Benjamin 392 power limit

[rsterne]

I think the first thing I would look at in a 392 is machining the front of the valve flat, shortening it to just ahead of the O-ring, and fitting a Flat-Topped Piston, similarly O-ringed.... all of that to eliminate headspace.... Inside, I would either fit a captive O-ring check valve (assuming there is enough length after shortening the front)…. or shorten the check valve and use the washer on the internal shoulder trick and fit a MUCH lighter check valve spring along with a shorter (and maybe lighter) valve spring .... This reduces pumping effort considerably because the stock spring pushes so hard on the check it takes a couple of hundred psi to crack it open, and that pressure never sees the inside of the valve....

If planning on exceeding 1500 psi, I would definitely use Mac1's High-Strength pump linkage.... If I wanted to use over 2000 psi, I would step down the diameter of the pump tube to that of a 1322, like I did in this conversion for a 2240....



Of course there would be a lot of scratch-building involved, and a much longer pump tube and custom linkage required.... but the pumping effort at 2300 psi would be the same as a 392 at 1500.... 3000 would be the same effort as 2000....

Bob

[Tom Tucker]

So we would want a longer pump tube with a narrower 5/8" I.D. and perhaps a thicker wall wouldn't be a bad idea if one were building from scratch.  Going longer actually makes a lot of sense to me when I consider that a 392 is a very compact rifle at about a foot shorter and 3-4lbs lighter than the typical magnum springer.  The longer form factor, let's call it a magnum pumper for now, would also allow for a longer pump handle, increasing the shooter's mechanical advantage.   So if the 392 has a stroke of about 7.5 inches and if the tube I.D. is reduced from ~.775" to .625" the stroke would need to be about 4 inches longer to match the 392's swept volume.  At 6 inches longer, the Magnum Pumper would have 17% more swept volume.  Now we're talking about a rifle that could easily achieve about 22-25 FPE on only 10 pumps.  With the 6" longer tube, it would only be natural to mate it to a 26-inch barrel that ends flush with the end of the tube.  Now we're getting very close to 30 FPE on 10 pumps, with the ability to pump it many more times, and it's still half a foot shorter than some of the break barrels on the market.

[rsterne]

In terms of strength, 3/4" OD x 0.065" wall tube (like the 1322 uses) is plenty strong enough for well over 3000 psi with a safety margin greater than 4:1.... because as you decrease the diameter you increase the thickness/diameter ratio, which governs the safe pressure.... However, if you want to turn the outside of the tube to fit inside the 0.775" tube on the 392, you will need to start with a larger OD.... You could use 20mm OD x 2mm wall and end up with a 0.630" ID, and a large enough OD to turn it to fit inside the 392 tube.... If you can only get US size tubing, you will have to use a 0.120" wall, as the OD will have to be 7/8", which would give an ID of 0.635".... With that much wall thickness, you could use 2024-T3 aluminum, which is available with that 0.120" wall.... or of course DOM steel tubing or 4130 CrMoly…. With a 0.775" OD and the remaining 0.070" wall, using 2024-T3 aluminum, at 3000 psi the safety margin is 3.0:1 to yield and 4.2:1 to burst....

Remember, however, that the ID of the 392 tubing is subject to whatever pressure you run, and that is the diameter of the valve and the end of the pump tube subject to the pressure, and hence the axial load on the fasteners.... I don't know the wall thickness or material of the 392 tube, so can offer no suggestion as to the MSWP of that tube, or the anchoring of the valve or pump tube.... I don't even know if you are operating with a sufficient safety margin at 2000 psi....

Bob

[Rob M]

extending the sweep on a pumper is not as easy as it seems.. the linkage geometry needs to be done in stages  were the stroke to be way longer and the arc of the pump arm to not reflect that.  In other words , it would need to be operated by a tall person with a long reach at some point.. 2 stage piston designs solve this but thats another bag of worms

[Rob M]

in other words, as the stroke distance increases in a conventional pumper design so does the guiding slot in which the linkage will reside. To exacerbate the issue is the point in Arm travel where pressure will begin to rise  ( it happens earlier when the arm is at a higher angle in relation to the tube )

[Rob M]

one concept ive read about was skipping 70 % of the stroke distance and simply starting at the very end with a massive piston ( 2 inches diameter ) this way the 40 to 1 mechanical advantage( or more )  of the linkage is already in full swing when the stroke starts.. I have not seen it executed by the way LD described it it seemed quite viable.

[Psipumper]

Those are good points. Being a big tall guy helps . I hope the balanced valve adds some finesse to my build. If there is a gray area where a faster off the seat could add any power would be nice. Before I tear it down, I should document the fps at pump 4 thru pump 14. I need to put clay on the hammer to measure lift of poppet at mid pressures. This may be needed to help in the new spring design.
Looking at replacing the white delrin spacer with a brass balance chamber. The check valve spring will be pocketed in the check valve. The balance chamber vent hole will be very small (#60) exit to throat area on angle instead of drilling thru stem.
Ideally the poppet return spring would be inside balance chamber if there is room.
Currently the throat is .236 with a .093 stem. Poppet diameter is .320.
Transfer port is .236 x .170
Bolt probe is .078
Check valve spring is 400 grams.
Return spring is 1300 grams closed.
The balance chamber o.d. is .373 so with a .05 wall a .273 i.d. is possible.
The metric 1 mm orings can be used.
Any ideas on the size of this end of the valve? I was thinking 5 millimeters.

[Rob M]

everything looks great.. Question.. would it be easier to make a new front end of the valve with the balance chamber built in ??i know the travel distances become super technical ,im just brainstorming aloud.  another key factor is the stem length.. By shortening the stem a few MM you have a longer hammer travel ( more fpe from the hammer stroke

[Rob M]

studying rhe pic further i realized im not really grasping the arrangement.. maybe bob will chime in

[Psipumper]

everything looks great.. Question.. would it be easier to make a new front end of the valve with the balance chamber built in ??i know the travel distances become super technical ,im just brainstorming aloud.  another key factor is the stem length.. By shortening the stem a few MM you have a longer hammer travel ( more fpe from the hammer stroke

No I don’t think a new valve end would help. The removable chamber makes it a simple drop in replacement for the existing spacer. The way the stem length relates to the bolt can’t be changed but whatever you shorten the stem can be added to the hammer. The reduction of unneeded stem travel could allow the placement of the spring inside the balance chamber, the poppet could be shaped and turbulent airflow over the spring eliminated.

[Rob M]

will be interesting to  see the numbers with other factors essentially the same.

[Psipumper]

will be interesting to  see the numbers with other factors essentially the same.

I have a few details to factor in but there is a wide window to work in. No need to work the details out in this thread. I am keeping my expectations low.

[Tom Tucker]

It should be great for trigger feel and accuracy.  Any improvement in velocity would be a bonus.

[Jwollheim]

I know this is an old thread but would any of y’all that have modded 392’s be willing to do some power mods on mine if I paid you? I reached out to mac1 several times but never got a response.

[Habanero69er]

Jake, if you emailed Tim McMurray, you probably won't get a response. You need to call him.