Author Topic: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion (Read 1744 times)

rsterne · « **on:** November 20, 2017, 09:05:06 PM »

This is a project I have been developing on the GTA for a while now.... Here is a rough drawing of the concept, which is to mount two empty 88 gr. CO2 cylinders under a QB78 main tube, reversed back towards the trigger group.... This will provide a nice wide base to the forearm for shooting off bags on a bench.... A pair of cylinders weigh 1 lb. 9 oz. and they each have an internal volume of 121 cc....

Well I finally got some shop time today, and I decided to work on the tank block to mount two 88gr. CO2 cylinders reversed under a QB78 main tube.... Using the previously posted drawing as a basis, I produced the following....

The upper hole is tapped 1/4" NPT for a threaded stub that will mount the block to the front of a QB78 main tube, retained using the original threaded CO2 cap.... You can see the two air passages crossing in the bottom of the hole.... They were drilled with a #21 drill (0.159"), which is the tap drill size for a 10-32 screw.... The total area of the two passages is equal to a 0.225" hole, so a similar size hole will be drilled through the 1/4" NPT stub to feed air into the QB78 tube.... Since that is larger than anything that would be used for the exhaust port, that means the entire 290+ cc's will be available to the valve....

There is an 11/32" hole drilled through in line with each 88 gr. cylinder, and that is tapped from the front 1/8" NPT for a gauge and a male Foster fill fitting.... The air passages will be plugged with low-profile 10-32 SHCSs, sealed with a #008 O-ring in the 5/16" recesses milled in the lower corners of the block.... At the moment the holes for the 88 gr. cylinders are only milled 1/2" diameter and 3/4" deep.... I will be centering each hole in a 4-jaw chuck in my lathe to machine the O-ring groove, and drill and tap the portion outboard of the O-ring to 16 mm x 1.5 mm to accept the cylinders....

The block weighs 5 oz., so the completed weight with the stub and the two cylinders should be just under 2 lbs.... The stock will have to be shortened a bit, so the total weight increase will be less than that.... I plan an operating pressure of 1800 psi maximum, in keeping with what CO2 can reach at 120*F, and because that is the MSWP of CO2 cylinders....

Bob

sixshootertexan · « **Reply #1 on:** November 21, 2017, 04:59:50 AM »

Looks good Bob. What gauge are you going to use? I want to make a gauge block to go on the end of my fill whip for my 850 Hammerli.

rsterne · « **Reply #2 on:** November 21, 2017, 08:32:20 AM »

An MRod or Disco style gauge.... hopefully about a 3000 psi scale....

Bob

rsterne · « **Reply #3 on:** November 23, 2017, 04:07:08 PM »

Today I drilled and tapped the 16 mm x 1.5 mm holes for the cylinders, and cut the O-ring grooves.... The most difficult part was centering each hole in my 4-jaw chuck, with it being such an awkward shape.... If I had it to do over, I would have left the block rectangular until after this operation.... Once completed, this allowed me to thread in the tanks for the first time, and they fit great, just a few thou apart.... I laid a 2240 tube on the top for this photo....

You can see how nicely the cylinders nestle up against the tube, the bottom of the cylinders is only 1-1/8" below the bottom of the tube.... Here is a photo of the back of the block, showing the threaded holes.... You can see the O-ring in place in the left hand tank hole....

The plugs for the air passages are in place.... The left hand one is tightened, the right hand one in the photo is backed off so that you can see the O-ring that seals them.... When I drilled the holes in the end of the cylinders to enlarge them, I remembered that since the male Foster is inline with that hole, and there is a check valve in the Foster that is 1/4" in diameter, I had to drill the holes in the cylinders small enough that it would not fall inside.... so I made them 7/32".... This is still way larger than the air passages in the block, which are just over 5/32", so they will offer no resistance to the flow.... The photo below shows the enlarged hole in the end of the cylinders, one of which is installed in the block....

I weighed the assembly, which is now complete except for the stub that inserts into the front of the QB tube (replacing the stock one).... The two tanks, block and plugs weigh 1 lb. 14 oz.... While that is heavier than an aluminum bottle, I don't feel it is excessive.... particularly when the purpose is for shooting from a bench....

Bob

Motorhead · « **Reply #4 on:** November 24, 2017, 12:02:15 PM »

Well executed bob .. i like it !

Alan · « **Reply #5 on:** November 24, 2017, 02:11:44 PM »

I was just thinking (probably a bad idea...), what if you could refill the CO2 cylinders with helium? At the same pressure of course.

One other thing. I wonder if anyone has ever pressure tested any CO2 cylinders?

rsterne · « **Reply #6 on:** November 24, 2017, 04:49:56 PM »

Yep, you could use Helium.... There is a video on the Internet of one of these 88 gr. CO2 cylinders being Hydrotested to failure.... It split along the side at.... are you ready for this?.... 14,000 psi.... That is over twice as strong as it needs to be for 1800 psi.... Now this is not to say that ALL 88 gr. CO2s are this over-engineered.... They are designed, like any CO2 cylinder, for a MSWP of 1800 psi, and you should adhere to that.... In addition, the QB78s are limited to the same pressure, and ONLY if you first check to make sure that the valve is tight up against the steel backing block behind it.... Two of the three valve retaining screws anchor that block, and one of those screws holds down the receiver, so you should NEVER remove the receiver with pressure in a QB7X for that reason, even on CO2.... In addition, the knurled collar that retains the front tube plug has a thin flange that is only good for 1800 psi as well....

Bob

rsterne · « **Reply #7 on:** November 25, 2017, 01:01:52 PM »

Fiddled around with the block some more today.... I milled some recesses and drilled and tapped four 6-32 holes to mount a Picatinny Rail on the bottom and a barrel band (not yet made) on the top front of the block....

The barrel band will be made from a piece of 3/8" thick flat bar, milled off on the back so that the part that mounts in the recess in the block is 1/4" thick, with the screws in counterbores so the heads are below flush.... The top part will remain 3/8" thick and will contain an O-ring groove to hold the barrel in place while allowing expansion....

Bob

Dairyboy · « **Reply #8 on:** November 28, 2017, 02:43:11 PM »

This is a very cool project I'll be following for sure!

rsterne · « **Reply #9 on:** November 28, 2017, 02:54:28 PM »

I received a gift of a QB78 tube and front cap in the mail yesterday, and set about making a new stub form 1144 Stressproof steel, to mount the block and seal into the front of the tube.... The front of the stub is threaded 1/4"-18 NPT and is 0.6" long with a shoulder at that point for the knurled nut to retain it, and the back portion is 0.745" diameter to be a nice slide fit inside the QB tube.... I had to drill the retaining nut out to 35/64" to fit over the stub, the flange on it retains the shoulder of the stub flush with the end of the tube.... There is an O-ring groove for a #113 O-ring 3/4" behind the shoulder so that the O-ring will be located behind the threaded portion of the tube, where it is full thickness.... Behind that, there are two flats milled so that it can be tightened into the block with a 9/16" wrench.... There is a 1/4" hole drilled through for an air passage....

Everything fit together as intended, but I found I was out of 70D O-rings for both the 88 gr. and the tube, and my supplier in Princeton has been neglecting his supply of 70D O-rings, so I didn't have those to do any testing, and besides, I don't have a QB valve at the moment anyways.... This project is pretty much done for now, until I accumulate the rest of the parts I need to assemble a QB78.... but I'm delighted at how it turned out.... and look forward to testing it at some point in the future....

Bob

sixshootertexan · « **Reply #10 on:** November 28, 2017, 04:24:04 PM »

Well done Bob.

rsterne · « **Reply #11 on:** January 13, 2018, 11:19:29 AM »

I received a very nice Christmas present in the mail from mangerover on the CAF.... in the form of a used QB78D in .22 cal.... It had a valve in it which I have to return (nice work Gipetto) to Curtis, but came with a spare stock QB valve, which is what I wanted to start with anyways.... I wanted to use a piece of 3/32" music wire for the valve stem, and a smaller OD poppet.... to increase the airflow without making the valve any harder to open.... so this is the gun that will get those parts.... Here is the back half of the valve and both poppets, old and new....

The hole for the valve stem has been bushed down for the 3/32" stem by using a short piece of K&S brass tube that is 1/8" OD x 0.014" wall,, available at any hobby shop.... The original stem was 3 mm, so I had to drill the back of the valve body out to 1/8" and the tube was such a tight fit I had to press it in place with my vice.... I stopped when the front of the tube was flush with the back of the throat inside the valve, and then just filed the back of the tube to length.... Once the ends were deburred, the music wire stem was a perfect sliding fit, easily as good as any PCP valve I have seen.... I made the poppet out of PEEK, with a 5/16" OD, just a simple cylinder with a spring seat turned on the front to fit the stock QB valve spring, which is also 5/16" OD.... The stock poppet is 3/8" OD, and since the valve is only 7/16" ID, there was only 1/32" around the poppet for airflow.... While that is enough to do the job, the gap around the new smaller poppet is 1/16", so there is twice the area around the poppet, for basically no restriction to the flow.... The only thing I did to the valve to improve flow was to drill the throat of the valve out slightly to 7/32" (0.219"), is was 5 mm (0.197") stock.... With the smaller stem, I now have the equivalent throat area to a hole of 0.198", which would be like the stock throat with NO stem in place.... The QB valve has a raised seat with a 1/4" diameter, so drilling the throat out that small amount did not affect the seat.... and the 5/16" OD poppet sits nicely on it.... I smoothed the stock, angled 3/16" exhaust port into the new larger throat, rounding the top front transition between the two slightly, using a small spherical burr in my Dremel.... The only other change to the back of the valve was to mill a 1/4" recess for the transfer port to sit in.... I will be using a tubular Teflon TP in slight compression, that seals directly on the valve and barrel.... so the barrel O-rings will be redundant.... The receiver will have the transfer port hole drilled out to 1/4" for the Teflon TP to pass through....

The front of a stock QB valve is quite restricted, with only a single 5mm hole in it.... While maybe large enough, I had a modded front end left over from a previous QB project and decided to use that.... The two front ends are shown below....

I had milled two 1/4" slots, one on either side of the front end, right back to the threads, and installed a pedestal to hold the front of the spring using the original hole in the front.... The air now passes along the outside of the spring instead of through the middle of it, on the way to the poppet and seat.... This QB (because it had a Gipetto valve) was missing the steel backing block for the valve, so I have to make a new one.... fortunately I have an M5 x 0.8mm and an M6 x 1mm tap....

Bob

Nvreloader · « **Reply #12 on:** January 13, 2018, 12:05:09 PM »

Bob

Thank you,
I am following along behind you, LOL.

I do have a question/concern,
When making the stem, do you have any problem with the stem sliding up/down the end of the stem,
I relies that you press the stem into the Peep head etc, this is enough to hold the head securely on the stem?

Tia,
Don

rsterne · « **Reply #13 on:** January 13, 2018, 02:20:51 PM »

Don, I just press the head onto the stem.... as long as it is a tight fit I have never had a problem with one coming loose.... The forces are acting to drive the stem into the head.... If you don't have a press fit, then you should grind some grooves in the stem and glue it on.... or even thread it on....

I made the backing block for the valve.... I used brass, and turned a recess in the front to press onto the back of the valve, leaving space for an O-ring between.... There is no constant pressure load on there, but if there should be a leak between the TP and the valve, it will prevent air from blowing back and affecting the hammer.... besides there was no machining required to make the groove, just make a shallower recess in the backing block where it presses on.... Since the block is tight up against the valve, I could drill and tap it in place to insure that all three screws share the end load on the valve.... The two on the bottom are such a good fit, and the stud that mounts the stock is large, I have no fears about them not having a huge safety margin at 1800 psi.... Here is the top view, with the poppet in place....

You can see the 1/4" OD recess for the transfer port, and just how big the stock QB exhaust port is (3/16").... I machined off the back of the block to allow 0.150" of lift before the hammer hits it.... Here is the bottom view....

You can see how much more room there is around the 5/16" PEEK poppet than between the stock 3/8" brass one and the ID of the valve.... Incidently, all I had to do to the new poppet to seal it was spin it around in a drill against the seat.... two nice shiny rings showing perfect contact.... Now that the valve was done, and my supply of 70D O-rings is back to usual, I was able to assemble the reservoir system, with the two 88 gr. cylinders, for the first time....

I initially had two leaks, one where I had forgotten to put Teflon tape on the 1/4" NPT threads where the tube stub threads into the block (that's what happens when you put a project down for over a month).... and the other, much harder to find, was the gauge I had was leaking internally.... That leak took a while to chase down, as none of the threads or O-rings were leaking, but I could hear a leak at the front somewhere.... The only thing I could figure was the gauge itself, and sure enough air was coming out the vent hole in the gauge.... Degass the gun, throw the gauge in the trash and replace it.... and now no leaks.... It is currently sitting with 1800 psi in it.... You will note that I completed the barrel band that mounts on the tank block.... It is drilled 5/8" to clear the 15mm barrel, and then a 3/4" recess milled half way through from the front for an O-ring to support the barrel.... This arrangement works well, it allows the barrel to "semi-float", allowing for movement from expansion or pressure changes.... but preventing it from getting knocked out of alignment if you hit the barrel on something.... I like the arrangement very much, especially on a gun like a QB where the barrel is not well supported in the receiver....

Bob

rsterne · « **Reply #14 on:** January 14, 2018, 04:55:11 PM »

I finished up the receiver and barrel mods today and made a new hammer.... Here is a photo of the hammers, old and new....

The cocking pin is in the same place relative to the front of the hammer, but the sear notch is forward 1/4".... and the back of the hammer is shortened to save some weight.... The new hammer weighs 43 grams instead of 55 grams.... a reduction of 22%.... The hammer stroke is now 3/4" instead of 1/2".... and using the "cock-on-open" parts shown above and a stock QB hammer spring I have 10 turns of preload adjustment, from 3 turns of preload to 7 turns of gap, using the 10-32 adjusting screw.... It ends up that was perfect....

The barrel port was drilled out to 11/64" (0.172") or 78% of the bore.... and a transfer port made from 1/4" Teflon rod with the same size hole.... I drilled a hole in the loading tray area of the barrel to allow it to be assembled to the receiver and the entire assembly fitted to the gun.... since the flat head screw under the loading tray can be accessed with a Phillips screwdriver through that hole.... That flat head screw now lives (permanently) trapped between the barrel and receiver.... I also installed two 6-32 setscrews in the sides of the receiver that tighten against the back sides of the barrel loading tray.... which really stiffens up the assembly.... The bolt was shortened (flow through portion cut off) and a 3/32" probe fitted that pushes the pellets just ahead of the barrel port.... I shortened the stock to just clear the back of the 88 gr. cylinders.... and assembled the gun for the first time....

I weighs exactly 8 lbs. as shown.... I have done some shooting, and the performance exceeds what I was hoping for.... A report will follow once I have a chance to organize the data.... but the maximum velocities are 1000 fps with 18.1 gr. (40.2 FPE) pellets and 910 fps with 25.4 gr (46.7 FPE).... with both pellets up on the plateau at 1800 psi....

.... The knee of the curve occurs with some gap between the hammer spring and hammer, so I expect to see the benefits of a Short Stiff Spring installation, even though I am using the stock spring (with a gap)....

Bob