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Assembling Twin Cylinders

Twin cylinder setHere is the recommended procedure to assemble a set of doubles using any modern design double O-ring style manifold and cylinder bands. Assembling a twin cylinder set is easy, and the process should take about 30 to 60 minutes if this is the first time you have assembled doubles. We recommend the actual assembly procedure for your twins be performed by only one individual, to avoid the possibility of working at cross purposes.

The tools required for 5/16"-18 thread bolt kits are an open end 1/2-inch wrench, plus a 1/2-inch box-end wrench. (Many who assemble often prefer to use a 1/2-inch hollow shank nut driver, but a wrench works fine for occasional assembly.) If you need to remove the port plugs from the modular valves, you will also need an adjustable end wrench (or slip-joint pliers) with approximately 1-inch jaw capacity.

  1. Drain any gas pressure from the cylinders until both are completely empty. Remove the cylinder boots and any unnecessary decals or stickers from sides of cylinders and from inside the tank bands. Make certain no decals, tags, tape or other material will be trapped between the bands and the tank.
  2. Disassemble the manifold into its three primary components: right and left outboard valves, and one centre bar with isolation valve. Make certain the centre bar lock nuts are hand tight against the body and note the orientation of the notched nut and its valve. Install one outboard modular valve into each cylinder, as tight as possible BY HAND ONLY, avoid use of wrenches. The DIN-to-yoke adaptor inserts should remain installed in 200 bar valves during doubles assembly. Improper use of a wrench during installation of the valve can warp the opening of the valve face, particularly with 300 bar valves.
  3. If the manifold ports on the cylinder valves have been plugged, then the plugs will need to be removed. The manifold port plug is the large hex head nut opposite and in line with the valve hand wheel. Use a small adjustable wrench to remove the manifold port plugs from both of the modular valves. Be sure to retain the port plugs should you ever wish to reconfigure the cylinders as singles. IMPORTANT: The modular valve with the hand wheel on the typical side has a manifold port plug that is notched to indicate that it turns in the clockwise direction to remove. The other modular valve manifold port plug turns in the usual counter-clockwise direction to remove.
  4. Put the cylinders on a table or flat working surface, placing them parallel to one another and with the valve orifices facing upward.
  5. Carefully orient the centre bar so that its threads correctly match those of the outboard modular valves. IMPORTANT: Serious manifold damage may result if the centre bar is not correctly oriented. The notched lock nut on the isolator (indicating threads that turn opposite the normal direction) will be on your left as you face the opening of the valves, i.e. on the diver's right when the double cylinder set is on the diver's back.
  6. Slowly turn the centre bar in the direction that causes it to thread itself into both outboard modular valves simultaneously. IMPORTANT: If one side does not engage, you must back the centre bar all the way out and begin again. Be patient. This will almost certainly require several attempts.
  7. When the centre bar threads engage properly, turning the centre bar draws the tops of the cylinders together. To keep the cylinders parallel to one another as you turn the centre bar, stop periodically to gently tap the bottom of the cylinders together. You can tell when tapping the cylinders together is necessary because the centre bar becomes difficult to turn when the cylinders are no longer in proper alignment. (This also helps explain why it is important you avoid using wrenches for this step and turn the centre bar only by hand. Any resistance you feel tells you that something is wrong.)
  8. Repeat slowly turning the isolator and tapping the cylinders as often as necessary until you reach a point where no more than 1/8-inch of threads are visible on each side of the centre section.
  9. Make certain the isolator knob is positioned at approximately the desired angle. (Again, if necessary, it is permissible to have as much as 3 mm (1/8-inch) worth of threads showing on each side of the isolator section. This may be necessary to ensure adequate clearance between tanks for the bolts.
  10. Now you are ready to install the tank bands and bolts. Bolt Kit
  11. This step creates a adjustable position head on the all-thread shaft, making the length of the shaft adjustable. On the end of each shaft, install a wing nut (turned upside down) followed by a regular nut. Lock these nuts against one another. This enables you to hold the shaft without damaging any threads. Place a box-end wrench (or a deep socket wrench) on the aircraft nut (the nut with the nylon insert) and another wrench on the regular nut. Turn the aircraft nut until it is positioned so that approximately 1/4-inch (6 mm) of shaft protrudes from its top (for the smaller 178 mm diameter 9, 10.5 and 12.2 litre cylinders the shaft should protrude approximately 1/2-inch (13 mm) and as much as 1-inch (25 mm) for very small 140 mm diameter 5 and 7 litre cylinders.) Unlock the regular nut from the wing nut and take them both off the shaft.
  12. Very carefully and gently pull the cylinders to the edge of the table. Let the cylinders extend beyond the edge so that the portion where the upper band will go is exposed. Make sure the valve orifices face upward. Take great care that the cylinders are maintained parallel to each other and no force is applied that might cause the isolator to become bent or warped.
  13. Place the top band right at, or just below, the shoulder of each cylinder. (The shoulder is part of the cylinder where the side begins to turn toward the valve.)
  14. (Note the bolt kit uses flat washers against the bands.) Place a flat washer on the end of the shaft with the aircraft nut. Push the shaft up through the band's bolt hole from below. On the other end of the shaft, place a flat washer, followed by the lock washer and regular nut. Put one wrench on the aircraft nut, the other on the regular nut. Holding the aircraft nut in place, tighten the regular nut until the band is moderately snug.
  15. Turn the cylinders around so their bottom ends are exposed. Position the bottom band so that the bolts will be spaced 11 inches (280 mm) apart, when measured centre to centre. Note the mounting holes in the bands are intentionally slightly larger diameter than the bolts themselves in order to allow the bolts a small amount of movement. When taking your measurement, make sure the bolts are centred and perfectly perpendicular to the holes. Repeat the previous step to install the bolt in the lower band and check the spacing with your backplate.
  16. Mount your plate and wing in order to make final adjustments to bolt height as necessary to assure ease of assembly but not so high as to risk abrasion with your exposure suit. The bolts normally should be approximately even with the outside edges of the bands depending on the depth of the V in your plate. In our experience, it's not unusual for the height to need to differ slightly between the upper and lower bolts, depending upon the configuration of plate, wing and accessories attached to the plate. Keep in mind that fine-tuning the bolt height to your specific configuration means that if anything changes or you loan your doubles to someone else the bolt height might need to be re-adjusted, so you might want to leave a tiny bit 'extra' height.
  17. Carefully examine the entire assembly. If the cylinders are parallel to one another (or reasonably close) and able to lie flat, finish tightening the nuts until they are snug. Do not over tighten, doing so will cause the bands to warp.
  18. At this point you should have two flat washers and two wing nuts which are used to hold the backplate in place against the cylinders. You may place one washer and one wing nut on each shaft to store them with the doubles set until you need to mount the plate.
  19. Some divers prefer that the isolator bar be locked into place, others prefer that it be able to turn freely. If you prefer to lock the isolator in place, turn the centre unit lock nuts so that they rest snugly against the outboard modular valves. Lock them carefully in place with a wrench. Do so gently. These components are brass and easily damaged by unnecessary force.
  20. Assembly is now complete. Leaks are unlikely, but if you wish to check then fill the cylinders with gas. Check for leaks by immersing them in water, or if that's not convenient then spray them with soapy water. Look closely for bubbles forming around the cylinder neck where it mates with the valve, burst disk plugs, manifold ports, hand wheels and outlets.

Assembling Doubles Video

Watch this six minute video showing how to assemble a double cylinder setup with a double-barrel, o-ring style manifold, which is by far the most common manifold available today. Some of the manifolds sold by The Scuba Doctor are the type described in the video.


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