How to Assemble a Set of Doubles

By: Dillon Waters

In selecting equipment for a planned dive, the technical diver’s first concern will be his supply of breathing gas, based upon the requirements for a particular dive, as well as a suitable reserve for contingencies. Technical diving may involve extended bottom times, perhaps at relatively deep depths, with additional time spent underwater during mandatory decompression stops. Sometimes a diver will be able to meet his gas supply requirements with a single high-capacity cylinder, though more frequently he/she will opt for double cylinders. In addition to volume, double cylinders offer an advantage to the technical diver in terms of redundancy. (TDI)

Assembling your own scuba gear can be a very rewarding and satisfying experience, but it is important to remember that there is no substitute for a trained professional. Until you are very familiar with the process, it may be wise to seek the assistance of one when assembling your double cylinder setup.

Step 1: Making sure you have the correct materials

  • 2x Matching Cylinders
  • Tank Bands (appropriate for cylinder diameter)
  • Manifold
  • 2x ½” Wrenches (or adjustable wrenches)
  • Bolt Kit
  • Tape Measure
  • Backplate, Harness, and Wing

Step 2: Preparing Your Scuba Cylinders

If the scuba cylinders you will be using aren’t new, you may need to do a few things to prepare them before moving forward. If your cylinders already have valves on them, they will need to be opened to ensure the cylinders are free of compressed gas and then removed. If there are any stickers that may lie between the bands and the cylinder, they must be removed as well. Be certain that your cylinders have been appropriately inspected by a certified professional and have a current hydrostatic test before moving forward.

doubles setup parts

Step 3: Preparing Your Manifold

Make sure the manifold is broken down into its three components; one left post valve, one right post valve, and a crossbar with an isolator knob. Turn the lock nuts (one will be notched noting that it has reversed threading) on the crossbar until they are hand tight against the inside of the body. Then, inspect the o-rings on the valves and the crossbar; replace and coat with oxygen-compatible lubrication if needed.  Be certain that your manifold has been appropriately serviced/prepared by a certified professional or the manufacturer before moving forward.

Step 4: Installing the Valves

Take one of your post valves and turn it clockwise into the threaded neck of your scuba cylinder. You should tighten the valve as firmly as you can by hand, making sure not to over tighten and crush the o-ring. Then, repeat the process with the second post valve and cylinder. Once both valves are installed, lay the cylinders down parallel to one another with the valve orifices facing up.

Doubles connection assembly
o-ring lube

Step 5: Installing the Crossbar

Doubles o-rings

Carefully place the crossbar in between the two post valves with the notched lock nut facing towards the right post valve (left/right posts will be referenced by their position when worn on the diver’s back). Then, slowly push the tanks closer together until the threads meet on both sides. It is extremely important to make certain the tanks are parallel to one another so that the crossbar sets correctly.

Doubles connection close

Next, begin to slowly rotate the crossbar, making sure there is even pressure on both sides, until you feel the threads connect. It is a common thing for only one side of the crossbar to connect – if this happens, just back the crossbar out and try again. Make sure both sides set evenly when threading.

Once the crossbar threads both tanks evenly, continue slowly turning the crossbar. This will bring the tops of the tanks together. If you feel ANY resistance while turning the crossbar, stop and gently tap the cylinders back together until parallel, then continue to turn. Do this each time you feel resistance.

Making sure doubles connection is tight

Continue to thread the crossbar until about 1/8-inch of the threads are left showing on each side, or until the isolator can no longer make a full rotation around. This may differ depending on the tanks, bands, or crossbar you are using. You may need to slightly loosen or tighten when adding the bands.

Doubles connection

Step 6: Installing the Bands and Bolts

Begin by disassembling the tank band bolts. Place a bolt in each band and then lightly secure it with a washer, a locking washer, and the nut.

doubles brackets

Now, slide the first set of bands up from the bottom of the cylinders. Be careful not to stress or rotate the manifold and ensure that the cylinders remain parallel. Slide them all the way up until the top of the band aligns with the portion of the tank right before the curve of the shoulder begins. Next, add the bottom band just as you installed the first. Position the bottom band so that the center of the bolt head is 11 inches from the center of the top bolt head.

Doubles tank setup

Once the bands are on, inspect the position of the cylinders inside the bands, they should be centered. If they are positioned or angled towards the inside of the band towards the bolts, then you will need to loosen the isolator while repositioning the cylinders. If they are positioned towards the outside of the bands, then the isolator needs to be tightened.

Before tightening the bands, place the backplate that you will be using on the tank bolts to ensure proper fit. If the bolt heads do not align with the holes in your backplate, you will need to adjust the bottom band.

Doubles brackets

Using your wrenches, loosely tighten the nut and bolt of each band, while periodically checking to ensure the isolator can still rotate freely and the tanks are still parallel. Tighten the isolator and reposition the tanks as necessary.

Doubles Bracket wingnuts

Step 7: Final Tightening

Once you are sure that the bands are in the correct position, you may tighten and secure them into place. Tighten the nut and bolts until snug and then turn once or twice more. Do NOT over tighten the bands –  the cylinders will expand when filled and put stress on the entire setup.

Position the isolator valve knob in its final desired position – this may be straight upward or angled slightly forward towards the diver.

Checking doubles connection

Tightening of the isolator bar is a personal preference. Many divers like to leave the locking nuts loose, allowing the isolator to have free movement to reduce the chance of damage if it comes into contact with anything during the dive. Many others prefer that the locking nuts be tightened to secure the isolator bar, keeping the isolator knob in place so that the location is always known when diving. Make your decision based on your personal preference and on the environment you will be diving.

Doubles valve knob tightening

Finally, add the wing nuts to the bolts for storage, then stand back and admire your craftsmanship. Your new set of doubles is now ready to be filled.

Doubles backplate


Krzeszowski, F. (n.d.). TDI Introduction to Tech: An Overview of Theory, Tools, and Techniques (Vol. 0816). International Training.

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6 replies
  1. Alan Fastner
    Alan Fastner says:

    Dear european readers

    ‘Step 4: Installing the Valves’ is described wrongly.
    With M25 thread the tightening torque is defined between 80 and 100 N*m and NOT hand tight!

    Best regards

    Alan Fastner
    SSI TXR Instructor 62184

      SDI/TDI/ERDI says:

      Hi Alan,

      Thank you very much for the clarification on the M25 thread valves for our European readers! Each different type of valve will have its own torque specifications and we recommend checking and adhering to those when installing any valve or manifold.

      • Fernando Escobar
        Fernando Escobar says:

        80 Newtons-meter = 59 Pounds-Foot / 100 Newtons-meter = 73.76 Pounds-Foot for the material with which the parts are manufactured seems to me exaggerated.

        You have to find out that data.

      • Brian Ferguson
        Brian Ferguson says:

        However your article basically states that valves should be “Hand Tight.” Can you provide the cylinder and valve manufacturers that state “hand tight” is correct versus providing specific torque specifications?

  2. Ted Reitsma
    Ted Reitsma says:

    Very nice and detailed article. As the premise is Deco time/extended range, then I much prefer side mount. There are a few benefits of this over doubles. One is I can lift 2 individual tanks where I can’t lift doubles. Second is that you can see the cylinder &first stage and any issues (bubbles) with it. You can feather much easier if have free flow. You can fit into tighter areas with side mounts as you can take one/both off and put in front of you. But each his own. You may want to mention NEVER lift doubles by the manifold.

    • Eric Batrex
      Eric Batrex says:

      I prefer no mount. You have all the benefits of side mount that Ted brought up even though this article has nothing to do with sidemount, and you can squeeze through portholes, small squeezes that would make the rec diver cringe, and trade tanks with your buddies like baseball cards while diving. But each his own. You may want to mention only real men backmount.


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