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 Post subject: Qb78 deluxe hpa
PostPosted: Thu Apr 27, 2017 11:25 pm 
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Location: Caronport, Saskatchewan
Bought this recently and after shooting it a bit, stuck a crosman 1322 valve seal in to replace the factory stem. It shot well over a 100fps higher with the new seal, which was amazing to me! Then I decided to convert it to hpa, but kept the co2 piercing needle on the end plug so I could still use co2 if I wanted. Put a tee in the end cap as you can see in the picture, with an Asa adapter and an empty 88g co2 cartridge. Just as it was it was shooting around 750fps at 1200psi but every shot dropped in velocity.
So I did a bit of research on the forums and seen the posts on cock on open, and Bob Sterne's ssg. Wanted to try that out, but didn't trust myself to do a good job cutting the bolt groove down at the back end, so I did the alternative mod of drilling a second hole behind the hole that the pin for clocking the gun goes in, and then put that pin in the new hole. I took the other pin out of the back block then cut a piece of a pen for a spacer on the spring guide, put it back together and kept having trouble clocking it. It'd cock 2 or 3 times then when Id get the gun all back together it wouldn't cock. I ground the bottom of the hammer a bit where the sear catches, put it back together, seemed to work good, then stopped working again. Finally realized that the spring was messing with the pin somehow, pushing it up or something. I ended up sticking a crosman 1322 poppet in the hammer before the spring, voila, haven't had trouble since!
Filled it back up to 1200psi and still was getting descending strings even after I shortened the spacer. The speed had dropped though to the 600's so I didn't want to weaken the tension anymore so I started filling the tube and tank higher. Supposed to be safe to 1800psi(so I tell myself anyway, from various people saying that co2 can reach those pressures on a hot day.), so I pumped it to 1400 psi, then 1500psi, then 1600 and finally 1700psi. This is higher than I wanted to go, but I finally got a nice long curved string starting at 633 fps and the next 22 shots were in the 620's and 630's, then next shots were 647, 633, 647, 636, 633, 641, and the next 13 shots ranging in the 620's to 640's rather randomly. After this I had to stop for an hour or so, and finally got back to it and decided to shoot 10 shots at the target to sight in my new 6-24 scope. (will post picture soon[WHITE SMILING FACE]️) after 10 shots I chronied it and got 618fps then 627. Again had to stop, then finally got back and shot 8 more shots before it dropped to 608. Stopped there, which was well over 60 shots, 64 I think! Pretty good I think for the tube and a little 88g cartridge, though I plan to tune this gun to somewhere in the 700's, so that will drop the shot count.
Anyway, my shot spread was a little weird and random, which makes me think I need to polish the hammer some more and the inside of the tube.
However I found another good reason for the wonkiness, when I finally found something to loosen my valve in the tube to get it out. The fiber dirt catcher was all coming apart and was jammed between the washer and the spring! Had a bit of a struggle getting it out! Threw the fiber away, and decided to butcher the valve by cutting two big trenches in the side of the valve for air flow, like Bob Sterne has done, though I did a horrible looking job, lol. Anyway sanded itand smoothed the outside and stuck it back together without the co2 striker and put a longer spring in with a little cap on the end for the valve stem to hit against. Put it back together with a 1" spacer on the hammer spring guide and pumped it up to 1600psi. Now it shot 847fps on the first shot and descended from there for 6 shots down to 804. Need to cut my spacer shorter to get it back down into the 700's. Not really sure if doing what I did to the valve will give me more efficiency or not, perhaps someone here can set me straight. [emoji4]
Oh I forgot I had also drilled out the bolt probe both directions earlier when I did the cock on open mod. Unfortunately I never tried it again on co2 without doing anything else to it, so I have no idea whether it helped, though it should have I hope.
Next job will be to make an ssg for it, try to tune it to 30 or 40 shots in the 700-750fps range! ImageImage

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Thu Apr 27, 2017 11:37 pm 
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Forgot to mention Ron from innovativemachining.ca, who has been a great sounding board and great guy all around, made me a new hammer, and I ended doing my modifying of the stock hammer while I waited for his to come because I figured it be fun to try and wouldn't matter if I wrecked it since I was getting another anyway. Turns out after a bit of trouble it's working great, though it is a bit shorter hammer stroke with that pin moved back.
I must admit if it wasn't for the fact you can't easily put on an rva or ssg with cock on closed, I'd keep it that way. I somehow enjoyed the feeling of pulling the bolt back the stock way, but this is fine too, more normal.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Fri Apr 28, 2017 12:15 am 
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Location: Winnipeg, Canada
1700 is pushing it a bit for a CO-2 tube. They lack the roll stamping of the XX guns, but that's still high.

The factory cap seal on my D started leaking around 1175. 1200 and the factory valve stem seal over compressed. I didn't take the CO-2 tube any higher than 1500... but that's just me. I'm not at all sure how well made the Asian CO-2 tubes are~ or what they will withstand.

When I had the RVA on it, I was getting upper 950's JSB Heavies with a near coil bound hammer spring at 1100 psi for one shot. :lol: Pump and dump :mrgreen: I had always thought about a regulated bottle for that set-up- it would have likely worked well. The valve was severely cut for that performance....

-D.S.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Fri Apr 28, 2017 1:01 am 
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Location: Caronport, Saskatchewan
Yeah, 1500psi is where I think I should limit it. Will have to monkey around with it to get a nice string at that pressure. Wow, 950 with heavies is pretty amazing! I never tried putting much pressure at all on my spring, haven't put an rva in yet. Will be fun to try it out. I agree a regulated bottle would be best. Though 60 some shots was pretty good out of that little bottle, and costs way less than a regulated one!
I got some different seals from canadashootingsupply.ca, black ones, came with a different valve port seal as well, all for 11.99, free shipping. Good guys there for sending small stuff free. They even sent me 1322 valves free, though I realized later they were detuned ones. Anyway so far so good with the seals, guess we'll see what happens in the future.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Fri Apr 28, 2017 9:23 pm 
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Location: Caronport, Saskatchewan
I put my medium spacer (around 1")back in and pumped it back up to 1500psi, and shot this string: 822, didn't register, 822, 825, 822, 838, 820, 827, 830, 809, 807, 812, 804, 804, 785

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Fri Apr 28, 2017 9:27 pm 
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Tried with a smaller spacer, lol, didn't even fire. Got to do either an rva or ssg soon [emoji4]

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Fri Apr 28, 2017 9:41 pm 
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Location: Caronport, Saskatchewan
Found an interesting article where a guy tested a qb78 tube with a custom plug that put the oring further down the tube away from the threads. He found a safety max working pressure of over 4000psi, for the tube with the extended plug. He had a hydraulic piston in the valve end of the tube so that's not testing how much the valve can safely hold. However, if a guy had the bulk fill cap with the oring further down a person should be able to fill it to 2000psi or so? I think Bob Sterne had recommended putting a higher tensile screw to hold the valve as well as the spacer behind the valve?
I'll have to find that website and post the link here.
Of course with my 88g cartridge, I think 1600psi is probably the max I think for a regular fill, perhaps even that is too much.

I looked at my end cap on mine, and the oring is actually sitting ahead of the threads as it is, though not by much. If a guy could machine a second groove further up for another oring, you'd think that would be fine. I however, do not have a lathe!


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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Fri Apr 28, 2017 9:44 pm 
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Here is the article quoted on a south africa airgun forum. Couldn't find the actual link.
Questions about PCP conversion of the QB78 have generally met with negative responses in UK forums. However the pressure cylinder of the QB78 is very similar to the cylinders on similar pump-up pneumatics from Crosman and others and it therefore seemed to me that the QB78 cylinder could safely be used as the basis for a similar relatively low pressure PCP to be charged from a hand pump. For those interested here are the results of my experiments.

After stripping the gun and measuring the cylinder it was apparent that the total volume available would be quite Small - about 50cc including the volume of the valve chamber. The shot count between fills would therefore be low, but the trade off would be that less pumping would be needed between strings of shots.

It was also clear that the metal of the cylinder was much thinner in the region of the threaded section at the fore end and that the original end plug made its seal in this region.

I suspect that this may be the reason for any reported failures of the pressure cylinder when subjected to higher than CO2 pressures, but to be on the safe side I decided upon a hydraulic pressure test of the cylinder.

A simple hydraulic pressure testing rig was formed by making a new front end plug to form a front seal at a point behind the threaded section of the cylinder and a travelling piston to fit the opposite end of the cylinder. The front end plug was fitted with a hydraulic pressure gauge and by filling the cylinder with water and forcing the travelling piston forwards using a two ton hydraulic jack with the parts located in a suitable cage I was able to apply pressures up to 400 BAR.

From boiler technology the formula for the safe pressure limit of a seamless thin shell pressure cylinder is given by: p=t*2f/d
Where p is the pressure in psi, d is the internal diameter of the tube in inches, t is the wall thickness of the tube in inches and f is the UTS of the tube material in lb/sq. in reduced by an appropriate safety factor.

The QB78 cylinder has an internal diameter of 0.75 in, an external diameter of 0.865 in and appears to be made from bright drawn carbon steel. Assuming a UTS of 25tons/square inch which is between 20 tons for mild steel and 30 tons for low carbon steel this gives a pressure limit, ignoring the safety factor, of about 570BAR. Again from boiler technology it is accepted that the safety factor should be such that the cylinder does not expand by more than 0.1 percent of its diameter under the maximum working pressure.

With this in mind I applied test pressures to the cylinder. It withstood an initial test pressure of400 BAR without damage. Then I applied pressures gradually increasing from zero, while carefully measuring the increase in diameter of the cylinder. At 300 BAR the increase was 0.00075 in, and so l determined this as the maximum limit.

I then reassembled the gun as standard except that the new end plug was provided with a non return valve and a union for the attachment of a hand pump.

Using an FX stirrup pump I Then tested the gun at various pressures to determine the power curve under pneumatic pressures

Somewhat to my surprise l found that the power was quite consistent over a pressure range between 150 BAR and about 80 BAR. The following figures were obtained for a string of 22 shots, using Gamo TS18 18grain long range pellets straight from the tin.

fps Ft/lb
588.2 13.8
584.8 13.7
594.5 14.1
601.7 14.5
604.6 14.6
606.8 14.7
615.0 15.1
606.8 14.7
615.8 15.2
615.8 15.2
613.5 15.0
612.0 15.0
618.8 15.3
606.8 14.7
609.0 14.8
606.8 14.7
603.1 14.5
606.0 14.7
593.1 14.1
594.5 14.1
587.5 13.8
575.4 13.2

I am quite happy with this result for Spain, but if I were repeating this exercise to meet UK legal limits I would try to detune the gun by restricting the transfer port, reducing the hammer spring, or providing an internal valve opening restrictor similar to that of this BSA Ultra."

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sun Apr 30, 2017 3:15 pm 
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Location: Winnipeg, Canada
I never did change the valve o-ring from factory~ and it does hold at 3 K psi. I did really reef down on the front section of the valve to expand that sucker 8) Only place I had to use h/t screws was on the fill adaptor on the aforementioned pcp tube. No issues to date, and I usually leave 900 psi or so in the tube to keep the seals compressed when the gun is not in use.

I'll take anything posted on the internet with a healthy dose of salt over the left shoulder. :wink: 4 K is well over basic design.

Interestingly enough, the regulated 3-K bottle AR-20/78 I was looking at during the gunshow had a 1.2 K burst disc on the downstream side. Serious food for thought there... Those ninja bottles with their tiny regulators are intriguing....

-D.S.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sun Apr 30, 2017 4:34 pm 
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Location: Caronport, Saskatchewan
1200 burst probably meant it was set at 850psi. Interesting.
Bob Sterne sets his at 1500 I believe, could be wrong. Anyway, that'll be my limit until I get a tube or regulated bottle. Even with the leak, I was getting 850fps at top end, without any rva to max out the spring. Finally put an rva on it now, off centered to get past the long bolt down the middle of the end cap. Just waiting to take it out and shoot it now!

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sun Apr 30, 2017 4:36 pm 
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Hoping no more leaks! That tubing I had was 3/16 inside so it is too big, will have to buy some smaller stuff if the seal canadass sent leaks.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sun Apr 30, 2017 5:12 pm 
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Location: Winnipeg, Canada
850 is very good for RVAless! Stiff springs :mrgreen:

I really should investigate those ninja bottles a little more closely. .85 K sounds about right~ it was being sold as a non-pal rig :lol:
My problem is all these projects that demand completion. Some require a bit of shell-out.
That previous experience with the hogged valve is scratching at the back of my head though~ very good potential for what the bottle would cost.
The ninja bottle didn't seem that big, or in the way on the sample I looked at- in fact, if not for the cut down stock, it would be close in weight to an un-molested original...

-D.S.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sun Apr 30, 2017 7:27 pm 
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Nice! Sounds like a nice project gun!
I did have a spacer behind the spring in mine, have to because otherwise it won't even fire since I did the cock on open mod.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue May 02, 2017 5:49 am 
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Location: Stony Plain, Alberta
If you put in that hammer and spring I sent it will give longer hammer stroke and fire at higher pressure. I have similar hammer and same spring with ssg in mine and it fired perfectly at 3000 psi. Don't forget to lengthen bolt cocking slot and flat on bottom of bolt if you do this.

Ron

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue May 02, 2017 10:36 am 
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Thanks!

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