GTA
Airguns by Make and Model => Benjamin Airguns => Topic started by: redlined_b16a on January 12, 2014, 09:39:13 PM
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Bought it new in 2007,shot it stock a lot since.
Recently I haven't shot it in about a year,still had the 2 pumps I left in it.
Went on an afternoon rabbit hunt and after an hour of walking around for nothing I shot the gun and barely any air came out.
so Let the mods begin.
aluminum valve corroded itself badly,clogged up valve seat.
I cleaned and polished all the valve,bored the valve body larger.Ported the valve air path and enlarged all ports,even the transfer port on the barrel.
Machined down the bolt probe to allow more airflow.Added almost 3/16" spacer to the stock spring to hit the hammer a lil harder.Cut off 2 full threads on valve half,added a thicker center o-ring.Trimmed 1/8" off of the check valve
before I could dump all 8 pumps,9 pumps I had a lot of air left over
I can fully empty out 10 pumps and the gun has really come to life.
Now how would a lil heavier hammer work?
What factory made hammer spring offers a harder hit?
will the factory valve stem/striker pin hold up to a harder hit?
would a .177 cal bolt with the smaller probe help out?
or should I call it quits before I start losing performance?
where can I purchase a older brass valve to mod?
Im happy with the performance and wish I had a chrony,I feel the gun is breaking the 700fps mark because I have a stoeger x10 .22 that people are claiming 700fps with 14.3 grain cphp's and Im putting down a lil more damage than that gun.
Its almost .22 rimfire loud though.
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It sounds like your Benjamin 392 is really putting a smack down on what ever it hits. The Benjamins and Sheridans are much more powerful than folks think. I have a 1976 Benjamin 342 that will take 10 pumps and have just a little air left in it. It produces around 525fps and about 20fpe with a 32.4gr Eun Jin pellet. Your gun is most likely hitting harder than you think.
Bryan
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To clear more pumps you are going to need to reduce the spring tension on the internal valve spring...
Good luck.
Greg
BRASS
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If you have Chairgun you can get an idea of your fps/fpe
set up some targets @ 15-20-25-30 yards use a quarter size bull aim center on all targets (no holdover or under)
and then match trajectory...
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Crosman does have brass valves for the 392 397. Quite cheap shipped to your door.
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I trimmed the striker spring 1 coil,ported and polished some more to open flow,opened the transfer port to 3/16" and grinded bolt probe down to .177 caliber size.
Now I can dump 11 pumps with a lil air left but not much,bout a half pump.
I wish I had a chrony.
I have a Ruger that puts out 16-17ft lbs and I can inflict more damage than that gun on sheet metal and aluminum.
also puts out more than my .22 stoeger x10 wich advertise 800fps but others results show 700 with 14.3.Kinda dissipointed in the x10
Tommorow i'll look for a new hammer spring and if I can get my phone working right I'll get some pics
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You could try taking a bit off the face of the hammer, effectively lengthening the hammer stroke. I haven't tried it on mine yet, but in PCPs more hammer stroke length usually means a harder smack on the valve stem. There's only so much you can do with the springs before the hammer spring holds the valve open, and you can't cock it. So the next step is to play with hammer weight/stroke length.
On the other hand, if you increase the volume in the valve as much as possible, you may be able to make more power without a harder hammer hit. Will require more pumping though.
Get you a complete brass valve from Crosman. They ran $14 or so last year and they come completely assembled, o-rings and all.
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I have researched the hammer weight idea and have run across diffrent threads all stating mixed opinions.
The people with mathematical equations say it won't make enough diffrence.
I guess I need several valves and a couple hammers along with a chrony to try stuff out.
I pulled the valve apart to change seals and now sometimes I can only fully dump 9 pumps
So perhaps I dumped 10-11 pumps before thru bypassing seals
However even 11 pumps puts out a lot more power than 9
No chrony though to see real results.
Only phone books and layers of cardboard
9 pumps 13 layers
11 pumps with left over air 17 layers
1/4" industrial cardboard packed kinda tight in a box
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Well I ported the valve I feel as much as possible
Ran a piece of paracord thru the port and used that as a file for about 30 minutes,This mirror polished the aluminum
bought a new spring from hardware store
same lenth as stock
stock spring is .036 thousands wire
new spring is .043
I fully compressed the original spring and measured it
fully compressed the new one and trimmed the new one about 1 full coil to make it have room to pull the bolt back to lock the hammer
the full lenth was too much.
I can now fully unload 12 pumps
13 pumps has a tiny bit of left over air.
now Im considering smoothing out the transfer port on the barrel but I have no tooling that can reach in thru the breach and do this.
That port is still a rough 90 degree angle.
How I wish I had a chrony
I think im satisified and I need to stop while im ahead.
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Replace the factory valve stem with a flow enhanced delrin stem and you will have achieved about all you are gojng to get out of it.
The delrin stem makes a huge difference in dumpable pumps.
If you have no way to machine your own Roy at Mountain Air sells them for $12.
I use 3/16" rod turned down to 5/32" but I leave a portion at 3/16" for a threaded seat for the poppet.
Use 3/8" delrin rod for your poppet. Thread it onto the stem and glue it in place then turn it to the proper outside shape.
Turn the stem directly behind the head to 1/8" diameter for 1/2" length to increase the size of the flow path to the transfer port.
I did this to my HB22 pistol. It would retain air at 10 pumps stock. It will now dump 20 pumps. It is all in the harder stem seal and enhanced flow path. You dont need a harder hammer strike.
HTH
:)
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interesting
I do own a small lathe,any link or pics
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I pcp mine im very happy with it, now im looking for a 20 cal or a 22 to mod them
Do you have a link for mountain air
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They are $10 each
http://www.mountainaircustomairguns.com/New_accessories_page.htm (http://www.mountainaircustomairguns.com/New_accessories_page.htm)
I dont have any pics but here you can see everything I mentioned.
http://www.mountainaircustomairguns.com/Large_accessories_pages/Large_crossman_stem.htm (http://www.mountainaircustomairguns.com/Large_accessories_pages/Large_crossman_stem.htm)
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so where might one find a better piston cup seal,possibly a urethane one.
tonite is was cold and I can fully dump 13 pumps
either the cold air has effects or I suspect my piston may be bypassing or compressing and cant pump any more pressure
I guess I wont be happy until I blow this gun apart
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the next mods are going to have to be stronger linkage from tim at mac1.....you are doing great in the power dept...but the linkage is probably flexing, and may fail....get tims steroid linkage
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gonna have to get stronger linkage next
I machined the bolt probe smaller,shimed the hammer spring about 1/16"
I can dump 14 pumps fully,15 has left over air.
Im highly impressed with the $3 mods I have done.
I can set up 3" targets out to 50 yards and hit them with no problems open sites.I shoot the rear of soup cans,Boy is it loud.
Ima come up with better sights,I have the air venturi weaver mounts but I don't care for scopes too much
so far the flight path seems straight
aim is dead on from 20-50 yards
guess my 2289 will be my next project
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Great thread.
Thanks for all your imput. Now I'm going to have to get a 392. :)
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the next mods are going to have to be stronger linkage from tim at mac1.....you are doing great in the power dept...but the linkage is probably flexing, and may fail....get tims steroid linkage
+1
I had Tim steroid my 342. It's producing 21+FPE/Eun Jin 28's/10 pumps. Well worth the $$$.
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I may end up buying another valve to do a step by step on my mods,I don't have a chrony but If Im fully dumping 14 pumps with a 14.3 grain I know its pushing near 800 fps or better,My x10 .22 has an internet claimed of high 600-low 700's with a 14.3 grain.My 392 is way more energy than the x10 puts out.
anyways my mods only required new orings,port and polish valve,tiny valve space increase,trim check valve and spring,Stronger hammer spring.Port and polish the transferport and finally made the bolt probe skinner to allow more flow.
all for $3 of parts and about 2 hours work.
I'd still love to own a Steroid tuned
My gun is a lot more fun to shoot and the fpe damage it does is awesome,I set targets out to 50 yards and the flight path is straight,aim is dead on.
its a lot louder though and has a lil kick