highly modified Benjamin 392 flat top piston

[redlined_b16a]

I ported and polished the valve, hammer, trigger, changed all springs.
Mods started simple and went crazy.
I fully dump 14 pumps and its currently pcp loud.
My stoeger x10 .22 caliber was cronied at 710 fps average with 14.3 grain.The 392 is alot stronger than the Stoeger.
Im looking to reduce the headspace by machining the valve and a flat top piston.

I researched and someone claims a flat top piston would do no good on a 392 cause making the valve flat would weaken it too much and no gain would be achieved.

I recently destroyed the pump linkage pin with the added pumping.
I bored out the holes, tapped 1/4" 20 thread, boared the linkage hole 5/16" and use a 5/15 x 1/4 inch bronze bushing and a custom screw in pin.Its super smooth now.

Back to topic, is there any reason a machined flat top would not work.I own a small lathe.

I bored my valve to create more air space, modded springs and valve ports to dump more air and hold more pumps.
My transfer port is also bored 3/16"

Its a beast.

[DavidS]

So long as you are careful about the dimensions, and keeping the full pump stroke, I see no reason that a flat top valve custom made would hurt.     I would think it would help a bit.

[jon.k]

Maybe you could make a new bottom half of the valve, extend it as a flat top to the end of the current cone, and reap the benefit of an extra ~2cc volume bump? I have no idea if it would be worth it, but it seems like a reasonable alternative to cutting off the cone on the valve and potentially weakening the valve body.
I only think it's reasonable because I have never used a lathe. Maybe it is way too much work?

[DavidS]

Maybe you could make a new bottom half of the valve, extend it as a flat top to the end of the current cone, and reap the benefit of an extra ~2cc volume bump? I have no idea if it would be worth it, but it seems like a reasonable alternative to cutting off the cone on the valve and potentially weakening the valve body.
I only think it's reasonable because I have never used a lathe. Maybe it is way too much work?

I would be careful at that one.  You would lose a little stroke length (the length from the base of the cone to the top of the cone) doing it that way.    It would give extra volume, though may require more pumping for the same power.

I am not sure on this. as the Benji 39X Guns I have never been inside of personaly, though logic would say ....

[Big Bore Bart]

Maybe you could make a new bottom half of the valve, extend it as a flat top to the end of the current cone, and reap the benefit of an extra ~2cc volume bump? I have no idea if it would be worth it, but it seems like a reasonable alternative to cutting off the cone on the valve and potentially weakening the valve body.
I only think it's reasonable because I have never used a lathe. Maybe it is way too much work?

   Not the bottom half, but the top half.    They are easy to make if you do have a lathe.(I do)     The lost stroke would only be ~.25", IMHO the added volume may be worth the extra pump required.

[jon.k]

Not the bottom half, but the top half.  

Yeah, I guess they don't call them FlatBottom valves!

You would lose a little stroke length (the length from the base of the cone to the top of the cone) doing it that way.

Even if you could get away with parting off the top of the existing valve body you wouldn't gain any stroke length because you'd have to lengthen the pump rod accordingly, and then it wouldn't be able to use the same air hole so you'd have to move the air intake hole toward the valve. On mine the pump cup doesn't clear the air hole until the pump arm is fully open.

[DavidS]

You would lose a little stroke length (the length from the base of the cone to the top of the cone) doing it that way.

Even if you could get away with parting off the top of the existing valve body you wouldn't gain any stroke length because you'd have to lengthen the pump rod accordingly, and then it wouldn't be able to use the same air hole so you'd have to move the air intake hole toward the valve. On mine the pump cup doesn't clear the air hole until the pump arm is fully open.

Not exactly what I am saying.  The end of the pump cup comes to the bottom of the valve nose, being that the bottom of the cup is what needs to clear the inlet slot at the other end of the stroke, to keep stroke length equal you would have to make the flat top of the new piston come to the same point the bottom of the cup used to, and thus make the new valve front so that it is short enough, which means making it so that it comes as far as where the bottom of the cone used to be.

Otherwise you lose a little stroke length do to the difference in the pump cup versus the flat top piston.

[redlined_b16a]

Ok today I broke my bronze 1/4" x 5/16" bushing I made for the new pump pin.
So I machined a new 5/16" x 7/16" hard brass bushing.Bored out the pump arm and the plates to fit a 5/16 x 1/4" steel sleeve.Used a 1/4" 20 bolt as a pin.
Doing this i offset the hole in the pump linkage so that it would lenthen my piston by .030" effectively putting the piston against the valve decreasing head space.

The real test.
The pump arm almost doesnt clise all the way, should went .020" instead.

Now it fully dumps 10 pumps and is as powerful as 14 use to be.
14 pumps cracks like it broke the sound barrier and has alot of left over air.
6 pumps equals the power of my stoeger x10 .22 which was chronied with an average of 710 fps with cphp 14.3 grains.

Very happy with the results I no longer need the flat top.
Whats next?

[redlined_b16a]

I also wanted to mention that the 39x valves are larger versions of the 13xx and 22xx valves.nearly identical.

[freedomreigns]

I've been using air guns and rifles for over 50 years so I've seen and owned several of them.
When someone says a rifle can't be modified, I go into action.
Every air powered weapon I've ever owned has been modified and each of them has shown great improvement.
The Benjamin 392 is no different.
I get over 675 fps with Benji 14.3 gr. pellets at 8 pumps, over 720 fps with 10, 12 pumps hits around 740 fps and 14 does over 755.
Most of the mods can be done without spending more than 49 cents and to do all the mods will cost you less than 5 bucks, if you have the simple tools and a good knowledge of how to use them.
I made a video and posted it on Youtube with a quick review and Chrony test of my 392 but I can't post external links here.
It is on my channel, NotSoDumb.
If you feel like modding your rifle, let me know before you do anything, especially if it is a later model due to Crosman making it a lot more difficult to get the valve out. The valve will not drop out the bottom of the tube anymore.
I will share all my methods with anyone who wants to learn them and if you watch my video, you'll see it's no joke.

[redlined_b16a]

Currently its tgemost powerful my gun ever felt,I can only dump 10 pumps,11 has a lil retained air.Increasing the link arm/reducing headspace eliminated the use of 14 pumps to reach full power.Now the gun is harder to pump.Its a mans gun now.
More mods to come though I plan to machine a flat top.Just in no hurry.I have 2 other 2289 and a 1377 to make pistons for first.

Im currently shooting with a 42mm bsa redot.Love that combo

[freedomreigns]

I did all the standard mods to my 392 also. I even cut the cone off the valve and made it a flat top without any problems at all.
Your cut needs to stop at least 1/64" away from absolute flat.
The check valve hole opens up to be the same as the end of the check valve itself and still has plenty of wall to glide on and seal perfectly.
I made the valve 1/16" longer by adding a small o-ring the the center where the valve screws together to make it closer to the pump.
I made a flat top pump by using a 13/16" x 9/16" rubber stopper.
I cut the top off the stopper and the bottom off then glued them together to make the cup.
My rifle does 875 fps with 14 pumps but it doesn't dump all the air. Instead of making sure it has no pressure before reloading, I simply pump it 7 more times and get another shot at maximum.
When I'm done shooting, I simply purge the valve then give it one pump to store it.
If your interested in seeing what I did, I made a video on Youtube.
Contact me and I'll give you the link as I can't do it here.
Have a nice day.

[freedomreigns]

My Benjamin 392 is outstanding. All the valve mods. Drilled out to 1/2" x 1-1/2". 3/16" exhaust with o-ring. Ported valve, modded check and hammer valve. Home made pump. Valve spring modded and inlet check valve shortened to 5/16". Flat top valve.
14 pumps-840 fps/14.3 premiers, 760 fps/18.13 JSB Exact Diabolo.