Aqualung valve seat difference?

[Ron]

Hey all,

I know that USD made two valve seats.

#501-09 is in the J-60 single valve, the K60 single valve, and the K63 single valve.
#525-19 is in the J-62 doubles manifold and the J63+ single valve.

I can see that visually one has a little hole in the seat, and one does not. Does anyone know what the heck the difference is for? I have a bunch of spare parts, and I'm just curious because I do not want to mess one of these valves up if the seat difference is significant. Plus I do breathe through them underwater, so there's always that.

[antique diver]

Ron, the one with the hole in the seat is for the J-valve. It may have had something to do with relieving air pressure to prevent the seat from extruding from the threaded carrier... BUT over the years I have seen a number of these extrude when opening the valve, resulting in at least a restriction of air flow. I never saw one of the K-valve models (without the hole) do that.

Back then I really never sat down and figured out the exact cause of the problem, but now is as good a time as any to think about it. Let's figure that out...

[Ron]

Not all of the J valve seats have that hole though. Now we could say that people could have rebuilt them without the hole seat variant previously, but the catalog has some J valves with the non-hole part number in it. You said that the hole was for pressure relief? What would cause seats to extrude? Isn't there tank pressure acting directly against the seat via the brass volcano orifice? Maybe you could break it down barney style for me.

[antique diver]

I never sat down and figured out why the hole was there, or why those seats would extrude at times. Let's all get out heads together and figure this one out.

Did you find a year where they first used the seat in question?

[captain]

Could be the hole is one of those seems like a good idea at the time thing but in actually wasn't needed and was later eliminated.
Prior to the SPG almost everyone used a J valve so that seat might have been what everyone was most familiar with. I have never used or worked on a K valve from that era and maybe they also had a hole.

[antique diver]

I have only seen the seats with holes installed in J-valves, and only the solid ones in K-valves. Worked on a lot of USD valves in the dive shop during 1970-2000 and a few since then.

I'm pretty sure that the hole had something to do with preventing air pressure from getting trapped under the soft seat, but if that was their purpose, I think they made a mistake. During the filling process, if you forgot to open the cylinder valve first, and only then open the fill valve, a fair number of these seats would extrude part of the way from the threaded brass carrier... resulting a very restricted or even stopped air flow throught the cylinder valve and into the cylinder. However, that can happen, but rarely, with other valves. I always trained our staff members to open the cylinder valve first to avoid that problem.

Come to think of it, the hole in the middle may have allowed the seat material to eventually deform/crush inward toward the center more than the solid seats... resulting in a tiny decrease in outside diameter, thus letting the seat get a little loose in the carrier. Could that explain their increased propensity for extrusion?

[Ron]

It seems like the safe bet then is to use the one without the holes. Trident, which Bryan carries, stocks the USD style valve seat. They are the ones without the holes. This means that we can rebuild our valves to "like new" condition when we choose to do so. Great news.

[antique diver]

You said that the hole was for pressure relief? What would cause seats to extrude? Isn't there tank pressure acting directly against the seat via the brass volcano orifice? Maybe you could break it down barney style for me.

Ron, Sorry that I missed that question in the above rambling thoughts.
Anyway, I think that the hole in the seat was a big mistake.

During the filling process,if the fill line valve was opened first, and the seat was not a good tight fit and seal into the carrier, high pressure air from the fill side could get under the seat from the outside edges. When the cylinder valve opened, the high pressure air under the seat could push the seat out since the lower air pressure in the cylinder could not push against the smaller area of the inner part of the seat.
(This really has nothing to do with that strange hole, since the same can happen with any seat that is old enough or crushed enough to slightly loosen in the carrier)

Now for a little more about that hole... I only mentioned extrusion during the fill process, but I have seen at least three or four of these seats that were old and/or badly deformed extrude upon opening the valve of a full cylinder with regulator mounted. The only way I can figure that could happen is due to the danged hole in the center. That hole could readily allow the high pressure air to get behind the loose-fitting seat, and push it out of place during the opening of the valve. I first discovered this on an old Mexican live-aboard back in the late 70's. One of our divers opened his valve (possibly only partially), didn't test his reg, and hit the water expecting air about the time he was 10' under. I was able to get that to repeat, then took the valve apart and confirmed the problem.

I'd sure like to hear from anyone else that has experienced a similar failure.

[Ron]

It sounds like some hole-less seats may be in order. I might buy a bunch from Bryan and just rebuild my valves. They all have original seats in them, plus that way I can track when the last time was that I rebuilt them and changed out the burst discs.

[captain]

There is also a difference in lengths between the K and J valve seats but I think the difference is only for the side knob valves and not the early top wing nut valves.