be properly threaded. Had him trim down 2 for testing purposes.
Overall length is 1 3/8”
8-32 threaded portion is 1/4”
Two I had trimmed down are narrower from the base of the threads to .60” from the threaded end. That includes the threads. There is no reason behind this length other than it seemed to make it so the shaft was narrow all the way into the valve body when the hammer sits flush against the other side of the valve.

I had to drill out the cup seal a little bit because the hole is pretty tiny. I guess you could get the stems turned down to 6-32 and it would probably work well. Nonetheless, I got it all put together and it sealed and fired. Took a few dozen shots and all seemed fine. Once the weather warms up I’ll try and do more thorough testing between the different stems and update it here. I was hoping it might help a little with HPA, so we will see.

[Ecapnation]

I've had in my parts bin a very similar piece for years.

Basically a stem that allowed the use of nelson cup seals.