High-pressure Caron Dioxide is extremely dangerous and the pages showing Steves work on and with re-filling Co2 cartridges and the 88 gram Airsource cylinders is presented for information only.

 If you chose to copy or to take inspiration from this work you do so entirely at your own risk.

In Short:- You build it, it breaks you get to keep the bits , if some one or some thing gets damaged it's your problem. Likewise if it just does not work, too bad it's still your problem.

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How to make refillable 12g CO2 cartridges

Why would anyone want to make 12g refillable CO2 cartridges? Disposable cartridges are not expensive to buy or are they? The current average price for 10 cartridges is about £6.50 that works out at 5.4p per gram. That is of course assuming you have 12g in each cartridge to start with, try weighing your cartridges before and after use then determine how much CO2 was actually in there. If you can find some where that refills fire extinguisher’s or brewery CO2 bottles they charge approximately £3.50 for 6.5 Kg of CO2, that works out at 0.05p per gram you will however have to supply your own bottle or extinguisher. Still think 12g disposable cartridges are cheap?

What happens when you want to shoot just a few shots? Do you leave the cartridge in your gun; some guns are OK with the cartridge left in place for short periods assuming the seals are OK. If you transport your gun with a cartridge in place I am lead to believe you are committing an offence. If you remove the cartridge you loose any remaining gas and also risk freezing the seals.

A refillable cartridge allows you to remove it when you have finished shooting with minimal gas loss, but at 0.05p per gram does it really matter. Are they expensive to buy? I don’t know. No one sells them (that includes me). As they are not available commercially I decided to make my own. There have been various versions along the way and I now have a design which I am more than happy with.

Hopefully the following will enable you to make your own refillable 12g CO2 cartridges. As for safety, in over seven years of experimenting with these cartridges I have not had any accidents or near misses. The original cartridges had external threads which allowed me to hydraulically test them to 200 Bar without failure. The current design has not been tested to this pressure and due to the threaded valve insert it should leak prior to any form of failure. The overall safety will be down to your own engineering competence.

There are three parts to the cartridge. The main body, the valve insert and the valve. The main body is of course a used disposable 12g CO2 cartridge; you were going to throw it in the bin when it was empty so the cost of this is nothing. I have found the gold coloured ones most suitable for conversion. The valve insert is made from a car tyre valve stem, it does not have to be a new one and your local tyre fitter should let you have all his scrap ones for nothing. The valve is also from a car tyre; however in its normal form it will not withstand the pressure of CO2 for long. You can use standard unmodified one if you wish, but they will fail. You will require small O rings and an applicator to fit them to the modified valve. These are O rings for Brocock TACS the applicator also being available from Brocock.

Apart from the Loctite to secure the valve insert and the O rings for the valve, these cartridges should only cost you your time to make. You will require a source of CO2 and filling kit, but this is not difficult to obtain or make. People, who already bulk fill, will have most of the kit already.

The Main Body

First you require a used 12g CO2 cartridge and a lathe. The three jaw chuck on my lathe is not large enough to fully accept a cartridge; hence I use the four jaw chuck. You can not always guarantee that the neck of the cartridge will be central to the body, so if you can use a four jaw chuck all the better.

Use a DTI and ensure that there is no run out on the neck as shown above.

Using a centre drill, drill the front face of the cartridge. If you are using the same type of cartridge shown above you will notice a minor diameter at the tip. This is a plug which seals the cartridge during manufacture. Use the centre drill to open up the end of the neck just short of the edge of this diameter.

Drill out the neck using a ¼ inch drill, this is the closest I have to 6.25mm. Take your time and use minimal or no cutting fluid or coolant. If you generate too much heat you will discolour the plating.

Finally tap out the inside of the neck to M7 x 0.75 once again take your time and use minimal lubrication. If you can cut the thread without lubrication, then you will not need to degrease the inside of the neck prior to securing the valve insert with Loctite. Do not attempt to cut the thread in one go, the swarf will cause the neck to deform and your efforts will be wasted. The neck is very thin and it is not unusual to see tap marks of up to three threads on the outside. If the neck was running out in the chuck you will have probably broken through the neck by now.

Shake out the debris from inside the cartridge, you should also find the remainder of the plug. If necessary degrease the inside and neck of the cartridge and blow out any remaining swarf with an air line. The body is now ready for the valve insert.

The Valve Insert

As mentioned previously the valve insert is made from a car tyre valve stem.

Remove dust cap and valve from the stem. If you are using used valve stems ensure they are not excessively bent. The tool that is normally used to remove the valve stem from a wheel can cause the stem to bend. The advantage however is that it sometimes removes the large piece of rubber on the end.

Using a knife cut the end off the valve stem, if it is still intact.
 Grip the valve stem lightly in the lathe chuck by its threaded end.

Centralise and support the opposite end with a revolving centre. Now tighten the chuck, making sure not to crush the end.

Bring the cutting tool in so that it just touches the brass, and then proceed to remove the rubber. Always cut from left to right, if you cut from right to left you run the risk of pushing the valve stem into the chuck and damaging it.

The first pass will leave a large flap of rubber at the revolving centre end. This will be removed on the second pass.

Position the tool back at the threaded end approximately 3-4 mm to the right of the brass/rubber join. Send the tool in until it can be seen cutting brass. Keeping to this depth make your second pass.

Withdraw the revolving centre and remove the washer shaped piece of rubber. Reposition the revolving centre, take tool back to the threaded end and commence your final pass cleaning off any remaining rubber.

Do not worry about the small amount of rubber left on the tip; this will not cause a problem. Move the tail stock out of the way and reposition the valve stem ready for machining to the correct diameter.

The valve stem now requires turning down to 7mm ready for threading.

Do not attempt to remove all of the existing thread as this will make the diameter to small. If you go less than 6.9mm, discard the stem and start again.

The slight thread that is still showing does not pose a problem. Once the correct diameter has been achieved, thread the end of the stem to M7 x 0.75.

Try the thread in your previously made main body to ensure it is not too loose or too tight. If you are producing a batch of cartridges you can pair them up for the best fit.

Place the valve stem back in the lathe chuck and part off to 13mm.

You will need to make a tool for holding your valve insert. This will allow you to remove the excess material left when parting off. You will also need it for modifying your valves. After facing off the excess material, your valve stem is ready for insertion into the main body.

Valve Insert to Body assembly

The main body and valve insert are now ready for assembly. First you will require some Loctite; my recommendation is that you use Loctite 542. This is a hydraulic sealant and so can withstand high pressure.

Apply two drops of Loctite to the internal threads of the main body and slow rotate the body until all the threads are evenly coated. The Loctite should sit just proud of the threads, if not apply a third drop and rotate once more.

For the next part I use a wooden chop stick with a sufficient amount cut from the tip in order to allow it to be screwed into the internal thread of the valve insert. With the valve insert secured to the end of the chop stick, apply two drops of Loctite and rotate the valve for even coverage.

You will have to keep the valve insert rotating otherwise the Loctite will drip as in the above photo. Whilst rotating the valve insert, screw it in to the main body and stop when it is just below flush with the tip of the neck. Using your thumb nail to prevent the valve insert from unscrewing, unscrew the chop stick from the valve insert.

Now leave the cartridge inverted for 24 hours to allow the Loctite to fully cure. For this I use an empty plastic milk bottle with 4 crosses cut into it with a knife then push the cartridge through the cross.

Once the Loctite has cured you can tidy up the end of the neck. This has been left until last so that least amount of material is removed. This way the overall length will be as close a possible to the original cartridge. Use a small stone and burnish the rim of the neck until all sharp edges have been removed. As long as you did not go OTT with the Loctite there should not be any Loctite on the inside of the valve insert. Your cartridge is now ready for its valve.

The Valve

As I said previously you can use unmodified valves in your cartridge, however they will fail. The valves below are guaranteed to fail due to their lack of surface area on the sealing face. The extreme pressure on the seal causes the seat to cut into and split the seal causing valve failure.

The following valves will last longer but will also eventually fail.

The surface area of these valves in much larger hence they last longer before failing. The seal ends up being forced inside the valve. You will also notice that all of these valves have theirs springs externally mounted. Removing the spring aids filling the cartridge with CO2, after filling the internal pressure of the cartridge will close the valve. When the cartridge is fitted in your gun, as the pressure equalises after the valve has been depressed. The valve will open fully as there is no spring trying to close it, thus aiding gas flow.

Next remove the seal, if you cut the seal off ensure you do not scratch or damage the stem.

The seat of the valve is too long for the seal that you will be fitting. As a result you will need to remove 1mm from its length. To do this screw the valve into a spare valve insert. Then using a bb to keep the stem depressed and as far from the cutting tool as possible. Secure the valve insert and valve in the holding tool you made to face off the valve insert. You can now remove the 1mm in the lathe.

Next you require an O ring as used on the nose seal of Brocock TAC’s and an applicator. Slide the O ring over the applicator and get it as close to the end as possible before attempting to fit it to the valve.

Your valve is now ready for fitting into your cartridge. You now have a refillable CO2 cartridge. All you need now is a device to fill it with. Unless I have missed something major (which I don’t think I have) then you should have no problem making your own cartridges. If you feel there is insufficient information to make them, then may I suggest that you shouldn’t try.