Here is a diagram showing the components of the DSV, including the three O-rings.
You will notice that I am showing the preferred O-ring and at least one option to account for possible tolerances on the clearance and fit.
Let start with the wagon wheel valve O-ring (Item # 6 in the diagram):
The design nominal O-ring is an SAE -027 (in either 70 or 50 durometer).
If there is a loose fit then the metric 2mm x 33mm will provide a tighter fit. In this case I recommend using the softer 50 durometer O-ring.
The mouthpiece opening O-ring (Item # 9A in the diagram):
The design nominal O-ring is a SAE -028 (in 70 durometer).
If it feels loose or there are any signs of a leak then the metric O-ring will provide a tighter fit.
The recommended metric O-ring is either a 2mm x 35mm (or a 2mm x 34 mm works and it can be easier to install).
This O-ring slides past the mouthpiece opening in the outer tube (Item # 3). So make sure that the inside edge od the opening has a smooth chamfered edge, not a sharp edge. After assembly, operate the DSV and check that the O-rind stays in place, in the groove, while fully opening and closing the DSV.
The two circumferential inner tube O-rings (Item # 9B in the diagram).
These O-ring can be either a -030 or a -029.
Either one works fine since this is not a sealing O-ring. These O-ring are not intended to create a seal, they are intended to create a constant gap between the two cylinders and create a bearing surface for sliding, with just enough friction so that the DSV has to be intentionally rotated. The gap is designed so that the two cylinders are do not slide and scrape on each other.
This O-ring are intended to keep sand or dirt from getting in between the tubes, but it is not intended as a water tight seal. The space between the tubes is a wet space and it has to be a flooded space.
There is a vent hole below the mouthpiece opening in the outer tube (Item # 3). Any attempt to seal the space between the tubes to keep it dry, by plugging this vent hole is a very bad idea. That could create a sealed void that will not equalize with the ambient pressure as you dive. Please, never consider closing the vent hole with a plug. It is not a good idea to create a sealed void.
The vent will also allow you blow the little amount of water in the mouthpiece tube before opening the DSV. By blowing the water from the mouthpiece tube, you can keep the hose-loop dry.
To disassemble the DSV:
Remove the retainer screw (Item #11) and rotate the inner in the closing direction and little past it so that the opining in the inner tube is far from the opening in the outer tube. Then slide the inner tube sideways. Only slide the inner tube sideways when the opening in the inner tube is far from the Opening in the outer tube, to avoid pinching or cutting the O-ring.
To assemble the two tubes just revers the process. Make sure the opening in the inner tube is not lining up with the opening in the outer tube. The inner tube opening should be facing up, near the label.
Lubricate all O-ring with good quality silicone grease. I do not recommend using any of the oxygen compatible grease. They do not work as well for this application and they are not needed.
