QB79 tank block improvement

[Gipper]

I must have a newer style tank block as mine has 3 orings on it.  I am afraid pinning may be a bit trickier with this block so am leaning towards additional screws instead. Any suggestions as to where to put the extra support?   Any issues if my additional screw holes are drilled into the vacant oring groove? 

Brian

[rsterne]

I think the latest block actually has 4 O-ring grooves.... Why, I have NO idea, one is all you need.... and that should be the innermost one (closest to the trigger).... Obviously it would be better if you can drill between the O-ring grooves, rather than losing the support of the metal for the screw, or shear pin, near the tube.... If you can't find enough room for a 4mm or 8-32 screw, then I would use a 3/16" diameter shear pin, which would be almost impossible to bend.... Locate the hole so that the inside edge of the pin is supported by the full diameter of the tank block stem, so that it cannot bend when the tank block is forced forwards by the air pressure.... I don't know how strong one is, but I would think even a 3/16" roll pin (spring pin) driven into place would be better than doing nothing.... Use Jason's trick of 200-300 psi of air in the tube holding the tank block forward so that when you drill the hole straight through the tube and tank block, the pin, when installed, will be sharing the load with the existing screws.... You will probably destroy the O-ring when you pull the block out after drilling, from the burrs on the hole in the tube, and don't forget to deburr all the holes before reassembly.... No guarantee this method will work, but it's easier to align than trying to drill the tube and block separately.... Worth a try?....

Bob

[Gipper]

You are correct it has 4 orings.  It has about 1.5mm between the front 2 and the same between the rear 2 and a fat 2mm seperating the 2 pairs.  Sure seems like over kill but it never leaked when I had it on a bulk fill co2 crosman.   Fortunatley the Ninja bottle I have on hand is only set to 850psi so should be good to go with the soon to be arriving AR2079..  I double checked the burst disk and it has a 5k on the bottle side of the regulator and a 1.8k on the downstream side.

Brian

[Ribbonstone]

Guess we are in agreement…sort of. 
\Pretty sure it’s not going to stop folks from running +1250psi tanks on their standard gas block fixated QB79’s.

It should make all of us think about better block fixation a little harder, but we’re battling the mind set of “it hasn’t launched the tank/block like a rocket yet, so it must be OK.”


Degassed, removed the gas block screws, and took a look at the 3 cut QB78 tubes.  Screws aren’t bent and the holes in the tube aren’t elongated. Only running 850psi, but I had to check them all anyway.

[SpiralGroove]

Thanks Guys for driving the safety issue home,
I was originally looking over this thread with a little bewilderment as it seemed to be a bit overkill.

However last night, I remembered when changing a trigger spring in my (unpressurized) QB78/HPA a few weeks ago, I had noticed a bit more forward movement of the tank block, stock screws and air cylinder/tank block screws than I originally remembered seeing?  My regulator is set at about 1400/1500 psi, so was a little alarmed .  Further inspection had shown the air cylinder tank block holes had elongated a whisker (see photo below taken -> tonight). I also remembered, that when first made/assembled, the holes in question needed to be widened one drill size over 4mm to screw into the block correctly .



To remove this shadow of doubt (I have a 10 year old son who sometimes shoots this gun), I will be either adding a pin as discussed or buy another QB78 tube.  If I go for the former (per Jason), I will drill the pin hole through the entire air cylinder/tank block with about 400 psi in the 13 CI tank.  After this process, the block will look identicle to my current picture, but be held simultaneously in 4 spots instead of 2, 6 if you count the stock screws.

Thanks all, much cheaper than hospital bills and just a little elbow grease  

[Austringer]

So glad I asked this question. Any future tank blocks I use will look like the one Bob pictured.

So Bob...as I’m still digesting everything you wrote, I have a clarification regarding this:

 "The standard minimum dimension for the distance of a hole from the end of the material is 1.5 diameters (to the C/L), which would be over 6mm, let's call it 0.250".... Using that dimension, and the full wall thickness of the QB tube, we have 2 x (0.250 x 0.059) x 2 x 38,400 = 2266 lbs. force.... That works out to 2266 / 0.439 / 3.5 = 1474 psi MSWP”

So since my modified tank block allows my 12.9 class m4 screws to be installed .250” on center from the edge of the full thickness tube, then my MSWP is 1474, right??

If true (by your non engineer best guess:)) then my 1475 psi set point is 1 psi over the safe limit. That being the case, I feel WAY better off than I did with the 79 tube and cheesy factory screws. Cant wait till my next build!!

Thank YOU Bob!

Troy

[rsterne]

Sound right....

Bob

[Austringer]

So as I am new to this also, I’m looking at the picture SpiralGrove posted and see his tank block has been pushed slightly out of the air tube. This is consistent with my limited experience upon both initial fillup of my QB79 airtube and to a lesser degree my modified 78 tube. On both occasions, the gap between the end of the tube and the shoulder on the tank block increased as pressure was applied. My thought was this is somewhat normal as the load seats on whatever bearing surfaces are present. In this case, it is screw threads against the hole in the air tube. The thinned tube of course is much thinner so has less bearing surface so it moved more. Screw threads are naturally going to cut into the tube. I’m guessing movement might also depend on the accuracy of alignment of the screw holes in the block vs the  tank. One hole may bear more load until the block moves a bit and the load is equally shared. I’m thinking this is normal to an extent. Too bad we can’t find screws with a .062” unthreaded shoulder?

Having said that, I flat out do not like the QB79 tube holes OR the cheesy screws. I’m sure I could safely live the rest of my life on that marginal MRWP many of us 1500psi’rs are working with, but I’ll be adding extra safety precautions to my future QB’s depending on what new ideas are out there. For now, I’ll be monitoring my inletted block and adding at least one screw where possible.

Troy

[rsterne]

I agree, this is for many a case of blissful ignorance, it certainly was for me until I did the math.... I'm actually a bit shocked that the gun wasn't given a larger safety margin when you consider the pressures that CO2 can reach on a hot day.... You are absolutely correct that the load will imbed the screw threads into the thin portion of the tube until the loads on both screws are evenly distributed.... which causes a gap between the block and the end of the tube.... If you look at a Disco valve, same thing, the screw heads are always right back against the rear of the holes....

Bob

[Buldawg76]

Bob
I did some measuring on my tank block since in your response to me you stated in may be a bit thin between the o ring grooves with the 17/64Ths heads of the 8-32 screws in done with the heads in shear, but if done like yours in the picture you have on page 2 of the thread where they are located towards the front o ring groove there is room to set the heads in shear with the tube instead of just the threads.

I have not fully decided whether I am going the two additional screw route or the hardened pin and third retaining screw like your design as of yet in my QB. I did notice the two front screws that come as standard Philips head screws with star washers in the photos you show have been replaced by you with better grade allen screws, so are they still 4mmx .07 pitch screws or have you switched to all 8-32 screws for both front screws and the third pin retaining screw as well.

Would a 3/16" drill bit shank cut to the proper length be considered the same as a hardened pin in respect to strength in shear in the tube or does it need to be an actual drill rod or a hardened pin.

Mike

[rsterne]

If you drill the tube for the heads of the screws, you may not be able to leave enough room between that hole and the existing one in the tube.... You should have at least one full hole diameter to the edge of the hole....

A 3/16" drill would be plenty strong enough because of the diameter.... Mine is a piece of O1 Drill Rod, but not hardened....

I used 4mm x 0.7mm SHCSs to replace the stock pan-head screws.... If you can get Grade 12.9, that is the Metric equivalent of Grade 8 screws in 8-32....

Bob

[Buldawg76]

Bob
I have a 78 tube so it has no holes at all as of yet but understand what you mean now with the holes for the 8-32 screws in shear will not leave the full screw hole distance to the factory 4mm screws so some more inspection and measuring is in order to insure I have at least 5mm of metal between the front two screw holes and the rear holes with heads in shear and if not then pinning with a 5/32 or 3/16" drill shank cut to proper length with a top retaining screw will be the route I go.

Now I just need to decide if it want to leave the 78 tube its full length and cut it right behind the threads for the cap or cut it back 2 to 3 inches to get the bottle more rear ward to help balance the gun better. Can you tell me the difference in length between a stock 78 tube and a 79 tube as all I have is a 78 to compare to and I know from what you have said the added length of the 78 tube will add needed volume for the power I am after from the gun. So I guess I am asking is how short can I cut the 78 tube and still have sufficient volume to achieve my sub 20 fpe goal with 10.34 pellets.

Mike

[nervoustrigger]

Just to add another anecdotal data point, 2 years ago when I set out to build my first QB78, I made a post asking about the safety margin of the tank block at 1500psi.  It was to be a rifle for my uncle and based on the feedback, I decided to pin the block.  I was so pleased with the outcome that I quickly turned around and built one for myself but this time I omitted the pin in order to monitor it for movement.  Bear in mind this is a model 78 tube cut to accept the tank block, meaning it has the full tube thickness.  Well, 2 years later it has not moved from the point it was when it was new.  This is a picture taken yesterday:



I had the rifle apart after a few months of use and there was no observable deformation of the holes in the tube under a 5x loupe.  The machine screws have a very short (~1mm) unthreaded section just under the head so there wasn't even any marring from the threads.  I had taken care to locate the holes and drill so as to maintain as much material as possible so I imagine that has helped.

That said, I still recommend that folks use a pin or additional screws because of assembly and materials variables, and because of possible abnormal use/abuse such as dropping the rifle or slipping on leaves in the woods and falling on it.

[Austringer]

Jason
Were the 4m screws with the 1mm unthreaded section difficult to find? I have not yet procured my class12.9 4m screws, so whether they are easy to find...or is a particular source, I'd sure like to know because I see this factor as being pretty important to maintaining minimal deformation of the screw holes and movement of the tank block, regardless whether or not the block is pinned. Really glad you posted this!

Troy

[nervoustrigger]

Hi Troy,

To be honest I didn't deliberately select screws with that feature but the ones I'm using are McMaster #95263A247

They are considered full thread but in my experience most screws will have a tiny bit of unthreaded section just under the head.  Here's a closeup of the screw in question:



If memory serves, I think I needed to shorten them 1mm or so.  Not because they butted against each other (0.863" - 10mm - 10mm = 0.075" of clearance) but because I think the factory taps the holes from each end so there's a bit near the very middle where it's not tapped fully.   So if you need to order, you may want to save yourself the trouble and instead get 8mm length McMaster #95263A234.

[rsterne]

Mike, I did a thread on the importance of tube volume on the QB78-79 question a long time ago.... http://www.gatewaytoairguns.org/GTA/index.php?topic=21681

I ended up shortening the 78 tube about 3" (basically half way between the two stock tubes, to move the tank back, and that was an excellent solution.... The only time you really need a full length 78 tube is if you are trying to build a .25 cal (or a really hot .22).... The volumes between the tank block and valve are about 10cc in a 79 and 50 cc in a 78.... using my guide of 1 cc per FPE, if the tube is half way between, at 30 cc, that is adequate for a 30 FPE gun.... Not many QBs shoot hotter than that and stay under 1500 psi....

BTW, I believe 8mm is the correct length screws....

Bob

[SpiralGroove]

Yeah Jason,
I'm sure at least 1/2 of the tank block movement was due to my poor hole drilling of the QB78 tube
I will take off the tank block again and take a photo of the actual tube.

Ohh...... the holes were drilled larger than I thought:


Kirk   

[Ribbonstone]

Sorting through 3 old QB78 tubes, looking for a clean one to replace the QB79 shown in post #5.

Took some measurements, and have to say, not all QB tubes are all that uniform.  To be fair, two of the tubes are from “back in the day”.

So I’m going to post an average.  You could end up at a thick part or a thin part of the tube, as they aren’t all that dead-even.

OD: .862” (=/- .03”)
ID: .748” (+/- .03”)

There for the difference is .115” (or .057” per side).

The diameter over the two flats on the QB79 tube pictured in post #5 is .836”. So at those machined flats make the tube wall thickness .044”.

Considering the flats serve no use, are just there more-or-less as decoration (maybe to give a little less of the “bolts in Frankenstein’s neck” look), it’s intentionally weakening the system for the sake of style.

Picked the cleanest of the two, am going to simply make a QB79 tube from a QB78 tube.  NO extra length, just without that flat and with tube screw holes that have less "slack" to the screw. BUT, I do only run at 850psi. If you run at higher pressures, would relocate the screws farther back.

[WECSOG]

Guys, thanks for the work you have done on this. I need to take my 79 apart to put a spring guide on the rear face of the block, and when I put it back together I intend to follow Jason's advice of pressurizing the tube a bit before drilling for a crosspin. Bob, your idea of a setscrew for the crosspin is ingenious! I'll probably copy that.

BTW, I really don't see how the stock screws can add any strength against the pressure. Seems to me the most they could do is direct the movement downwards in the event the block blows out of the tube.

[Austringer]

WECSOG

If you drill the back of the tank block for a spring guide like Jason did (and as I did to gain plenum volume), you will likely not have enough material left to safely drill for a cross pin. Measure twice, drill once just to be sure.

Drilling the back of the block is why I had to inlet my tube into the block in order to beef up my screw hole strength as outlined at the very beginning of this thread.

Troy