My Benjamin 392

[rsterne]

Today I drilled out the barrel port, slimmed down the bolt probe, and drilled the holes in the tube for the side valve screws.... It turned out to be a LOT of hand work, mostly because I could not grasp the soldered barrel and tube assembly in my very small milling attachment on my lathe.... The side holes for the valve I had to drill undersize in approximately the right place, install the valve with the bottom screw, mark the position of the side screws, and then Dremel the holes over so that they fit the screw heads nice and snug.... It took a long time, but the results are great.... When I install the side screws, they are snug in the holes in the tube, and the bottom valve/trigger screw threads in with NO resistance, it lines up perfectly.... I am a LOT happier with the strength of the valve mounting now.... 

Drilling out the barrel port was likewise problematic, because I could not mount it in the lathe milling attachment.... I drilled it out one number drill size at a time until I got to 11/64" (0.172"), which is 79% of the bore.... Since the exhaust hole in the valve measures 3/16" at the top, because it was milled on an angle, I used a 3/16" center drill to put a slight taper in the barrel port, so that there is no step at the valve, but the port at the boreline stays at 0.172"....

Even the bolt probe was a problem, because with the bolt handle attached (it appears to be one piece) I could not chuck it in my lathe.... I ended up removing the O-ring, and sanding the probe smaller on the edge of a disc sander, and at the same time tapered the front of the O-ring gland, because it partially restricts the (now larger) barrel port.... Here is a photo of the thinned out bolt probe and the side valve screws (which are just above the stock)…. At the back of the tube you can see the hex head on the bolt for adjusting the hammer spring preload.... At the moment it is set for stock preload....



While I was working on the tube, I investigated why the valve was so tight when in position.... I had to drive it back the last inch, and it was almost impossible to turn to get the screws lined up.... It turns out the the roll-stamp on the left side is so deep that it dented the tube on the inside.... I had to file and sand off the bump on the inside (not an easy job), but it turned out really well, and now the valve slides in to position the way it should.... 

Here is a photo of the new front pins.... They have a single nut peened and loctited in place on one side, and double nuts on the other.... I will eventually get a couple of Ny-Loc nuts instead, they will be simpler and look better....



The last photo shows where the pump handle sits with no pressure in the air tube.... I adjusted the gap in the valve to 0.050" to achieve this position.... It is easy to remember, because the top of the pump handle is level with the bottom of the stock....



These changes took the whole afternoon.... After dinner I did some testing, the results will be in the next post....

Bob

[rsterne]

OK, time to test this beast.... All the port mods are now done, both in the valve and barrel, including a slimmer poppet stem (compliments of the MRod poppet), larger valve throat and exhaust port, and a slimmer bolt probe.... The smallest port in the system is the barrel port, which is 0.172" (79% of bore)…. It was 0.154" stock, so I have increased the area by 25%.... Everything else in the system can flow more air than that.... I had the hammer spring preload adjuster set so that the preload was exactly the same as stock.... The gun does not retain any air even at 14 pumps (more about that later)…. I checked the velocity every 2 pumps, ending at 14, and here is the results....



The dotted lines are the previous results, with the valve mods done, but not the barrel port and bolt probe.... Obviously they were hampering the airflow.... The larger ports make a big difference at higher pump numbers, as expected.... I then tried different pellet weights at 8 pumps and 12 pumps.... Here are those results....



The solid lines are at 8 pumps, and the dotted lines are at 12 pumps.... The gun now exceeds 22 FPE with the 25.4 gr. Monsters at 12 pumps....  …. I did one additional test, using my 27.4 gr. BBT HPs at 14 pumps.... I got 631 fps, which is 24.2 FPE.... I'm pretty happy with that, still using the stock pump....

I then backed off the hammer spring preload, because I was curious when the gun would start retaining air.... I backed it off a full 1/2" from stock, and it still showed NO velocity loss at 8 pumps.... but it was a LOT quieter.... I then tried 10 pumps, still no retained air.... At 12 pumps, it finally retained a small puff of air.... It looks like I can make an SSG, probably using the stock spring, and still have it dump 14 pumps.... That may be my next project.... 

Bob

[grand-galop]

Great work on the 392...
Even a simple pump rifle can be turned into a diamond..

[Bryan Heimann]

Man, those are great results for so far for what you have done.  Doesn't seem like a whole lot of modification as far as power goes.  I had no idea you could just about double the power with only a couple of mods. 

Looking at what else you have done there, i would bet you're only getting started.  So exciting!

[antithesis]

I was just getting ready to put mine back together and see what kind of freak I created, but now I may wait a lil longer😎

[rsterne]

To be honest, I don't think there is a lot more in the gun in terms of performance at the same pressure.... Yes, it might be possible to make the ports larger, but getting to full bore-area porting would not be easy, and in truth probably not needed unless you are chasing enough power to shoot slugs.... With the new Redesigned 25.4 gr. JSB Monsters, we have pellets capable of up to 50 FPE (more if you have a gun that likes the 34 gr. Beasts)…. That is twice the power I have now, which realistically means you need twice the pressure.... Even if it didn't destroy the gun, I sure wouldn't want to pump it.... 

Some of the largest gains in any pumper come from working on the pump.... A new, adjustable piston, and eliminating as much headspace as possible, are the keys to getting higher pressures with the same number of pumps (but harder to pump)…. or the same pressure with fewer pumps (a big win, in my experience)…. As an example, here is what you can get with a 13XX, just by changing to a flat-topped piston....



The 13XX pump, in stock form, is cheaply made.... The piston is plastic, and flexes and compresses as the pressure increases.... and the rubber pump cup doesn't match the shape of the front of the valve very well.... These two combine to drastically increase the headspace (dead volume between the piston and valve), which means more pumps to reach a given pressure, and an upper limit on that pressure of about 1250 psi, no matter how much you pump (green line).... At 10 pumps, it reaches about 750 psi, compared to about 1120 psi (50% increase) for a properly adjusted flat-topped piston meeting a flat valve face (red line)…. There are many intermediate steps you can do to help the stock plastic piston, starting with steel rods glued into the slots on the side (to stiffen it), and ending with an all metal adjustable piston, with new, specially profiled pump cup, meeting a reprofiled valve front.... The blue line shows a pretty good version of that, which is almost as good as my FTP....

The pump in the 392 is far better/stronger than the one in a 1322.... However, the piston is not adjustable in length (I made up for that by unscrewing the valve 0.050")…. The Steroid Pumper by Mac1 uses an adjustable piston.... In addition, the inside of the cup does not match well the front of the valve.... Charles (Psipumper) spent a lot of time reprofiling them to match, to eliminate headspace.... I don't know if the Steroid addresses that or not?.... I am eventually going to make an FTP for my gun, but it's not as easy as on a 1322, because the check valve cavity extends into the cone on the front of the valve.... If you machine the valve flat, level with the front of the O-ring, I'm pretty sure you will get into the check valve.... I believe that Charles recently had a check valve extrude though the front of a valve because the hole was too big (not a 39X if I remember)…. This means that I'm pretty sure I will either need a new valve front end (with a flat face), or a piston face that is not flat (but profiled to minimize headspace)….

Since I would like to explore a retained air pumper, with a much larger valve volume, I will likely do both at once.... a new valve front end, and an FTP.... I would like to incorporate a gauge in the gun, ideally before I change away from this valve so that I have a baseline pressure curve.... I'm going to give some thought to where I might put that gauge, so that when I build a new, flat-fronted valve front end, everything will still fit properly.... Lots of measuring and thinking to do before drilling a hole in the tube and valve for a gauge.... 

I wish that Crosman had built the Millennium Pumper, and changed over to the steel tube from the Disco, and used a beefed up linkage, like the Steroid.... There would have been so much more potential that way, plus the ability to readily change barrels, mount a scope, etc.etc…. The soldered construction of the 392 just puts so many limits on tinkering....

Bob 

[rsterne]

Incidently, with the new valve screws, the bearing load on the brass tube wall now has a safety margin of over 2:1 at 1500 psi, and they have a margin in shear of 3:1.... and that doesn't include the original bottom valve screw.... If that is included, the bearing load increases to 3:1.... The most highly stressed component would now appear to be the single 1/8" pin that joins the pump linkage to the piston.... However, should that fail, it would not be a catastrophic failure, likely it would just bend and be difficult to remove.... I still consider the 39X to only be about a 1500 psi gun, however, to maintain the usual 3:1 safety margins.... Without pinning the valve, I would expect the first sign of trouble to be the tube wall behind the lower valve screw distorting.... Calculations show that to be marginal at only 1200 psi.... 

Bob

[Bryan Heimann]

That is king if crazy, considering.  I thought that pneumatics had to be built with 3:1.  Is that only for higher pressure pcp guns?

Edit: ^ based on reading yours and others posts/builds

[rsterne]

I agree, I was surprised at how close to "failure" the 392 was on that one point, where the valve screw bears on the edge of the brass tube.... My calculations at 1200 psi (admittedly higher than Crosman recommend) placed the pressure where the back of the hole would start to deform at only 1225 psi.... Bear in mind, this type of failure is not catastrophic, the brass would "flow" and the valve would slide back slowly.... That would increase the headspace, and lower the pressure at the 8 pumps Crosman recommends (maybe 800 psi in an unmodified 392, I don't really know)…. Ultimately, if the brass continued to distort, the screw would push on the front of the trigger group.... By that time, a stock 392 would be so sick somebody would have taken it apart to find out why....

I just have peace of mind, by adding the 2 side screws to take the load.... 

Bob

[Rob M]

Bob , have you installed a gauge ??

[antithesis]

Bob,

Please be detailed when you tell us how you decide to install a gauge, I just couldn't figure a practical means of doing so, and I'm dying to sponge off of your talent 😁

[grand-galop]

I have check the benefit of the transfert port and it's clear that the .22 gain over .177..
Not only for fps and fpe, but momentum of the calibrer is ibeal for plinker and hunter..

[rsterne]

No gauge yet.... and yes I will detail that when I get around to it....

This afternoon I made and installed an SSG (stopping spring guide).... Here is a photo of it, ready for installation....



The concept is quite simple.... There is a 3/16" spring guide rod 4" long that slides through the adjusting bolt.... On the front is a nut threaded and glued in place, and then turned down to fit inside the hammer.... At the back the rod is threaded 10-32 and a pair of nuts are used to adjust the preload on the spring.... In this case, I have 0.50" of preload, which is about 3.5 lbs.... There is a small O-ring on the rod between the rear nuts and the head of the adjusting bolt, to absorb the impact when the spring guide stops, and reduce the noise of that collision.... The entire assembly threads into the aluminum mounting sleeve in the top of the trigger group.... I chose 3/8"-24NF threads so that the assembly may be removed to change the preload or spring without disassembling the gun.... Here is a photo of it installed....



Turning the hex head bolt in towards the gun reduces the gap between the end of the spring guide and the bottom of the hole in the back of the hammer.... The idea is to have a small gap between them, so that the spring stops pushing the hammer just before it hits the valve stem.... The hammer then carries on from its own momentum, strikes the stem and opens the valve.... The hammer is NOT preloaded by the SSG, it can "rattle around" in the gap between the valve stem and the spring guide....

The magic happens when the valve closes after firing.... The hammer is thrown back against the now stationary spring guide, but because of the preload on the spring, and the mass of the guide rod, instead of recompressing the spring and getting thrown back at the valve and reopening it (the infamous hammer bounce)…. it just rattles around, without enough energy to open the valve a second (or third) time and wasting air.... Changing the gap adjusts how hard the hammer hits the valve stem, because as you increase the gap, you reduce the amount the spring is compressed when you cock the gun.... I tested the gun with the SSG in place, and I got a pleasant surprise....



The solid blue line is the velocity with 15.9 gr. pellets as I increased the SSG gap.... As you can see, the velocity did not begin to drop off until I had 6 turns of gap (1/4")…. When I compared the velocity with what I got yesterday, without the SSG, I was surprised to see that with no gap (and indeed yesterday without the SSG) I got 659 fps, which increased to 666 fps as the gap was increased to 3 turns (1/8") and over the next 2 turns it returned to 659 fps.... I have no concrete reason why the velocity should increase about 1% with the SSG, but I suspect that without the SSG, the hammer is bouncing off the back of the valve, and that is actually reducing the dwell a whisker.... Anyway, the important thing is that I can have 5 turns of gap and not lose any velocity with the SSG installed....

Now remember that without the SSG the gun did not retain any air, all the way to at least 14 pumps.... Now it retains a puff of air at 8 pumps with the SSG installed, even at zero gap.... At 5 turns of gap, it is retaining enough air to produce a 2nd shot of over 400 fps (the dashed blue line)…. That is roughly the velocity the gun shoots at 3 pumps.... so in theory I could pump 5 times and get back to the pressure I had without the SSG with 8 pumps.... That is why they call this type of gun a "Retained Air Pumper" (RAP, aka Air Conserving Pumper or ACP)….

As I continued to increase the SSG gap, the velocity of the first shot dropped, and that of the second shot increased.... By the time I got to 9 turns of gap (3/8") the second shot was slightly faster than the first.... Not only that, but there was enough air left to produce a 3rd shot, with the gap set to 7 turns or more (dotted blue line)…. I repeated the testing at 12 pumps, the results are in red above.... There were also 3rd shots at larger gap settings at 12 pumps, but I didn't record them.... 

If I adjust the SSG gap to about 8.5 turns, I should be able to tune the gun to have two equal shots of about 550 fps (10.7 FPE) without pumping in between.... However, since there is a bit of air left after the second shot, I can't just pump 8 pumps to repeat the procedure.... However, I might be able to pump it about 6 times after every 2 shots, and keep getting a string of pairs of ~11 FPE shots, for only 3 pumps per shot.... instead of 5 pumps per shot to get the same power without the RAP design and the SSG....

The SSG opens up a whole range of possibilities of different ways to tune my 392.... With 2 turns of gap, I could pump to 12 pumps, and get a shot of 755 fps (20 FPE)…. There would be enough air for a 2nd shot of about 550 fps (normally about 5 pumps), so I should be able to add 7 pumps (instead of 12), and keep shooting at 750 fps for as long as I want to pump.... The best way to do an RAP is to have a gauge.... That lets you know exactly how many pumps you need to get back to the pressure you need for the power you want.... The really cool part is that you have fewer pumps required to get back to full power....

If you need any further proof that an SSG really changes things, just listen to the difference.... Without it, you can hear the hammer bounce, with that telltale burrrppppp sound.... With the SSG, you just hear one POP.... and as you tune it to lower power levels it gets a LOT quieter.... 

Bob

[Rob M]

excellent , so when all is said and done you can adjust the gun based on demand on the fly ( depending on your memorization of the settings or having a cheat cheat on the gun _

[Bryan Heimann]

Let's go, Time to start hogging that valve out and all that great stuff we KNOW you're gonna do it!  And install that guage!

[rsterne]

Valve ports are as big as they are gonna get.... Valve volume is another thing.... I may hog it out, or just bite the bullet and make a 2-piece valve.... still undecided.... I'm waiting for a 1/16"-27 NPT tap to install the gauge.... no idea when (or if) that will arrive, it was ordered from a local automotive/industrial supplier....

I don't know how big you can bore the 392 valve, but I don't think I have ever seen one at 6 cc without opening out through the sides.... I believe Charles (Psipumper) has done one at 5.75 cc.... I saw a 2-piece valve that was supposed to be 0.9 CI, which is 14.8 cc.... I guess one where the center was slotted and hogged away would be somewhere in between.... I think my next step is to cut another O-ring groove ahead of the screws.... Then I will do some measuring, make some decisions, particularly about the gauge location.... and maybe cut another O-ring groove on the front section that will end up behind where the gauge will be....

Bob

[rsterne]

I pulled the 392 apart again today and measured it up for a new pump piston with a flat face and an O-ring seal.... and decided that would also need a new valve front end and check valve assembly.... The only thing that really makes sense is to make that front end as a 2-piece valve, from a time point of view.... That part of the project will wait until I get some pressure measurements with the existing valve and pump to provide a baseline....

I measured up the valve and cut two new O-ring grooves, one on the back half, just in front of the existing exhaust port and valve screws, and the other on the front half, far enough back to allow a gauge between it and the front O-ring.... I used a # 113 O-ring for the front one, and a # 016 for the back one.... The rear groove is just behind the valve seat (where the valve is solid).... and the front groove is just ahead of the shoulder where the washer sits between the check valve spring and the valve spring (where the valve wall is the thickest).... There is plenty of room between the front two O-rings for a gauge port and two side mounting screws, which will be necessary when I make the 2-piece valve.... Here is a photo of the valve with the distance between the O-ring grooves dimensioned....



I clamped the main tube in my milling attachment on my lathe, centered off the bottom valve screw hole, and drilled a gauge hole 2.25" ahead of that.... Since I am using a 1/16" to 1/8" NPT adapter for the gauge, the hole in the tube is 5/16" which will just clear the adapter threads.... I installed the valve, with the two halves unscrewed with the same 0.050" gap as before, and scribed the gauge location between the front O-rings.... That looks after not only the location, but the clocking of the threads (at the gap selected)…. I then drilled and tapped the valve at the marked location to 1/16"-27 NPT.... The hole is in the check valve section so there is lots of thread depth.... I cleaned and reassembled the valve, and reinstalled it in the gun, and threaded in the gauge and adapter.... It looks like this....




I laid out and drilled a 1" hole through the stock with a hole saw, and then using a Dremel sanding drum fitted it to a fairly close fit around the gauge.... Once the gun is assembled, the gauge sits in a hole in the stock much like on a Disco....



I'm quite pleased with the appearance.... The gauge I had was a 3K gauge, and I should probably get one that has a 2000 psi scale.... However, it will at least let me find out what pressures I am pumping to now.... I don't know how accurate the gauge is, but it doesn't really matter, as all I want is to be able to pump to the same pressure for each shot, and to compare changes to the pump and when I change the valve.... It looks like 12 pumps with the stock pump, adjusted the way it is, and stock valve volume, is about 1200 psi.... The valve volume is slightly larger because of the gauge and adapter, maybe 10%.... It doesn't leak down while taking a couple of shots to test it, so I have put 5 pumps in it (just over 500 psi) to leave it overnight to see if it has a slow leak....

Bob

[Hoosier Daddy]

^ Mod of the year award!!! ^

[Rob M]

nice work Bob.. the 1/16npt was a perfect solution

[Bryan Heimann]

Very nice!