Author Topic: My Benjamin 392  (Read 4866 times)

rsterne

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My Benjamin 392
« on: December 25, 2019, 03:25:15 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.... She banished me to the shop to stay out of her way for a couple of hours this afternoon, so I did some baseline work to see what I am starting with.... As you can imagine I don't plan to leave it stock....  ::) …. Here is the velocity at each number of pumps, using the new 15.9 gr. JSB Hades pellets, which I thought might be a good weight for it....



What really surprised me is that it retained a tiny puff of air at 7 pumps, and enough at 8 pumps to deliver a shot of over 200 fps.... By the time I had pumped it 10 times, the second shot was over 450 fps....  :o …. This means that at 8 pumps and over, some extra power is lurking inside.... I know the valve spring is pretty stiff, it may only take replacing that with a weaker one to find that lurking FPE....

Next I used 8 pumps and tried a wide variety of pellet weights, from the 11.9 gr. RWS Hobby all the way up to the 34 gr. JSB Beast.... I shot 2 shots and recorded the average, and between every shot I cocked the gun and fired it to dump all the air.... The "puff" of air of the 2nd shot was minimal with the lightest pellets, but increased with pellet weight, as you might expect.... The fact it is retaining air indicates insufficient dwell to dump it all before the pellet reaches the muzzle, and heavier pellets, going slower, haven't moved as far from the breech, so the valve should have released less air (and therefore retained more)…. Here is a plot of velocity and energy vs. the weight of the pellets....



The behavior was much as expected, with the heavier pellets having more FPE.... What was interesting was that the rate of increase of FPE changed drastically at a pellet weight of about 19-20 gr.... Below that, you gain 2 FPE (17%) by increasing the pellet weight from 11.9 gr. to 18.1.... but above that the gain is less than that (1.8 FPE = 13%) at 34 gr.... You are still gaining energy with heavier pellets, but considering that the velocity is dropping below 600 fps, IMO there are two reasons not to use pellets heavier than about 20 grains.... BTW, I didn't have a pellet between 25.4 and 34 gr., so I used a 29.6 gr. BBT slug, as cast, and it fit right into the data.... This shows that a slug can deliver velocity and energy exactly the same as what you would expect from a pellet of the same weight....  8)

Anyways, this is a good baseline to have so that I will know what is happening when I change things.... I think the first change will be an O-ring spacer inside the valve to reduce the headspace, and likely the 2-spring mod. to reduce the pumping effort, or simply a lighter valve spring....

Bob


  • Coalmont, BC

steveoh

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Re: My Benjamin 392
« Reply #1 on: December 25, 2019, 05:42:22 PM »
Good stuff!
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Alan

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Re: My Benjamin 392
« Reply #2 on: December 26, 2019, 05:32:37 AM »
I have a 397, which according to Benjamin, can develop 10 FPE. No matter what I did, I couldn't get the velocity/weight combination to get over 8.5 FPE. However, a simple mod I found on the net (modifying the valve slightly), pushed it over 11 FPE. So there is hope for you Bob!
  • Roswell, New Mexico
Alan

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rsterne

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Re: My Benjamin 392
« Reply #3 on: January 02, 2020, 01:15:50 PM »
Somebody asked what my 392 looked like.... Here it is the way it came out of the box on Christmas morning....



I have a spare valve, so I pulled it apart and installed the 2-spring mod.... I also unscrewed the valve a bit, leaving a gap between the two halves which will decrease the headspace between the rubber cup on the piston and the tapered end on the valve.... Here are the photos....



Above shows the gap in the valve, which I initially set at 1/16".... In the lower photo you can see the stock valve spring, which pushes against the end of the check valve.... There is a shoulder down inside the valve front, just behind the back of the check valve, and a #8 flat washer will drop in and stop against that shoulder.... I used a much lighter spring in front, and turned the end of the check valve down to create a spring seat.... I had to do some work on the spring with a pair of needle-nosed pliers to create a taper in it to prevent it working its way through the hole in the washer....



The new valve spring is a LOT lighter, only 0.035" wire and 1.75" long instead of the stock spring, which is 0.049" wire and 2.13" long.... Stand by for a report on the results (not what I expected)….

Bob
  • Coalmont, BC

rsterne

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Re: My Benjamin 392
« Reply #4 on: January 02, 2020, 02:24:16 PM »
So, to continue.... yesterday I disassembled the gun for the first time, and found to my surprise that it had the older brass valve.... I guess that dates it a bit, but dimensionally the valves seemed identical.... I was really not happy with how hard it was to drive out the roll-pins on the front plug, and it's even harder to get them back in straight, because they are drilled offcenter on the tube (the front ones high and the back ones low)…. I will be making steel shear pins to replace them, because I will be taking this gun apart so many times the brass tube won't survive the continual driving of the pins back and forth....

Incidently, I did a safety study of the parts at 1500 psi, and it looks like the limiting factor for pressure loading is the single #12-36 valve mounting screw.... Even though the 0.880" OD x 0.053" tube is only brass, the hoop strength is decent, and even the roll-pins are comparable (because there are 4 shear planes)…. compared to the loading on the edge of the tube at the valve screw....  ::) …. I don't know what the pressure is at 8 or 10 pumps (I may install a gauge to measure it)…. but past about 1200 psi the brass tube at the back end of the single hole where the valve is attached (the front trigger mounting) may start seeing distortion.... This isn't a "sudden" failure mode.... but should according to calculations be an indicator that you are pushing things too hard.... It makes me curious what a "Steroided 392" at 14-16 pumps may be seeing, and if any distorting is showing up at that valve screw hole....  :o

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....



The bottom one (on the poppet) which I am going to try next, is the valve spring from an MRod (I think)…. It is 1.43" long and made from 0.044" wire.... The crude testing I did indicates that it should just hold the valve closed against the stock hammer spring.... Here is the comparison of the current 2-spring setup and the stock 392 valve spring....



Oh, I almost forgot, with the 1/16" gap in the valve there was quite a bit of pressure on the pump arm just before it closed.... The rubber cup was hitting the (now further forward) end of the valve when the handle was nearly 3" from closed.... I will screw the valve a bit further together, leaving about a 0.045" gap next time.... Now to make some new pins for the front block, and reassemble the gun.... Hopefully this time it will pump and fire....  ::)

Bob
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Alan

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Re: My Benjamin 392
« Reply #5 on: January 03, 2020, 05:19:21 AM »
I'm not an expert on the 39X valve variations (aluminum vs. brass), but Crosman told me several years ago, that the aluminum valve was only used for a couple of years. I bought my 397 almost 5 years ago, and it has a brass valve.

The valve "mod" I bought off eBay, uses a different spring (I didn't measure the differences between it and stock). And, the plastic (Nylon?) valve is shorter overall (≈.125"). At the same time, I installed a SuperSear from AoA, which turns out to be the best thing I did! Makes the gun much easier to fire. The stock trigger setup was horrible to say the least.

Before the mods, the best I could do, shooting the RedFires (7.8 gr.) was ≈675 FPS at 7 or 8 pumps. After the mod, the velocity was ≈780 at 6 or 7 pumps, and a bit less with 8 pumps. It shoots best (accuracy wise) with 5 pumps, at ≈750 FPS.
  • Roswell, New Mexico
Alan

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rsterne

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Re: My Benjamin 392
« Reply #6 on: January 03, 2020, 02:14:01 PM »
I made a pair of shear pins for the front of the pump tube today.... They are short pieces of 3/16" drill rod, threaded both ends with 10-32 thread.... On one end a nut is glued and pinned in place.... No more wearing out or damaging the brass tube wailing on those roll-pins....  8)



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.... In case you didn't know, that is why the 39X valve has a small hole in the back on the right side of the valve stem.... The tool is simply a piece of 3/4" OD 6061-T6 aluminum bar stock, with a 1/2" deep 3/16" hole in the center to clear the valve stem, and a 3/8" deep, 1/16" hole that is 0.21" off center with a piece of 1/16" piano wire glued in.... The screw is simply a handle on the bottom to allow you to turn the valve so that the threaded hole in the valve for the trigger/valve screw lines up with the hole in the tube....

Bob
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Alan

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Re: My Benjamin 392
« Reply #7 on: January 03, 2020, 03:12:23 PM »
You sort of struck a chord with me (Robert), so I got out the 397 and the Chrono. Well.... I can still get almost 800 FPS with the RedFires, but it takes one more pump. Perhaps a seal as I haven't fired the gun for about 10 months.

Opinion: I think too many pass up pump guns because they think they're inferior to PCP guns. That fact I won't challenge. However, I have an after-market adapter which allows me to attach a suppressor to the 397. With it, the 397 is VERY (!) quiet, to the point, my next door neighbors don't know I have fired it, even if they're in their back yard, less than 10 feet away! I just love the stealthiness it offers when an errant squirrel just happens to come around.

Each and every airgun has a nitch. I've found my 397's nitch, and I suspect you will find one for your 392.
  • Roswell, New Mexico
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rsterne

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Re: My Benjamin 392
« Reply #8 on: January 03, 2020, 03:57:57 PM »
Now that I have the new pins and the valve tool, I can reassemble my 392 without fear of damaging it.... It looks a bit clunky with the nuts on the front pins.... but after the gun is finished (are they ever REALLY finished? ) I could make something a bit prettier.... Anyways, I reassembled the gun with the 2-spring mod installed, and the two halves of the valve with a 0.045" gap.... The pump arm now closes to within 1.5" of the tube after a shot (pump tube empty), so the rubber cup is being compressed against the end of the valve just a bit.... It turns out that the new valve spring is not QUITE strong enough to allow me to pump the gun without cocking it.... It is really close, so close it might work when I install the MRod poppet, which is 1/16" thicker, so will compress the valve spring more.... It's not an issue to have to cock the gun before pumping (unless you forget)…. all that happens if you don't, is you hear the air farting out the muzzle....

I checked the velocity with the 15.9 gr. pellets at every pump, and continued until the valve retained a "puff" of air.... Instead of that happening at 7 pumps, it now happens at 12 pumps, which shows what a difference the valve spring can make.... The gun is pretty hard to pump at 12 pumps, I don't think I will be doing that on a regular basis.... but at least I now know that it is not retaining air at 10 pumps, which it certainly did before.... Here is the previous chart, with the new data on it for the velocity and energy, shown as dotted lines....



From 7 pumps and up, the velocity and energy have increased, now that the gun is not retaining air.... I tried a few pellet weights, again at 8 pumps, to see what the gains were in velocity and energy, and the gun gained more with heavier pellets.... However, the inflection point in the curve remains at about 19 gr.... above that weight the FPE increases slower as the weight is increased.... That may change with larger ports.... and it may also change with increased valve volume....



I'm pleased with these modest gains from such simple mods that anyone can do.... I guess the next step is to increase the flow, by fitting an MRod poppet, and drilling out and smoothing the throat and ports.... Stay tuned....  ;)

Bob
  • Coalmont, BC

rsterne

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Re: My Benjamin 392
« Reply #9 on: January 04, 2020, 06:40:00 PM »
Work progresses slowly on my 392.... I wanted to fit an MRod poppet, which has a smaller stem, 1/8" instead of 5/32".... At the same time, I decided to enlarge the valve throat from 7/32" (0.219") to 15/64" (0.234")…. This will increase the throat area from the equivalent of a 0.155" hole to that of a 0.198" hole.... which is a 63% increase in area.... I also wanted to enlarge and angle the exhaust port to ease the flow from the throat into the exhaust port.... Part of the plan to install the MRod poppet was to bush the hole in the back of the valve with K&S brass hobby tubing, but Charles (Psipumper) suggested that I drill the back of the valve straight through at 0.234" and then press in an insert which was drilled 1/8" for the stem of the poppet.... By grinding an angle on the front of the insert, it would fill some of the vacant space from the original vertical exhaust port, and the volume behind that in the throat....

Charles suggested that I drill the hole for the stem first, to insure it is straight and square to the valve seat.... but since I have a lathe, and I have made lots of my own valves, I figured I could drill the hole afterwards from the back.... First, however, I decided to drill and tap holes in both sides for 8-32 low profile SHCSs to better locate the valve in the tube.... I was concerned about the single valve screw, not so much for the strength of the screw itself, but the load on the thin brass tube.... I calculated the safety margin on the support given by the tube wall to that screw, which was the only thing keeping the valve from sliding back.... To my surprise, I found that at 1200 psi it was only 1.02:1....  :o

That means that if the pressure exceeds 1200 psi, you can expect the screw to start deforming the brass tube on the back of the hole.... IF that happened, the valve would move back, and the exhaust port would no longer line up with the port in the receiver.... Since it is always nice to have a 3:1 safety margin, I decided to add a screw to each side of the valve, set down against a flat spot milled in the valve body.... Once I drill holes in the sides of the brass tube, I will triple the ability of the brass tube to prevent the valve moving back.... The holes are in the same plane as the port and existing valve screw, and the head is slightly smaller than the countersink for the valve screw.... so they won't interfere with the installation of another O-ring ahead of the screws, in case I want to skeletonize the valve....

I set the valve up in a square indexing block (against an end stop) with a 5C collet, using a #20 drill to line up the exhaust port with the headstock.... Then by simply turning the block I could drill the side screw holes in the same plane and at 90 deg. to it and the existing valve screw hole.... Once those were drilled and tapped, and the flats for the heads milled, I turned the block so that the exhaust port was lined up again with the headstock.... I then rotated the milling attachment 20 deg. and plunged a 5/32" mill through at the location of the existing exhaust port.... I examined the inside to confirm that there was lots of room, changed the angle to 25 deg. and milled the front of the angled port.... I then raised and lowered the valve 0.012" to widen the exhaust port and make the hole where it meets the receiver round.... It ended up at 3/16"....

I removed the valve from the collet block, and chucked it in the lathe, with the exhaust port lined up with the chuck key hole.... That means I can put it back into the 3-jaw chuck in the same orientation each time.... I used a 3/16" stub drill and drilled along the hole for the stem, from the back of the valve.... I then went to 7/32", which was the size of the existing throat (it lined up perfectly), then a #1 drill (0.228") and finally the 15/64" drill to finish it to the size of the new throat.... I then made an insert out of 6061-T6 aluminum, about 0.002" over, ground an angle on the front (to match the exhaust port) and heated up the valve body to press in the insert.... Long story short, it didn't work, and stuck about halfway in.... I had to cut it off, machine it off flat with the back of the valve, and then drill it out again.... There was no damage to the valve, so I made a new insert, the same size as the hole in the back of the valve.... polished it a thou smaller and then pressed it in with Loctite 638.... It ain't going anywhere....  ;)

I then used a small spherical burr in my Dremel, and working through the exhaust port, smoothed out the front of the insert and faired the exhaust port into the throat.... Now, when you look into the exhaust, you can see part way through the throat of the valve, and the passages are nice and smooth.... An 11/64" (0.172" drill) wobbles around loose in the exhaust port, so it is larger than that.... The last step was to drill the 1/8" hole for the stem of the MRod poppet.... Here is the finished back half of the valve, along with the MRod poppet and spring....



The insert in the back of the valve covers the 2 side screw holes, and the original lower valve screw hole, sealing them off.... They are only under pressure during a shot, so no danger of the valve leaking down anyways.... One added bonus of the insert is that it supports a greater length of the valve stem, so it doesn't wobble around like the stock one did.... I haven't drilled the holes in the tube for the side screws yet, nor machined the extra O-ring groove.... but I think I will test the gun at this stage.... I have not yet drilled out the receiver/barrel port, nor turned down the bolt probe, so I will be able to see what difference that makes, if any, after testing the new valve port and poppet.... Incidently, I think there is room to angle the exhaust port 30 deg. instead of 25.... but it might get pretty close to the valve seat.... BTW, the MRod poppet seems to seal up perfectly, just blowing in the front of the valve it has no leak detectable....

Bob
« Last Edit: January 04, 2020, 06:51:39 PM by rsterne »
  • Coalmont, BC

rsterne

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Re: My Benjamin 392
« Reply #10 on: January 04, 2020, 09:32:25 PM »
Here is the testing with the valve mods so far.... larger port, throat, and MRod poppet.... I am only checking the velocity every 2 pumps.... The MRod poppet and valve spring, sitting on a #8 washer on the shoulder in the valve front, is just about perfect for this gun, IMO.... The poppet is smaller in OD, it has a smaller stem, the gun is easier to cock, and you can still pump the gun without having to cock it....



As you can see, the improvements continue.... In addition, I can now pump 14 times without retaining air.... I didn't try more than that....



I am no longer going to test anything lighter than 14.3 gr. pellets.... and I am thinking about pellet weight in the reverse, compared to before.... Instead of not looking heavier than 19 gr. because the FPE doesn't increase as fast, I think I should not be looking at pellets lighter than 15.9 gr. because the FPE deceases quicker.... Sometimes I get it backwards, and this is one of those times....  ::)

The gun is now producing 21 FPE at 12 pumps with 25.4 gr. pellets....  8) …. The next set of mods is to drill out the receiver and barrel port, which is now the smallest port in the gun.... While I'm at it, I will slim down the bolt probe as well.... I think that will complete the "breathing" mods, then I will take a look at the pump.... or, I may install a pressure gauge....

Bob
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Alan

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Re: My Benjamin 392
« Reply #11 on: January 05, 2020, 05:02:14 AM »
Question remain... How many would go to this amount of upgrade work? And of course, still want to pump it after you're finished?

I like my 397, no more than I have changed it over stock. The main reason is, it is dead quiet. After you're finished, I'd like to know how much the report is??
  • 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 #12 on: January 05, 2020, 08:28:30 AM »
It's getting louder, for sure.... but that is mainly a function of how much you pump it.... Remember, in Canada, quiet guns are not allowed....  ::)

Bob
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steveoh

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Re: My Benjamin 392
« Reply #13 on: January 05, 2020, 09:01:44 AM »
Bob, you never cease to amaze me with your skills, creativity and enthusiasm. Then there’s your willingness to share your findings and knowledge with us. Thank you!

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rsterne

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Re: My Benjamin 392
« Reply #14 on: January 05, 2020, 12:53:01 PM »
It would be hard to come up with something that hasn't been done before on a 392.... and the limitations of the plartform (eg. soldered barrel and receiver) make major (eg. caliber) changes a challenge, which is why I decided to stay with the basic .22 cal platform.... I doubt if my gun will be a record setter in any way.... but it's a fun project, and few have documented each step in their mods....

Bob
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