Author Topic: My Benjamin 392  (Read 5341 times)

rsterne

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Re: My Benjamin 392
« Reply #30 on: January 11, 2020, 12:06:47 PM »
Well, the gun didn't leak at all overnight....  8) …. However, I am getting a puff of air out the side screw holes on firing, particularly at high pump numbers....  :(

Today I tested the velocity and pressure at every pump stroke up to 15 pumps.... That was just over 1500 psi, which I am going to use as a self-imposed limit (at least for now) because of using the stock pump linkage, and because I want to maintain the safety margin on all components, particularly the valve screw pockets in the tube.... I did the same on my .25 cal Disco Carbine.... I pumped it to 1800 psi to test it, and then backed off to 1500, for longevity, and also because it was harder to pump than I liked at 1800....  ::)

I calculated the additional volume from adding the gauge, which was about 0.4 cc, so the total volume has now increased to about 4.6 cc.... This means that for any given number of pump strokes the velocity and FPE have dropped a few fps because the pressure has dropped by about 10%.... This is because the swept volume is the same, but since the valve is larger the pressure gain on each pump stroke is a bit less.... In other words, it takes about 11 pumps now to get the same pressure as I had before at 10 strokes.... However, if you take that into consideration, the gun is developing more power at the same pressure than before, because of the extra valve volume, which causes a lower pressure drop during the shot.... I set the SSG at 1 turn of gap for this testing, and recorded the pressure and velocity for the first shot, and once the gun began retaining air, I did the same for the second shot (dotted lines)….



This gives me a really good baseline for further work.... As I increase the valve volume, the pressure gauge is a necessity to know how many pumps to use to get back to the same pressure.... It will be equally important when I start to work on the pump, not only to measure any gains in pump efficiency, but to make sure I am not overstressing the components.... When I compare this pressure curve to the graphs in Reply #66, the closest match is a headspace of 0.3 cc (because I adjusted the valve length a bit longer) and an efficiency of about 75% (when corrected for the increased valve volume)…. The next step is to remove the valve and bore it out to increase the volume....

Bob
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calinb

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Re: My Benjamin 392
« Reply #31 on: January 11, 2020, 01:29:23 PM »
I have never had a Benjamin 392 pumper before, and Eric at Scopes and Ammo was clearing them out last fall, so I had my wife pick one up for me for Christmas, and I opened it today....<snip>
Bob, I first read your post on the other forum and then came here (where my post is less likely to get pulled, I think). I still need to study the rest of this thread but gotta go plow some snow right now. This thread looks like it's picking up more steam than the other one too. Good stuff here!

I have both a 392PA and 397PA and one of the last Racine Silver Streaks made, which I had Tim McMurray Steroid for me. The old Yellow and Green forums had a treasure trove of info, but between their decommissioning and the loss of countless photobucket pics, I can't find most of it any longer. (The tapatalk archives seem to be almost unusable to me.)

In a nutshell, there are two different directions to take these guns: Maximize air dump and power (Tim's approach with the Steroid mods) or take the air conserving pumper (ACP) mods approach (as promoted by Steve Woodward on the old Yellow) for multiple shots on an air charge. Tim and Steve argued a lot online. Tim posted a ton of info about pumpers and his Steroid performance mods on GTA (actually shared with everyone pretty much what he does). Hopefully it's still all in the GTA somewhere, but ya' never know on any airgun forum, with all the fighting and banning that goes on!

https://www.airgunsofarizona.com/BenjaminACP.htm
http://mac1airgun.com/steroidbenjaminsstreaks.html

Tim did my Streak Steroid, which is the only way to get an official "Steroid" (and also did a beautiful job silver soldering the barrel under the rear sight--a common separation point) but I also have a bunch of Tim's spare parts and a bunch of ACP mod parts just sitting in a box. I'll probably "Steroid" my 392PA eventually but I might install some ACP parts in my 397, including a small CO2 cylinder as an air tank under the pump fore end.

The newer 392PAs tend not to be very accurate. Tim warns to not spend money on them or extend their power and longevity with mods (Steroid), if they are not accurate and thus not worthy. I've tested a couple dozen types of pellets in my 392PA and the JSB Jumbo Heavy is the only accurate pellet that I've found; my 392 is very pellet picky (maybe due to its very shallow rifling, which is common in these guns). I've slugged all my barrels. BTW, I once spent a few hours searching the web and compiling a spreadsheet of online accuracy reports on these rifles (dozens of online reports). I simply indiscriminately put the data into the sheet and found the 392 reports to be significantly lower in accuracy, on the average. The 397s accuracy reports were the most favorable and Sheridans were close. If you can't get U.S. quarter size 10-shot groups at 25 yards (center-to-center measurement), try JSB Jumbo Heavy pellets!

My 397PA is just the opposite--a tack driver with just about any pellet. My Silver Streak is also accurate but there are not many 20 cal. pellets available to test (still need to test the brand new JSB 20 cal. Heavy pellet). I use the 13 gr. JSBs or sometimes H&N Hunters in it.

I think I have a bunch of large Internet forum and document archives about the "Bens" and "Dans" that I downloaded somewhere. If you browse through them, you should get some ideas. I'll PM you, if I can find them on my disk drives. I studied the subject for countless hours online several years ago but never found the time to benefit from my learnings in project completions. :(

BTW, Most people just install an adjustable piston to remove excessive headspace. You can mod your stock piston or buy one from Tim (but I know you'll mod your own).

http://www.mac1airgunshop.com/sheridan-rocker-bolt-lug-p/m1stadpis.htm

-Cal

« Last Edit: January 11, 2020, 01:48:59 PM by calinb »
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rsterne

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Re: My Benjamin 392
« Reply #32 on: January 11, 2020, 04:19:29 PM »
Hey, Cal.... thanks for the summary, much appreciated.... I wasn't sure which way I was going when I started this project, but it looks like I'm going to end up with a Retained Air Steroid with a twist of Canuck....  ::) …. I will be making my own adjustable flat-topped piston, like I did for my Millennium Pumper once I exhaust the possibilities with the stock pump....

Today's project was to bore out the valve, removing as much metal as I could safely, without cutting holes in the mid section....  Even though I currently have the two extra O-rings needed to do that, I wanted to see how much I could increase the volume and still have the valve look stock on the outside (except for the gauge port)…. At this point, the two extra O-ring grooves are not needed, even for the gauge, since it seals to the valve with Teflon tape.... Here is a photo of the outside of the valve at this point....



The gauge port is exactly 2.25" ahead of the lower valve screw hole, measured center-to-center.... The  first thing I did on the valve to increase the volume was to cut off some of the threads.... I left the stub 3/8" long, measured from the shoulder…. I then bored out the inside of the valve front to 5/8" ID, from about 1/16" ahead of the shoulder where the O-ring sits down to the shoulder where I had the spring seat.... The length of that bored out section ended up at 5/8".... I had to make a new double ended spring seat to hold the check valve spring and the front end of the valve spring, I just turned it out of aluminum.... It is 3/8" OD on the front end, 0.450" in the middle, and drilled to 0.29" to accept the outside of the much lighter check valve spring.... I also shortened the check valve some more, to make up for the extra volume of the new seat.... The back of the seat is 1/4" OD to fit inside the valve spring, and the through hole is 3/16".... Here is a photo of the "guts" of the valve, ready to install....



The back half of the valve also got bored out to 5/8" ID, which just removed the threads inboard of what I needed to secure the two halves together.... The 5/8" ID carried on all the way back to the solid area where the seat is.... That section of the 5/8" ID is about 3/8" long, starting from 5/16" inside the valve, leaving the same threaded length as on the stub, once you allow for the 0.050" gap when assembled.... Here is a photo looking inside the valve halves, to show where they were hogged out....



I calculated the increase in volume from the machining, and it works out to 2.1 cc.... That would be a 50% increase on a stock valve, but with my gauge port and adapter, my total volume is now 6.7 cc.... That is 50% larger than with the gauge installed, and a 60% increase over stock.... To get to the same pressure, it should therefore need 50% more pumps than the graph I posted in my last post.... We'll soon see, as the next thing to do is reassemble the gun and repeat the last set of tests....

Bob

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rsterne

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Re: My Benjamin 392
« Reply #33 on: January 11, 2020, 06:51:35 PM »
After dinner I ran the same tests I did with the stock valve with gauge.... I measured the pressure and velocity of the first and second shot every 2 pumps up to 20, where the pressure exceeded 1500 psi.... I was still using the same 15.9 gr. pellets as all the other similar tests.... As expected, I require more pumps to get to the same pressure as before, because the valve volume is 50% larger, at 6.7 cc.... I am still running the SSG at 1 turn of gap.... Here are the results....



It looks like I have gained significantly in pumping efficiency.... With a 50% valve volume increase I had expected 50% more pumps to get to the same pressure, but these results are significantly better than that.... I got 1520 psi with 20 pumps, whereas I expected that to take 22 pumps.... This 2 pump advantage over what I expected occurred all the way along the pressure range, from 600 psi and up.... The first few pumps are MUCH easier, which I think is from increased pump efficiency from filling the larger valve.... There is probably also an efficiency gain because the headspace, which hasn't changed, is a smaller percentage of the larger valve volume....

The velocity and energy is significantly greater at the same pressure with the larger valve volume.... This is much like what happens when you install a larger plenum in a PCP.... In addition, the residual pressure for the second shot is also greater, and should increase even more when I dial back the SSG.... At 1520 psi (20 pumps) I got 827 fps with the 15.9 gr. pellets (24.2 FPE), and the 2nd shot was 622 fps (13.7 FPE)…. for a total of nearly 38 FPE.... I tried the 25.4 gr. Monsters at the same pressure, and hit 698 fps (27.5 FPE) with a 2nd shot of 524 fps (15.5 FPE) for a total of 43 FPE.... The next thing to test is to see what happens to the velocity of both shots when I dial the hammer strike back by increasing the SSG gap.... It will be interesting to see how many pumps per shot are required to get a continuous string of 20 FPE shots....

Bob
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Alan

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Re: My Benjamin 392
« Reply #34 on: January 12, 2020, 05:16:33 AM »
Geez! Twice the FPE! A lot more pumps to be sure, but it does show what could (can) be done if one is willing. All of this makes me wonder....

Benjamin advertises their 397 as developing (up to) 14.9 FPE. Maybe they pumped up their test model with Helium, because as close as I could get, was just over 10. Even after I "slightly" modified the valve, I only got a bit over 12 FPE. Makes me wonder what a few factory mods could do?
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calinb

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Re: My Benjamin 392
« Reply #35 on: January 12, 2020, 01:01:40 PM »
Anyways, on reassembly with the aluminum valve with the 2-spring mod. I found out that the hammer spring was holding the valve open, so I couldn't pump it.... I think this was causing the hammer to jam on firing (the hammer moving too far forward and catching on the forward bump on the sear)…. so I could not proceed further.... I pulled the gun apart again (cursing those roll-pins again, I really have to change those out)…. and disassembled the valve and found another valve spring, in between stock and the one I tried.... Here are the 3 springs....
I'm not sure that I understand all of the above, Bob. It's common for the hammer to hold the valve open with modified guns (Steroid instructions say to cock the gun before pumping and, as I recall, older Sheridan manuals may have said the same thing).

What was causing the hammer to jam on firing and how does it relate to getting the first pumps into the gun?

Are you pulling the valve out through the muzzle end each time you disassemble? You should pull it out from the breech end and then there's hardly ever a reason to remove the roll pins (only for work on the pump piston section). My 392 required a lot of work inside the tube to get the valve out easily though. The problem is Crosman stamped the lettering too deep on the outside of my tube and distorted the tube. I managed to get it out eventually (even without resorting to punching it back from the muzzle end, which would've required removing the roll pins, of course). It's standard technique to use the pump pressure to help to drive it backwards--just be careful of the valve becoming a projectile! I made a mandrel to fit inside the tube and worked the lettering bumps mostly out, but I also had to use a long sander and take off a small amount of brass inside the tube too, followed by polishing. It was a PITA but valves SHOULD come out the breech end!

I'm slowly working my way through this thread. More later when I find more time today but I wanted to mention this before you pull your gun apart from the front too many more times!

Edit: You actually don't have to fully cock the gun. Just hold the bolt back a bit to get a pump or two into the gun. Then the pressure will hold the valve closed against the hammer pressure. I usually just fully cock and then de-cock.
« Last Edit: January 12, 2020, 01:11:53 PM by calinb »
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calinb

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Re: My Benjamin 392
« Reply #36 on: January 12, 2020, 04:01:27 PM »
The other item is a tool to rotate the valve when installing it so that the screw hole (and therefore the transfer port) line up properly....
Clever tool, Bob! I've never needed it, because I can usually keep the holes lined up while I push the valve into place the short distance from the breech, rather than having to push it in nearly the full length of the pump tube from the muzzle end. If I miss alignment a bit, I just give the pump arm a quick partial pump, push the valve back out and try again. I usually get it the first time though.

It would be hard to come up with something that hasn't been done before on a 392....

For sure!

So, I added that screw in the spring hole, and added a 1/8" long #6-32 setscrew in the top of the trigger housing to push against it.... Bingo, I had an adjustable sear engagement, that looks like this (sear shown upside down)...

But you've probably done it, Bob. I've never seen this adjuster approach before now. Very nice! I've seen adjusters installed to push near the top of the bolt safety extension arm of the trigger (extension arm only slightly shortened to enable de-cocking, but not completely removed).

The extended tang on the back of the super sear turns the trigger into a true 2-stage design.... where as you squeeze through the 1st stage you are reducing the sear engagement.... I don't know if it resets reliably if you release it after only pulling through the 1st stage or not (a balk fire), perhaps you have tested that?…. Is your super sear the original, made of hardened steel.... or the new one made of aluminum with just a pressed in  roll-pin on the catchment point?.... That idea might work OK, but just the thought of doing that to save a bit of money on production irked me.... so I decided that was not something I would purchase....
I installed the original SS into my 397PA. It resets reliably. I installed one of the newer aluminum versions on a friend's gun. It does not reset reliably and I never solved the problem. SS designer Steve Woodward told me that it should reset, but it does not. I cautioned my friend to always recock after a balked trigger pull, as is necessary with many airgun triggers.

I ground my 392PA sear to realize a bit of two stage action, along these lines:
https://www.airgunsmith.com/392/triggertune.htm
I also slightly re-profiled the striker ("hammer") engagement surface of the sear to reduce its "positive" engagement against the striker. I still have a bunch of Kasenit so I surface hardened the sear after these grinds. A muzzle loader smith told me there's nothing available that's as good as Kasenit today. I repositioned the weaker hardware store trigger return spring ABOVE the trigger pivot point to remove all the trigger slack and slightly help my finger to oppose the strong hairpin spring.

My third trigger in my Steroid Silver Streak has Tim's treatment. All three of my gun's triggers work well.
I assume that existing ACPs, such as that sold by AoA, must have some way of tuning the hammer strike.... but if so, I don't know what they use.... I think that this version will work great....
Steve Woodward has designed and documented fully adjustable hammers, but his ACP sold by AoA only offers two power settings via two cocking positions on a version of his SuperSear. This Dual Power sear is only available on the AoA gun, AFAIK.
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rsterne

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Re: My Benjamin 392
« Reply #37 on: January 12, 2020, 04:26:37 PM »
Hey, Cal.... I'm so used to taking the valve out through the front (13XX with that raised lug to prevent the valve moving back, and Discos where the slot in the back carves up the O-rings) I never thought of pulling it out the back.... ::) …. Yeah, that roll-stamp really messed up my tube, I had to file/sand it away.... now I could probably take it out the back but old habits die hard....  ;)

When you get further into the thread, you will see that I'm running an SSG, so the hammer has a gap to the valve stem all the time (even uncocked)…. I could actually use a much lighter valve spring now, but the MRod spring is working fine and I can't be bothered changing it.... The required hammer strike with the smaller MRod poppet with the 1/8" stem is so low the gun is easy to cock, no need to change.... If you don't know about SSG's, stand by, this post will rock your world....  :o

OK, guys, now we're into the meat and potatoes of this build....  ;D …. Today I wore out my arm completely, shooting out all the air in the valve at 4 different pressures and up to 15 different SSG gap settings.... Here are the results, on 2 charts so they aren't so crowded.... In all cases the solid line is the first shot, the dashed line the second shot, and the dotted line is the third shot.... By the time the 3rd shot got to about 500 fps there was a 4th shot, and although I recorded it, I didn't graph it, it would have made the graphs even harder to understand....



On the above chart, the red lines are at 6 pumps (about 600 psi), and the blue lines are at 10 pumps (about 900 psi)…. You will note that the plateau (665-670 fps) on the 10 pump curve extends to 8 turns of gap (over 5/16") and on the 6 pump curve (550-555 fps) past 10 turns of gap (about 7/16")…. Reducing the gap for more consistency by using a lighter spring and/or less preload would be better.... MAN is the gun easy to cock with the SSG installed, and would be even nicer with a lighter spring and smaller gap.... Now the details for these pressures....

At 6 pumps, all three curves cross at 14 turns of gap, giving a 3-shot bell curve of 385, 400, 360 fps (a 10% ES)…. This is only 5.1 FPE average, and I didn't explore it further.... At 10 pumps, I can get two shots of 555 fps (10.9 FPE) without repumping, at 10.7 turns of gap.... It takes 7 turns to repump back to the same pressure as I get at 10 from empty, or if I only shoot 1 shot, just 3 pumps.... and I can keep shooting at 550-555 fps with 3 pumps required per shot.... That actually works out to 1.02 FPE/CI, based on the volume of air pumped (58 cc or 3.54 CI per stroke)…. which is very good for a pumper....  8)



On the second chart, the red lines are at 14 pumps (about 1200 psi), and the blue lines are at 20 pumps (just over 1500 psi)…. The plateau at 14 pumps (742-748 fps) extends past 6 turns of gap (1/4"), and at 20 turns the velocity starts to drop from 824 fps after 3 turns of gap (1/8")…. With the SSG set at just over 3 turns of gap, I still have maximum power available, but over half the air is retained, so I only have to pump 10 pumps to get back to 24 FPE, instead of 20 pumps.... The same thing applies at the 14 pump level, set for where the velocity just starts to decrease, it only takes 7 pumps to refill to 19.5 FPE, instead of 14 pumps.... For maximum power work that means the hammer spring and preload are OK, but I could still probably reduce the preload, or use a lighter spring, particularly for a 2-shot tune at 14 pumps.... 

At 14 pumps the first and second shots have the same velocity (640 fps) with the SSG set to 8.5 turns of gap.... That gives two shots at 14.5 FPE without repumping between, and takes 10 pumps (instead of 14) to repeat those 2 shots.... If I choose to fire a single shot, I have to alternate 4 pumps and 5 pumps to maintain the velocity as close as possible to that 640 fps mark.... Looking at the curves for 20 pumps, I can get two shots of 725 fps (18.6 FPE) by setting the SSG gap to 5.7 turns.... It takes 14 pumps to get back to just over 1500 psi, or I can shoot single shots and pump 7 times each.... Here is a short string, showing the velocity and the number of pumps in between, mixing 2-shots and single shots between pumping....

Pump 20 times
2 shots, 732 & 726 fps
14 pumps
738 fps
7 pumps
721 fps
7 pumps
727 fps
7 pumps
2 shots, 723 & 722 fps

The gauge makes this easy, I just pump until the pressure is just over 1500 psi, and I can take 1 or 2 shots, and stay within less than a 3% ES....  8)

My conclusion is that this bored out valve, with a volume of 6.7 cc, is too small to be able to get a useful 3-shot bell curve without pumping.... By increasing the volume by using a 2-piece valve, that could probably be accomplished, at the expense of a lot more pumps for the initial fill.... However, it is entirely possible to get 2 equal shots at any power level desired from 10 FPE up to probably 20 FPE (by using heavier pellets), and be able to shoot a mix of single or two shots by repumping to the appropriate pressure level.... It requires roughly 1/3 as many pumps as the initial fill for each shot fired when tuned for 2 equal shots.... If tuned for full power, the SSG allows enough retained air to cut the number of pumps require to continue shooting at full power in half....  :o

The gun set up the way it is right now is pretty close to accomplishing my original goal.... I can get 2 shots within a 2% ES at 19 FPE (688 fps with the 18.1 gr.) at 20 pumps (1500 psi), with 7 pumps per shot required to refill.... I was hoping I could get 2 shots at 20 FPE....  ::)

Bob
« Last Edit: January 12, 2020, 06:13:17 PM by rsterne »
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calinb

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Re: My Benjamin 392
« Reply #38 on: January 12, 2020, 09:37:57 PM »
Hey, Cal.... I'm so used to taking the valve out through the front (13XX with that raised lug to prevent the valve moving back, and Discos where the slot in the back carves up the O-rings) I never thought of pulling it out the back.... ::) …. Yeah, that roll-stamp really messed up my tube, I had to file/sand it away.... now I could probably take it out the back but old habits die hard....  ;)

When you get further into the thread, you will see that I'm running an SSG, so the hammer has a gap to the valve stem all the time (even uncocked)…. I could actually use a much lighter valve spring now, but the MRod spring is working fine and I can't be bothered changing it.... The required hammer strike with the smaller MRod poppet with the 1/8" stem is so low the gun is easy to cock, no need to change.... If you don't know about SSG's, stand by, this post will rock your world....  :o

Yup--my mind is blown by your SSG, Bob! I have a couple of hammer debounce devices (HDDs) that I got from Scot Laughlin several years ago. I've never installed them and maybe they'll stay in the box. Your work with the SSG looks like it might be a superior approach for me too once I get around to modifying my 397 for air retention.

BTW, before you started to delve into the ACP features, you were already roughly at Steroid power levels, based on my extrapolations from my own slightly less efficient 20 cal. Steroid (compared to 22 cal.) and also Tim's numbers from his old GTA posts...and unlike Crosman, Tim doesn't lie and exaggerate his Steroid performance levels!

One thing that readers should keep in mind is, according to maplogs.com, you live at 2438 feet. The standard pressure lapse rate is about 1 inch of mercury per 1000' elevation so you have to pump almost 9% more than you'd have to pump at sea level to realize the same pressure. (A steroid rated maximum of 14 pumps becomes 15+ pumps at your elevation.)

Sorry to hear your 392 had Crosman's had too-deeply stamped markings on it too. It's a common problem but it sounds like you corrected it in a manner similar to my fix.

My conclusion is that this bored out valve, with a volume of 6.7 cc, is too small to be able to get a useful 3-shot bell curve without pumping.... By increasing the volume by using a 2-piece valve, that could probably be accomplished, at the expense of a lot more pumps for the initial fill....
How about installing a "tank" under the stock. I got a couple of 16g and 20g CO2 cylinders to try to fit up for that purpose. I don't know of anyone who's installed a 20 gram one yet but I think it might fit, if the underside of the pump hand guard is routed out a bit too.
« Last Edit: January 12, 2020, 09:44:24 PM by calinb »
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Re: My Benjamin 392
« Reply #39 on: January 13, 2020, 10:34:32 AM »
Cal, I don't know if you saw the two .25 cal Pumpers I made combining a Disco tube and 392 pumps parts or not.... First I built a carbine, using a 20" MRod barrel, stock 392 linkage, shortened Disco tube and lengthened valve (6.1 cc).... That is chronicled in this thread on the GTA....

https://www.gatewaytoairguns.org/GTA/index.php?topic=37401.0

On page 3 of that thread shows my installation of a 16 gr. CO2 cartridge (21 cc) to quadruple the reservoir size.... I still have another one here....



The success of that build (which used virtually all Crosman parts, and could have been made by Crosman for a tiny investment for the few new parts necessary) led me to build a longer version, which I called the Millennium Pumper.... It used a 25" TJ's barrel, full length Disco tube, Steroid 392 pump linkage, and a 2-piece valve even larger than what I had in the carbine.... It ended up at 28.5 cc....  :o

https://www.gatewaytoairguns.org/GTA/index.php?topic=39328.0

The MP could be tuned for one shot at 51 FPE (with a 40 FPE backup), two at 45+ FPE, or three at 40+ FPE with 25.4 gr. JSB Kings.... It had a Foster fitting for initial filling because it took 80 pumps from empty to reach the working pressure of 1800 psi.... If you repumped after a single 40 FPE shot, it was only 11 pumps, so not bad at all....  8)



I think an SSG could have saved a bit more air, and improved on that slightly.... maybe.... hopefully....  ;)

Bob
« Last Edit: January 13, 2020, 10:36:32 AM by rsterne »
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Re: My Benjamin 392
« Reply #40 on: January 13, 2020, 01:19:45 PM »
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....



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

« Last Edit: January 13, 2020, 01:22:26 PM by rsterne »
  • Coalmont, BC

Insanity

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Re: My Benjamin 392
« Reply #41 on: January 13, 2020, 01:41:20 PM »
Bob there is most definitely something wrong with you. Dont go see a Dr or anything and changing to normal on us lol.

Alan

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Re: My Benjamin 392
« Reply #42 on: January 13, 2020, 05:23:23 PM »
Incorrigible! At least that is what "I" think!

Bob, you're an example for the rest of us to follow. Unfortunately, darn few have the requisite skills you employ everyday. We're not just awed, we're jealous!
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: My Benjamin 392
« Reply #43 on: January 13, 2020, 07:10:46 PM »
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....



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)



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

« Last Edit: January 13, 2020, 08:00:10 PM by rsterne »
  • Coalmont, BC

calinb

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Re: My Benjamin 392
« Reply #44 on: January 14, 2020, 01:32:03 AM »
Cal, I don't know if you saw the two .25 cal Pumpers I made combining a Disco tube and 392 pumps parts or not.... First I built a carbine, using a 20" MRod barrel, stock 392 linkage, shortened Disco tube and lengthened valve (6.1 cc).... That is chronicled in this thread on the GTA....

https://www.gatewaytoairguns.org/GTA/index.php?topic=37401.0
Hi, Bob. No, I missed your carbine project and your follow-on rifle. I spent the morning reading both threads and this evening I got caught up here. Impressive! I would consider duplicating the carbine, but I don't have a Disco tube and buying piecemeal parts from Crosman gets a bit expensive too. I do have a 392, an extra brass 392 valve and some parts from Mac1, including Tim's standard length Steroid pump arm kit. Maybe I'll leverage what I'm learning from you here and mod my 392 similarly. I have several small pressure gauges, including one with a 2000 psi scale. I've always wanted an FX Independence but I never spend that much money on my air rifles!

On page 3 of that thread shows my installation of a 16 gr. CO2 cartridge (21 cc) to quadruple the reservoir size.... I still have another one here....
If you decide to fit a cartridge up to the 392 and would like to try for a bit more volume, I'll send my spare 20 g. cartridge to you. Maybe that approach is too similar to what you've already done, but I have two of them. Just let me know and I'll mail it up to you. The 20 gram-er is a tight fit in length without interfering with the pump forearm, but a bit of wood can be removed from the underside of the pump forearm too, as I recall, in order to accommodate the rounded bottom of the cartridge. I was planning to place the 1/16 NPT fitting for it and the gauge very close to the exhaust valve (much closer than I show in my photo attachment above), but it would probably require either threading the 1/16 NPT through the existing 2-part valve threads or shortening the number of threads on the male valve half (which would also increase valve volume a little :)). I was hoping the latter would pencil out.
I'm pretty sure it would have still sealed OK, but the diameter of the exposed Delrin would be about 7/32"
Not that it matters but I'm pretty sure Tim said he uses Teflon check valves in the Steroid and he makes two of them from a single Crosman 392 valve. So your 392 valve should be Teflon too, unless I missed somewhere in this thread where you swapped it out for Delrin. (I saw your photo where you turned the end of the stock check valve for a spring seat). Tim uses Delrin for his exhaust valves. BTW, if you machine your valve seat to fit-up one of Tim's exhaust valves, you'll gain a small amount of valve volume and you'll never need to worry about extrusion of the stock valve stem rubber. I can send you one of Tim's Delrin exhaust valve stems along with the 20 g C02 cartridge, if you think you might be able to use a new exhaust valve too. :)

« Last Edit: January 14, 2020, 01:34:29 AM by calinb »
  • N. Central Idaho