My Benjamin 392

[anti-squirrel]

I'm not a power or speed freak with a pumper- but I do like big heavy pellets lumbering along at a sedate 450 to 550 fps.  I suspect this will bring efficiency to the pumper among other things, IE: more oomph with less ugggh, which is "efficiency"

Frankenpumpers impress the heck outta me even more.

[Hack21]

My curiosity makes me wonder what the upper limit on performance would be if reinventing the 392 as a SSP. If the stroke could be increased and the valve redesigned, then I imagine the performance might be respectable even if the platform itself didn't make a lot of sense. That is an execution that I haven't seen yet.

[rsterne]

I think the problem with making an SSP using the 39X as a base is the large diameter pump tube.... SSPs require high pressure crammed into a small volume.... That high pressure would require a lot of force generated by the linkage.... To do that, without making it very hard to pump, might require either a 2-stage pump arrangement (for the last part of the stroke)…. or an articulated pump linkage which increases the force on the first part of the pump stroke while reducing it below the peak (or rather achieving more leverage in the last portion of the stroke)….

Bob

[Hack21]

I think the problem with making an SSP using the 39X as a base is the large diameter pump tube.... SSPs require high pressure crammed into a small volume.... That high pressure would require a lot of force generated by the linkage.... To do that, without making it very hard to pump, might require either a 2-stage pump arrangement (for the last part of the stroke)…. or an articulated pump linkage which increases the force on the first part of the pump stroke while reducing it below the peak (or rather achieving more leverage in the last portion of the stroke)….

Bob

Bob,

I agree with all of your comments. The limiting factor likely is the amount of force that the operator can provide when trying to close it up. I haven't measured the P17 dimensions, but I am guessing that it is 20 to 25mm piston and that the stroke is 100 to 125mm. The pump arm might effectively be a third the length of the 392. Others gain velocity by adding barrel length, which the 392 already has. If a 550 fps SSP 392 were possible, then I imagine that it would be quiet and a fun paper puncher.

[Mole2017]

and likely the 2-spring mod. to reduce the pumping effort

Haven't heard of this mod, and too much pumping effort is my main complaint with my Steroid 392 with billet lever from Mac1, plus with the wood stock 397 I just bought.
...

Here's a diagram from the old Yellow...

Whoa...that's the way they did it in the old Benjamins. I guess they "simplified" it at some point to save money. However, that doesn't mean the old ones don't retain air. A 317 I'm working on now has a huge spring and retains air after just 3 or 4 pumps. But a maybe I can find a replacement spring for that...

For that matter, I also have a 392 that may have a single spring valve configuration and would benefit from this trick. Thanks for the info!

[Techie]

Thanks for that diagram!  I guess that is why my 312 is so much easier to pump than my 392.

[Rob M]

I think the problem with making an SSP using the 39X as a base is the large diameter pump tube.... SSPs require high pressure crammed into a small volume.... That high pressure would require a lot of force generated by the linkage.... To do that, without making it very hard to pump, might require either a 2-stage pump arrangement (for the last part of the stroke)…. or an articulated pump linkage which increases the force on the first part of the pump stroke while reducing it below the peak (or rather achieving more leverage in the last portion of the stroke)….

Bob

Phil riggs made a 397 SSP on the green forum about 8 years ago .. It was single stage ,but had an articulating linkage to increase MA as the stroke became more difficult.. i think he was getting 550 fps with light 177 pellets.

heres the related link or whats left of the archives

https://www.tapatalk.com/groups/thegreencrosmanforum/397-ssp-bet-t43917.html

as it turns out , most things that dawn on us in 2019 were done in the past, just few sparse records

[Rob M]

in any event , im looking forward to your 392 build..

[Hack21]

I think the problem with making an SSP using the 39X as a base is the large diameter pump tube.... SSPs require high pressure crammed into a small volume.... That high pressure would require a lot of force generated by the linkage.... To do that, without making it very hard to pump, might require either a 2-stage pump arrangement (for the last part of the stroke)…. or an articulated pump linkage which increases the force on the first part of the pump stroke while reducing it below the peak (or rather achieving more leverage in the last portion of the stroke)….

Bob

Phil riggs made a 397 SSP on the green forum about 8 years ago .. It was single stage ,but had an articulating linkage to increase MA as the stroke became more difficult.. i think he was getting 550 fps with light 177 pellets.

heres the related link or whats left of the archives

https://www.tapatalk.com/groups/thegreencrosmanforum/397-ssp-bet-t43917.html

as it turns out , most things that dawn on us in 2019 were done in the past, just few sparse records

Rob M,

Thanks for sharing. I didn't realize that a SSP 39X had been done before.  I am also excited to see what Bob does with this and I'm sure it will be well chronicled. Thanks again.

Eric

[Hoosier Daddy]

... in a make your head spin sortta way.
But I LOVE it!

[rsterne]

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

[rsterne]

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.... 

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

[Techie]

Can't see any of the PhotoBucket pictures.  Can you just post them on the GTA site? 
I did click on the PhotoBucket icon.  It then told me I can't see any PhotoBucket pictures unless I disable my ad blocker.  I did this and then it let me see one picture.  It then took me to a website with a Verizon advertisement, and then to another ad; no more of your pictures.  I won't be clicking on any PhotoBucket links any more.

Edit: After writing the above, I went back to your post and now all your pictures appear inline with your text!  Never mind...

[rsterne]

Sorry, but I'm not about to change 1000's of images to a different hosting service.... I wouldn't live long enough to complete the task.... 

Bob

[Techie]

Like I said in my edit above, all is fine now...

[Tom Tucker]

Any plans for an o-ring around the exhaust port?  That will be the next thing I do to mine...

[rsterne]

No, but I plan to install another O-ring on the valve just ahead of the exhaust port.... That will allow me to cut away the middle of the valve with either "windows" or completely remove the middle and mount the check valve separately as a 2-piece valve.... I may also install two additional screws, one per side, to hold the back half of the valve.... I was NOT happy with the stress on the brass tube around that single bottom valve screw....

I would think a tiny O-ring, such as that, might get shredded as it passes the transfer port in the receiver.... In addition, how will you machine the bottom of the O-ring groove in an arc, parallel to the inside of the receiver, to maintain the proper "crush"?....

Bob

[Tom Tucker]

No, but I plan to install another O-ring on the valve just ahead of the exhaust port.... That will allow me to cut away the middle of the valve with either "windows" or completely remove the middle and mount the check valve separately as a 2-piece valve.... I may also install two additional screws, one per side, to hold the back half of the valve.... I was NOT happy with the stress on the brass tube around that single bottom valve screw....

I would think a tiny O-ring, such as that, might get shredded as it passes the transfer port in the receiver.... In addition, how will you machine the bottom of the O-ring groove in an arc, parallel to the inside of the receiver, to maintain the proper "crush"?....

Bob

I don't have the means to machine the arc for an o-ring groove.  I suppose I planned on spending many hours with wet/dry, and a lot of trial and error, along with lots of cursing prayer.  Perhaps that's why I haven't done it yet 

Anywho, I'm anxious to see where your project ends up, Bob.  I'm sure a few records will be broken.  I will be following closely.

[RBQChicken]

Bob,
I've found that 3/16" x 1" steel "dowel pins" work beautifully in replacing the roll pins.  I get them at my local hardware store.  I don't know if it was SpiralGroove or that other feller ( Rob22 something or other) that gave me the idea.

[Rob M]

No, but I plan to install another O-ring on the valve just ahead of the exhaust port.... That will allow me to cut away the middle of the valve with either "windows" or completely remove the middle and mount the check valve separately as a 2-piece valve.... I may also install two additional screws, one per side, to hold the back half of the valve.... I was NOT happy with the stress on the brass tube around that single bottom valve screw....

I would think a tiny O-ring, such as that, might get shredded as it passes the transfer port in the receiver.... In addition, how will you machine the bottom of the O-ring groove in an arc, parallel to the inside of the receiver, to maintain the proper "crush"?....

Bob

so a split valve design.. will this be air retaining or single shot ??