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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 12, 2020 11:03 pm 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6882
Location: Rocky Mtn Hse Alberta
Haha, yes I suppose. Would like a larger setup on a Crosman 1400.
This thread might renew an old hankering for a QB pumper.
Cheers

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Walter


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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 12, 2020 11:33 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 9058
Location: Coalmont BC
Many years ago I built a .22 cal Uber-Pumper around a Crosman 140/1400 tube and linkage.... with a much enlarged 2289 valve....

Image

At nearly 26 FPE with 14.3 gr. pellets at 900 fps it had more performance than I have yet teased out of this 392.... With a .177 barrel on it, shooting 7 gr. pellets, I actually broke Mach 1 with it.... :shock:

Image

At anything past 14 pumps it was a bear to pump.... but power it had.... :roll:

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Mon Jan 13, 2020 1:40 am 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6882
Location: Rocky Mtn Hse Alberta
Fantastic build there Bob!
Love it

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 Post subject: Re: My Benjamin 392
PostPosted: Mon Jan 13, 2020 2:15 pm 
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Location: P.G. B.C.
Yeah - that's amazing - amazing power as well.

Speaking of which, where's Joe?

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 Post subject: Re: My Benjamin 392
PostPosted: Mon Jan 13, 2020 4:20 pm 
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Location: Coalmont BC
I tried to figure out what the efficiency was based on the pressure drop in the valve, for the first 2 shots at each pressure level.... It was a bit complicated, because I had to base the remaining pressure on the velocity of the third shot, but I think the chart below is pretty close....

Image

I stopped the data at the SSG gap setting where the second shot was faster than the first.... The FPE I used was the total of those 2 shots.... and I calculated the volume of air used by taking the pressure drop (in bar) times the valve volume in CI (6.7 cc = 0.409 CI)…. This is similar to the way it is calculated for a PCP.... I confirmed by taking the total FPE when the gun was tuned for 2 equal shots and the number of pumps to top it back up, and the data is consistent with that....

The other way of calculating the efficiency of a pumper is to use the volume of air compressed by the pump (swept volume of 3.54 CI times the number of pumps)…. which always results in a lower value.... The difference is because of pumping losses, and the cooling of the air after pumping.... For the 4 pressures tested, at the SSG setting where there were 2 equal shots (3 for 6 pumps), I got the values below.... These values are always lower for high pump numbers in any MSP....

6 pumps..... 3 shots totaling 15.5 FPE requiring 4 pumps to replace = 14.2 CI.... 1.09 FPE/CI
10 pumps.... 2 shots totaling 21.8 FPE requiring 7 pumps to replace = 24.8 CI.... 0.88 FPE/CI
14 pumps.... 2 shots totaling 29.0 FPE requiring 10 pumps to replace = 35.4 CI.... 0.81 FPE/CI
20 pumps.... 2 shots totaling 37.2 FPE requiring 14 pumps to replace = 49.6 CI.... 0.75 FPE/CI

I'm pretty happy with these efficiency levels.... in particular the overall efficiency for when there were 2 equal shots, which is the way I would tune the gun most of the time.... That runs 1.29 to 1.43 FPE/CI, so the SSG is certainly doing its job.... 8)

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Mon Jan 13, 2020 10:11 pm 
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Location: Coalmont BC
Today I decided to make an adjustable piston.... The valve has a cone on the front, and if I machined it off flat, like you would for a 13XX valve, and then made a matchng flat-topped piston, it would have exposed the check valve.... I'm pretty sure it would have still sealed OK, but the diameter of the exposed Delrin would be about 7/32".... I had one like that on a 2200 valve years ago, and it partially extruded through the hole.... After careful examination, I decided that I could machine the cone to a much shallower angle, which would eliminate some of the distance from the O-ring to the end of the valve, and shorten the small hole that feeds the check valve.... Both of those will eliminate some headspace.... Then I made the adjustable piston with the same angle, but concave instead of convex.... The angle that worked the best to leave the smallest lands near the O-rings, and not expose the check valve, was 20 degrees.... Here is what the end of the valve and piston look like....

Image

If you look closely at the center of the valve, you can see that the check valve hole is so short you can see the end of the check valve.... When I machined the piston I started out with a slightly shallower angle, pressed the valve against it, and of course it rocked on the center of the cone on the valve.... I kept changing the angle, about 1/2 deg. deeper at a time, until the valve would no longer rock when pressed against the end of the piston.... This should work like an FTP, but with shallow matching angles.... The total distance between the O-ring on the piston and valve are the same as what the land on the valve was by itself.... It is actually less than the distance from the valve O-ring to the edge of the original cup seal was.... I don't see how I can reduce the headspace any more without the possibility of ruining the valve by machining into the check valve....

Here is the adjustable piston.... I even used an O-ring grooving tool with an hemispherical end to minimize any air gap under it.... Just behind the O-ring I machined a groove 3/8" wide and 0.10" deep.... This is wound with 3 layers of kitchen string (with the ends pulled underneath to secure them)…. This works like the felt on a 140 piston, and is soaked in a 50/50 mix of "Moly-Slip" transmission additive and 10W motor oil.... I use that mix for almost anything that needs lubrication, (except inside HPA reservoirs, of course) and have used it to soak the felt (or string pad) on many pumpers in the past, including my Millennium Pumper.... I talked to the 4th owner of that gun the other day, and he says it still functions perfectly.... 8)

Image

I adjusted the length of the piston before I installed the O-ring and string pad.... With the valve installed I made it so that I could not quite install the pump pivot bolt.... At the moment the length is such that the pump handle stops about 1/2" from the tube.... Once things settle in I may lengthen it 1/4 turn to get a small amount of preload.... The adjusting threads are 1/4"-28NF, so 1/4 turn is only 0.009".... Now to assemble it and see if it takes fewer pumps to get to 1500 psi.... ???

OK, I put it back together, and I'm getting about 50 psi more at 20 pumps.... I have saved nearly 1 pump at 1500 psi.... There is very little difference at lower pressures, but it is definitely more efficient over 1000 psi.... I pumped 20 times, adjusted the SSG to 5 turns out, and I got 2 shots within a 2% ES, averaging 705 fps with the 18.1 gr. JSB Heavies.... It only takes 13 pumps to refill for another 2 shots.... That reaches my initial goal of 2 shots at 20 FPE.... 8)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 15, 2020 1:38 am 
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Location: Coalmont BC
I readjusted the piston 1/4 turn tighter.... The pump arm rebounds about 1.5" from the tube, the bottom of the wood forearm is below the stock, so not quite as much rebound as with the rubber cup.... That has reduced the headspace a bit (just that 1/4 turn gave another 50 psi at 20 pumps)…. Here is a new velocity and pressure vs. pumps chart, with 1st, 2nd and 3rd shots charted.... I was running 2 turns of gap on the SSG, which I tested and found to produce no velocity drop with the 15.9 gr. pellets, even at 20 pumps....

Image

I am now getting 1500 psi (maybe 1510, it is above the line on the gauge, not below it) on 18 pumps.... With the rubber cup that took 20 pumps, so I have definitely increased the pumping efficiency.... 8)

I also tested it at 20 pumps with the 25.4 gr. Monsters, and I got 707 fps, which is 28.4 FPE.... I think that surpasses most of the Steroids I have seen numbers on, so I'm pretty pleased with that.... I also tried it a 20 pumps with 3 turns of gap on the SSG with the 15.9 gr. pellets, and I only lost 3 fps on the first shot, but the second shot was 691 fps instead of 640, and the 3rd shot was 326 fps instead of 210.... So, even if you tune for full power, backing off on the SSG will retain more air, in this case about 200 psi less used on the first shot, yet no loss in power.... That means fewer pumps tp refill.... 8)

I made a much lighter hammer today too, and did some brief testing.... It weighs 40 grams (instead of 65), and I can no longer get full power with it at 20 pumps.... The reduction in weight means I need more spring to get the same power in the gun.... I tried it at 18 pumps (1500 psi), and instead of requiring 5.3 turns of gap like the stock hammer to get 2 equal shots, with the light hammer I only need 1.5 turns.... I got 4 shots with the 15.9 gr. pellets of 726, 731, 562 and 245 fps.... and the report is noticeably quieter.... The gun makes a nice crisp SNAP on the first shot and a louder BANG on the second shot, but NO burp of hammer bounce at all.... You don't get any with a properly tuned SSG anyways, but the lighter hammer is definitely quieter and more efficient.... With the heavy hammer the third shot is about 550 fps, and there is not enough air for a 4th shot (just a pop, not enough to drive the pellet)…. This is proof positive that the lighter hammer / SSG combination is saving air.... 8)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 15, 2020 3:31 pm 
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Joined: Sun Jul 28, 2013 8:25 am
Posts: 4
rsterne wrote:
This works like the felt on a 140 piston, and is soaked in a 50/50 mix of "Moly-Slip" transmission additive and 10W motor oil.... I use that mix for almost anything that needs lubrication, (except inside HPA reservoirs, of course) and have used it to soak the felt (or string pad) on many pumpers in the past, including my Millennium Pumper.... I talked to the 4th owner of that gun the other day, and he says it still functions perfectly.... 8)


That would be me and yes the Millennium Pumper works great! I'm having much fun with it and I look forward to small game hunting in the spring with it. Thank you Bob for crafting such a fine piece and thanks to the two previous owners who looked after it so well.

I'm going to mix up a batch of your lubrication mix and use it myself.


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 15, 2020 9:38 pm 
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Joined: Sat Nov 05, 2016 10:52 am
Posts: 3142
Location: Caronport, Saskatchewan
Great results with that 392!
Do you find a cold gun gives different results than one that's been pumped recently?
I found this trouble trying to get consistent double shots with the LR700W. I'd get it nicely tuned. Then try it cold and it'd be different.
Regards,
Wes

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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 15, 2020 9:41 pm 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6882
Location: Rocky Mtn Hse Alberta
Any accuracy results from your 392? Curious to know how yours handles the 'swoop'?

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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 15, 2020 10:48 pm 
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Posts: 9058
Location: Coalmont BC
Walter, my winter weather prevent any accuracy testing until about May.... maybe then, with a little luck....

Wes, when tuning for 2 shots, if you are pumping and shooting and tune for 2 equal shots, and then let the gun cool, the pressure will drop.... That will result in the first shot being faster than the second shot.... Adding one pump after it cools down, to bring it back to the same pressure as it was during the testing warm should cure the problem....

My Uber Carbine, tuned for 2 equal shots of 600 fps with 14.3 gr. takes 19 pumps.... If you let it sit, you have to add one more.... Alternately, if you pump it 20 times, and test while hot, the second shot will be faster.... but if you let it cool they will be the same velocity.... It is the cooling of the charge, and associated pressure drop, that is the culprit.... That is why a gauge is nice, you can detect that....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Thu Jan 16, 2020 8:50 pm 
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Location: Caronport, Saskatchewan
Very good. I recall that cure now with the LR700W as well.
Thanks,
Wes

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 Post subject: Re: My Benjamin 392
PostPosted: Sat Jan 18, 2020 11:05 pm 
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Location: Coalmont BC
I started working on the new valve front end today to convert to a 2-piece valve.... The body for the front end is made from scratch, turned from a piece of 6061-T6 aluminum.... The space between the O-rings is 1/2", to give room for the gauge port and the two low-profile 8-32 high tensile screws.... They are set down on spot faces on the valve, so that the head will be supported by the brass tube.... I made the check valve from a piece of 1/4" Teflon rod with a light spring.... The valve spring is the Conical spring I use inside an MRod valve.... In order to provide a spring seat for both, I made a thin stepped washer, and it is supported in a column high enough to provide about 3/8" preload on the valve spring.... I can shorten the column if that is too much.... The tublular column only has a 1/32" wall so that it doesn't take up much volume inside the valve....

Image

The seat for the check valve is an 8-32 setscrew, threaded into the front of the valve so that the cup point just protrudes into the check valve hole.... It is drilled though with a 3/64" drill to minimize head space, glued in with Loctite 638, baked to cure it, and then machined off flush with the flat front on the valve....

Image

I still have to drill the holes in the tube for the mounting screws and then I can check for leaks before I do the final machining on the back of the valve to shorten it just in front of the seat.... I also have to make a new flat faced piston for the pump....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 19, 2020 9:25 pm 
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Location: Coalmont BC
Today I drilled the holes in the tube to mount the front part of the 2-piece valve.... It was mostly hand work again, because of the difficulty of holding the soldered together tube and barrel assembly in my small milling attachment on the lathe.... :roll:

Once I got it mounted properly, I then made a new flat head for my adjustable piston to match the valve.... I didn't yet machine off the back of the valve in case there was a problem, and I'm glad I left it so that I could back up to the previous valve.... I struggled for over an hour trying to get the valve to seal in the tube.... I was sure it was the front O-ring on the rear valve half, and replaced it with a 90D, but it still leaked.... I finally gave up and went to reinstall the previous 6.7 cc valve, and while it was out I checked the poppet and it was leaking a bit.... enough that I couldn't get pressure into the gun....

I spun the poppet against the seat with a drill and it sealed right up.... I then tested the gun with the new FTP and valve front, but with the rear half of the valve still intact.... I estimate the volume at about 9 cc.... Anyways, it seems like the new FTP has less headspace, because it pumps better at high pressures.... Here is a chart of the velocity and pressure at up to 24 pumps with this interim setup....

Image

I was using 1 turn of gap again on the SSG to make sure the velocity was peaked.... I'm quite pleased with the results.... The velocity with the 15.9 gr. pellets now peaked at 871 fps at 1580 psi, which is 26.8 FPE.... That is 2 FPE more at about 20 psi less pressure because of the increased valve volume.... 8) …. With the 25.4 gr. Monsters I got 720 fps (29.2 FPE), and I will have more volume yet when I cut off the front part of the rear valve half just at the seat.... ;)

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Mon Jan 20, 2020 10:23 am 
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Location: Caronport, Saskatchewan
Wow that's impressive!


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