Assembling Double 12l Steel Cylinders

We have recently had to build a new set of Doubles for a friend and thought it might be useful to document the process for those attempting this for the first time. If you are at all unsure about doing this, take the components to your local dive store and have them assemble them for you.

Step 1: The Components
We ordered two Faber 12l Steel cylinders with no boots and no valves. By ordering them with no valves from the manufacturer they should in theory arrive O2 clean but we had them cleaned at the shop prior to picking them up. Additionally, we ordered a Halcyon 300BAR DIN Isolator manifold (3/4" Thread) and Halcyon 18.4cm diameter tank bands.

It is beyond the scope of this article to argue the pros and cons of the varying methods of setting up double cylinders. Suffice it to say that the DIR community at large has settled on the 300BAR isolated DIN manifold as the manifold of choice.

The bands are the 3" Highland-Mill type which have been proven to provide the best possible support for dual systems. When choosing a band, ensure that the centre of the band, where the bolt binds the band to the cylinder when tightened, has been sufficiently reinforced so that it will not deform when torqued.

Step 2: Disassemble the manifold
The first job is to carefully disassemble the manifold so that the left and right post valves are free of the isolator. Be careful to maintain the orientation of the isolator so that you don't inadvertently attempt to re-connect it backwards and cross the threads. The right side of the isolator is reverse threaded, if the isolator is reversed it will not screw into either post, but inadvertently trying to force it can damage the delicate threads and render the whole manifold useless. The right post on the isolator has a notched nut to help identify the correct orientation.

Step 3:
Attach the drain plugs (dip sticks) to the cylinder valves. These are important as they prevent debris or liquid ingress into the valves and impeding airflow in the event of the cylinders being inverted.

Step 4:

Sparingly lubricate the threads of the valves with Oxygen compatible grease. Attach the cylinder valves to the cylinders, when it sets as firmly as you can set it by hand, use gentle taps with a rubber mallet to fully seat each valve.

Step 5:

Lightly grease the isolator's internal O-rings with Oxygen compatible grease if required.

Step 6:
Offer up the isolator to the cylinder valves and push together the cylinders to close the gap until the threads meet on both sides. Ensure that the cylinders are aligned in parallel to one another.


Step 7:
Carefully begin to rotate the isolator ensuring that there is even pressure on both sides until you feel the threads connect. It is important that the threads engage on both the right and left sides at the same time. Frequently one side will catch before the other, which creates an uneven fit. You may have to back the isolator off a couple of times to ensure that both posts engage evenly.


Step 8:
Slowly rotate the isolator into the cylinder valves, the threads will tighten as the cylinders are drawn together and pulled out of parallel so gently squeeze the cylinders together to realign them as you proceed. Eventually you will reach a point where the isolator will no longer rotate a full turn, at this point align the valve knob to its desired final position.


The gap between the right and left post valves and the isolator should be equal on both sides. Note the notched nut on the right side of the isolator valve.



Step 9:
Disassemble the tank band bolts, fit a bolt to each band and lightly secure with a washer, a split washer and a nut. Carefully slide the bands onto the cylinders from the bottom ensuring that they remain in parallel and you do not stress or rotate the manifold.


Raising the tail of the cylinders on a small strip of wood helps facilitate moving the bands down the cylinders and into position.


Step 10:
Align the top band with the start of the shoulder of the cylinder, right on the point where they begin to curve away to the neck. Secure the bolt with a spanner and gradually tighten the nut. Stop periodically to ensure the the cylinders remain in parallel and that the isolator valve can rotate freely and is not being strained. Tighten till moderately tight such that the cylinders will not shift when being handled.


Step 11:
Once the top band is secure prepare the second band as you did the first. Slide the band up the cylinders until the bolt head is 11" on centre from the top band bolt. Use a backplate to correctly position the bottom band along the cylinders.

Secure the bolt and gradually tighten the nut as before, ensuring periodically that the isolator valve remains free to rotate. Also periodically offer up the backplate to the bolts to ensure that they remain the correct distance apart.

As the band tightens make any final adjustments to it position with gentle taps of a hammer.

Step 12:
Once the backplate fits do the final tightening of both bands. Tighten till snug and then rotate one or two extra turns. Remember that the cylinders are empty and will expand when filled. Over tightening the bolts will cause them undue stress. Ensure there is still free movement of the isolator and that the threads are not under stress. Note: the isolator should be able to move freely. Do not tighten the locking nuts as this will impair the ability of the isolator to rotate slightly if it comes in contact with an object during diving, thus increasing the likelihood of damage.

Reassemble the remaining bolt hardware and spray the assembly with marine lubricant to lubricate and protect it.


And that's it, you now have a beautiful new set of DIR doubles all ready for their first fill.