Leaking QB78 tank block driving me crazy!
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Leaking QB78 tank block driving me crazy!
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Topic: Leaking QB78 tank block driving me crazy! (Read 4108 times))
nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Leaking QB78 tank block driving me crazy!
«
on:
November 11, 2013, 01:38:18 AM »
I am converting a QB78 to HPA with a Ninja tank for my uncle as a Christmas gift.
Things were going really well at first. With porting and valve modifications, it went from 8fpe stock to 20fpe on a standard Ninja tank (850psi or so). And up to that point the tank block sealed just fine. But the goal is to hit 25fpe+ so I ordered a SHP (super high pressure) tank for it. For safety, I cross-drilled the block and installed a rod (1/8" cobalt drill bit shank) to augment the grade 12 M4 screws that hold it to the tube.
Now I can't seem to get it to seal to save my life.
Here's what I've tried:
1. Original O-rings were damaged so I replaced them with size 016 cheapies from my O-ring assortment. Leaked before it was pressurized to 500psi. No damage to the O-rings when I pulled it apart.
2. Applied 3 coats of varnish to the block per Ribbonstone's recommendation. Made super extra certain the inside of the tube was deburred and polished and installed new O-rings just in case. Once again leaked with less than 500psi.
3. Ordered metric O-rings with a slightly larger cross-section (2mm vs 1/16"), buna 70 Shore A durometer. Doing the math, they should be about 16 thousandths larger but best I can tell, they only measure 5 or 6 thousandths larger. You guessed it, still leaks.
4. At this point I figured there had to be an axial scratch in the tube that was allowing air to escape by. Couldn't detect anything with a strong light or by feel. But just in case, I decided to varnish the inside of the tube. And yes, it still leaks.
The tank block is one of the newer 4 O-ring types. After cross-drilling, I have 3 of the 4 grooves populated so I would have thought getting a good seal would be child's play. I could not have been more wrong.
What's next, guys? Are 90 durometer polyurethane O-rings the cat's pajamas?
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MS
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tkerrigan
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Posts: 1176
Real Name: Tom
Re: Leaking QB78 tank block driving me crazy!
«
Reply #1 on:
November 11, 2013, 01:53:40 AM »
If you google 016 o-ring groove measurements, you should be able to get dimensions to check the groove, just in case it is cut too deep. 90 durometer o-rings would definitely be good to try. If you still have the original o-rings check with your finger nail by putting the o-ring on a table top and comparing how deep the nail dents it compared to the 70's, the 90's don't dent very much compared to the 70's. Regards, Tom
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Ribbonstone
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Posts: 12030
Re: Leaking QB78 tank block driving me crazy!
«
Reply #2 on:
November 11, 2013, 02:31:23 AM »
You tried al the tricks I've tried (was worked with the old style/two o-ring blocks).
Only one wouldn't seal even with the varnish trick. Deepened one of the o-ring seats (very slightly, the gas passage is close to the outside bottom) and a larger o-ring used.
Rely on the pin rather than the hex screws. Screws are a good idea, but they don't need to be bust-a-gut tight. If you over tighten the screws, are basically pinching the end of the tube, the end of the tube tends to turn into an oval, promoting a leak.
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Louisiana
nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #3 on:
November 11, 2013, 02:44:55 AM »
Thanks, Tom. I just ordered some 90A polyurethane in the -016 size. I checked the originals like you said and they are definitely a higher durometer than the metric ones I ordered so hopefully that will do the trick.
Ribbonstone, that's a good point about overtightening. That occurred to me earlier today so in my last attempt I only snugged them, and measured the OD of the tube and it was within 0.001" of round, much to my surprise. But unfortunately it still leaked.
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MS
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Rdsail
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Posts: 1571
Re: Leaking QB78 tank block driving me crazy!
«
Reply #4 on:
November 11, 2013, 08:12:49 AM »
I know that you said to you debarred. But Take a second look at the screw holes. Use 600 grit paper and then polish it. I had a similar issue. I would damage the orings every time. polishing fixed the hole problem. Al make sure that you use a fair amount of lube.
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Richmond, VA
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OleTomCat
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yes
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Re: Leaking QB78 tank block driving me crazy!
«
Reply #5 on:
November 11, 2013, 08:44:51 AM »
Have you tried a soap bubble test to see just where you are leaking?
Mix a weak solution of dish soap and water and drip a few drops around it will bubble up where it leaks....
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Sfttailrdr46
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Re: Leaking QB78 tank block driving me crazy!
«
Reply #6 on:
November 11, 2013, 09:20:53 AM »
Also Lowe's and or Home Depot sell gas leak detecting liquid which has glycerin this makes stronger bubbles and since it is denser than dish liquid solution works slightly better
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nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #7 on:
November 11, 2013, 09:39:41 AM »
Thanks, guys. The leak is coming from between the block and tube at the top side of the tube, which is partly why I thought the screws on the side might be compressing the tube into an oval if overtightened.
I'll quadruple check for sharp edges. There were some that damaged the original O-rings so I polished to 600 grit. The last two attempts, I cleaned and inspected them very carefully and they looked flawless though...
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MS
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TimmyMac1
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Posts: 1793
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Re: Leaking QB78 tank block driving me crazy!
«
Reply #8 on:
November 11, 2013, 09:43:26 AM »
Buy the good stuff (Mil Spec 90's) in an 015. The 016 is too tight and only works with the soft stuff. There are two grooves. I'd back it up with a 70 durometer 016 well lubed. O-rings need to move in the groove so when they are Chinese tight they often roll when you slip them in and leak along the parting line for the tooling. You need to install with liberal amounts of dielectric grease like the DC 111 valve and seal lube.
http://www.mac1airgunshop.com/urethane-mil-spec-90-durometer-orings-p/90urms.htm
TimmyMac1
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"No Guns Would be a Riot"
nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #9 on:
November 11, 2013, 11:48:46 AM »
John, hmm, I'll have to stop tonight and see if my store carries them. I am really anxious to get it fixed. Thanks for the pointer.
Tim, thanks for the confirmation on the 90A polyurethane sizing. As stubborn as this rifle has been, I may be in need of some of your O-rings before this is all said and done.
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MS
Conduct yourself so that when a man accuses you of something dishonorable, no one will believe him.
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Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: Leaking QB78 tank block driving me crazy!
«
Reply #10 on:
November 11, 2013, 12:50:43 PM »
It’s the holes for the screws (and in your case, for both screws and a pin) that kill o-rings. Even a little nick. Too large and too soft an o-ring will tend to want to “puff up” into the hole as it passes by, almost wanting to be sliced/nicked. Hard o-ring just the right size for the tube will pass pretty easily, but still seal when pressurized.
One last thing might help in smoothing the rough edge of each hole.
Needed:
A short length of either 3/8 or ½” wooden dowel.
A small pad (about 1/3”) of fine abrasive paper (or emery cloth) glued to the dowel.
Steps:
Strip the gun until you get to bare-tube.
With the screws and pin out, you can see though the hole, and get that little pad of paper right on the hole itself.
Run it by hand in a small (like ¼” movements) back and forth and with a little rotation (again about ¼”) JUST AT THE HOLE. You aren’t trying to make a “dent” in the tube, just to be sure the edge of the hole is polished and level to the inside of the tube.
Repeat for each hole.
Then clean the devil out of the tube. Twice.
Reassemble
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Louisiana
nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #11 on:
November 11, 2013, 04:45:40 PM »
Ribbonstone, thanks for the words of wisdom regarding polishing the holes. I had actually done that already and checked each hole by pressing hard with the end of my fingers and dragging across each hole in each direction. I figure I'm coming across as a hard head but I am really very confident the O-rings are not getting nicked, scored, or gouged. With Super Lube, I was getting what I feel like was the right amount of resistance when inserting the tank block, without the cringe-inducing hangups that sometimes happen when there is a burr. FWIW, I've had my son's QB79 apart a couple of times with no issues but it does have one hard O-ring so maybe that is the key.
I laid down a couple of coats of varnish yesterday and that seems to have softened the edges of the holes further so I don't want to polish again just yet. I'm planning to pick up some harder paintball O-rings on the way home tonight and try them.
Sooner or later I'm going to conquer this obstinate contraption!
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MS
Conduct yourself so that when a man accuses you of something dishonorable, no one will believe him.
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TimmyMac1
Expert
Posts: 1793
"You've Come A Long Way BB"
Re: Leaking QB78 tank block driving me crazy!
«
Reply #12 on:
November 11, 2013, 06:41:11 PM »
On Dynamic applications like caps being removed each time the gun is charged the only o-ring is the Mil Spec 90's. They are beyond a scale 90 UR and will not absorb CO2 at all. The International Oring Service stuff will get saturated with CO2 and degrade. For Bulk Guns you want to leave charged the only way to go is the Mil Spec 90's. Everything else will last 1/2 as long Max. Most 90A or less will get soaked with the liquid CO2 and expand on the low pressure side. I like stuff to last. Mil Spec 90's are the best I've ever seen and all I use for containing CO2.
TimmyMac1
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nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #13 on:
November 12, 2013, 10:19:42 AM »
Well, online it said the local Walmart did not have the paintball rings but I couldn't find them on the shelves and had them run the UPC and was told they don't carry them.
Some 016 polyurethane 90A will be delivered today so I'l try again tonight. From what Tim says they will likely be too tight but given the difficulty getting this bugger to seal, I'm going to give them a try.
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MS
Conduct yourself so that when a man accuses you of something dishonorable, no one will believe him.
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nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #14 on:
November 13, 2013, 01:32:34 AM »
Cool beans, the leak is finally fixed! Or at least it is slow enough that I can't hear it and it has held 2000psi for the last couple of hours.
For anyone wondering, the -016 polyurethane 90A durometer O-rings did indeed fit...very snugly.
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MS
Conduct yourself so that when a man accuses you of something dishonorable, no one will believe him.
Barrel accurizing guide
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Gertrude
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Real Name: Ron
Re: Leaking QB78 tank block driving me crazy!
«
Reply #15 on:
November 13, 2013, 02:01:09 AM »
Jason,
Can you post up a pic of this gun please ?
I'm stating the fight off the "Gotta Get a New Gun" Demons,... And hopefully I'm going to
-lose the battle-
somewhere around Santa Clause time.
I've been thinking about a Bottled- '79, but I suppose a Bottled -'78 might be just as fine ?
I'm just starting to learn about them.
What are the pro's/con's of a bottled 78 vs. a bottled 79 ?
either way,.... provide us with a little AG porn puleeezzzee ... I wanna see it !
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drewciferpike
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Re: Leaking QB78 tank block driving me crazy!
«
Reply #16 on:
November 13, 2013, 02:30:03 AM »
Quote from: tri-5-ron on November 13, 2013, 02:01:09 AM
What are the pro's/con's of a bottled 78 vs. a bottled 79 ?
78 - More air "on deck" between the reg and the valve. Ability to develop more power, if you're going for mega-power-mods (or if you want to convert to .25, etc.)
79 - Bottle doesn't stick out as far. Con of this is that you don't have as much stock to hold, but we don't mind.
I went with a '79 for my dad's .22, as it is for hunting, and we both didn't like the bottle sticking out so far. With an 1100 psi reg, the thing is a hammer. If he wanted to go powerpowerpower, a 78 would have made more sense, but the thing is definitely more than he "needs", anyway. Honestly, he'd like to eventually figure out a way to quickly adjust power, as he can't use it in the warehouse.
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nervoustrigger
GTA Senior Contributor
Posts: 8701
The Grin Reaper
Real Name: Jason
Re: Leaking QB78 tank block driving me crazy!
«
Reply #17 on:
November 13, 2013, 03:11:27 AM »
Ron,
Drew pretty well summed it up. My son's modified .177 cal QB79 on 850psi air makes 15fpe and it's a positively lethal squirrel gun. But this new rifle is for my uncle for Christmas and I wanted it to be capable of taking possums and raccoons with authority like my B51. So to do that I wanted to develop 25-30fpe without too much fuss which is why I went with the QB78 and a higher pressure tank.
No pictures yet, though. I have to modify the stock. I'll post some up when I do in
the previous thread I started
.
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MS
Conduct yourself so that when a man accuses you of something dishonorable, no one will believe him.
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Leaking QB78 tank block driving me crazy!