Air-Venturi (for Gamo) measurements

[CO_AirGunner]

I received a package from Pyramyd Air with an Air-Venturi gas ram to fit my Gamo Whisper.  Since I've had just about ZERO luck finding any real detailed info about this upgrade anywhere but the people pitching it, I've decided to remedy that problem myself.

Upon receipt of the gas ram, I took measurements of the ram and of the two plastic parts (I think they're delrin) included with the ram.  Without my rifle apart, I can't yet tell you guys how it fits.  I'll let the pictures do the talking:

(hmm, attachments can only be 150k...  Well, off to photobucket then)

  The hole in the top of this part is 0.520" in diameter.



I forgot to photograph it, but the diameter of the ram portion of the Air-Venturi is 0.313". I don't know what the ID of the piston in the Gamo powerplant is (anyone help me out here?), but without having that info or having one apart in front of me I can't say for sure how this is supposed to be installed.

Right now, I'm guessing that the ram portion goes forward and pushes the piston and the cylinder is towards the rear near the trigger given the two plastic parts and their shape. I'm wondering if I'll need to cut a part off of the rear spring guide?

Anyway, hope this helps someone out!!! I'll be posting a how-to on the installation in the near future. Keep a watch out for it!

[snatch]

Jared, I'm looking forward to see how this goes for you and what you think about it. I just had mine tuned with a new spring so I won't be doing this mod for a while but I'm thinking I might pick one up now just to have on hand. I poked around to see what I could find out about them and the install but everything that I found suggests that you have it installed by one of their professional technicians (of coarse). Keep us posted. Good Luck.

[CO_AirGunner]

There is one concern I have right now.  If the gas ram is meant to be installed as I'm pretty sure it is (ram pointing forward), I'm worried about the small frontal area of the 0.313" piece (a meager 0.077 square inches) pushing against the back side of the piston - could this possibly cause deformation and eventual failure of the piston head?  I don't know, but it is a concern I have right now.  Anyone know how thick the front of the piston is?

[outofstepper]

For whats its worth, I'd also be very interested in seeing the process.

Just one note, the finish on those delrin parts (OD) doesn't look great.  
Someone has a chattering lathe.  Maybe they're a bit oversized and
the hone themselves in?

Whats on the business end of that rod -- female threaded on the inside?

[pgsmith]

I would love to see a write-up on the instalation, what is needed/used from the gun, and what is not. I have been thinking about doing this exact thing with my Big Cat, but have not been able to find anything about the install.

[CO_AirGunner]

Ya, the delrin parts could definitely be better.  They're probably made too quickly (time IS money), and as machining delrin is child's play there really is no excuse for a chattering tool.  I think they're just mainly spacers to keep the piston centered over the gas cylider when the gun is cocked, so I don't think it should matter.

Also, I wonder if the thin 0.050" piece I measured at the top of the delrin piece on the front of the cylinder isn't meant to be there.  It might just be a "web" piece left over from parting the piece from the bar stock, but I can't tell for sure (I don't see what purpose this thin little piece could even have).

Interesting you mention the front of the ram.  It appears that something was cut off of the end of that rod and then the end was touched up on a grinder.  It is relatively flat and appears to be perpindicular to the shaft, but it is not perfect as there is a slight roundness on the end where they took it to the grinder.  Not sure how that will affect anything.

I measured the cocking stroke on my Whisper, and it is approximately 4 and 1/8 inches.  That means that there should be a little bit of extra room (since the ram measures 4.444"), so if the end of the piston turns out to not be terribly thick, I might add a disk of some metal up in there to thicken it a little (maybe 0.1" or so) and help to spread the load.  I would then reduce the length of the ram accordingly, and possibly even drill and tap a hole in the end and attach the disk to the top the ram.  Not sure though - I'm just thinking out loud here as I haven't even taken the rifle apart yet.

[outofstepper]

For what its worth, most stock air springs -- that you'd buy from an industrial
supplier (like McMasterCarr) -- to keep the lid open on your beer cooler -- have
threaded male ends.

By the sounds of it, the thread has been cut off and ground flat.

How do you attach anything to the end of that rod?
Or don't you?

I'm sure it'll make alot more sense once you get your gun open.

And for anyone else who might be thinking what I'm thinking -- stock
air springs in this size cost about $50.

[airiscool]

Count me amoungst the curious too.

After hearing about how the springs can sometimes just break, I thought I'd get a spare to have on hand if the Whisper. Then I heard about the gas  rams and I'm gonna order one of those instead.

Man, that is alot of chatter marks for Delrin.   Maybe they intended them to be  "speed grooves" to make it go faster like the dimples on a golf ball ??  

Paul.

[CO_AirGunner]

It is apparent that nothing gets attached to the end of the rod.  I think it is meant to rest on the underside of the piston where the tophat usually sits.  That's why I'm a litle concerned.

I was one of the ones thinking EXACTLY what you are thinking, but I couldn't find any information about this thing ANYWHERE (which is why I've posted it here).  This particular Air-Venturi gas ram doesn't look like anything special despite what I've read about it from PA, so I don't know if maybe they've just found a size from somewhere like McMaster or Grainger and are charging 100% more or not.  We shall see.

I would like to run some kind of compression test on it (to get the force of this unit) to see if there is something comparable out there, but I don't have the means for it.  All I can tell you is that I am not able to compress it without mechanical help (I had to use a bar clamp to get it to compress).  I'm guessing it is somewhere in the 250-300 lbf range.  If I can figure out a way to get a reasonable force measurement I'll let you guys know.

[brokenarrowjbe]

http://www.nzairgunners.com/nzairgunforum/showthread.php?t=2216
CFX after gas ram install from new zealand. Good luck. John

ALso see this thread http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=13803&mid=110924
MAy try PM'ing Daveed
Good luck, John

[CO_AirGunner]

Thanks for the link to the NZ thread.  I've noticed the Theoben rams have that flat washer on the end of the ram rod.  That's basically what I'm thinking about doing with my Air-Venturi gas ram as I'm worried that putting all that force directly onto the bottom of the piston with the small diameter of the rod could lead to problems.

[outofstepper]

AirGunner,
any news?  I find it hard to believe you have an air spring sitting there and its not just
burning a hole in your pocket

Let us know how the mod is progressing -- pictures would be great!

Looking forward,
-Tony

[CO_AirGunner]

LOL it is indeed burning a hole.  I actually tore the Whisper down Thursday night in anticipation of working on it Friday morning.  Unfortunately, my piston seal was badly damaged as installed from the factory (see my frantic thread looking for a new seal http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=16420&posts=5&start=1), and I can't put the gun back together knowing that I'll need to tear it down again to replace the seal.  So, I'm waiting on my order from Maccari to finish off the mod, and the order should be here anytime.

The good news is that the Air-Venturi install is going to be easy as pie - it is literally a drop-in mod minus having to cut down the original rear spring guide, which shouldn't be hard to do (if you can do a tune, you should be able to cut the guide down).  It turns out that my original worry about the front of the airgun's piston not being thick enough is totally unfounded - the end of the piston (in my Whisper at least) is a nice solid chunk of what appears to be a stainless steel about an inch thick, so it shouldn't have any deformation issues.  If anything, the gas ram rod might have problems with the relatively small end mushrooming over extended use unless it is hardened to a high enough extent.  I don't know though; something to watch out for.

The gas ram does indeed get installed into the gun as I expected.  The cylinder end sits to the rear near the trigger, and the ram rod points forward and rests against the back end of the piston.  The Delrin part that is installed on the forward end of the piston (around the ram rod) is a centering guide that rides inside of the gun's piston sleeve.  It should be interesting to see how this part wears with use.  The other Delrin part (the stepped one) serves to center the cylinder inside of the gun's receiver.  The hole that is cut in the back of it (hmm, I thought I had measured it) fits nicely around the larger diameter part of the rear spring guide.  I will need to cut the rear spring guide down to a stub slightly less than 0.069", which will also help to center the delrin part.  According to my measurements, the piston sleeve will not quite reach the smaller diameter of the rear stepped Delrin part with the gun cocked, so I'm not sure why PA went through the trouble to make this a stepped part unless it is a common part with other installs.

Anyway, keep a watch out for the new installation thread due sometime within the next week!

[airiscool]

Airgunner,
Thanks, for the detailed write-up.

I'm ordering  one today for my Whisper.

Paul.

[CO_AirGunner]

Well, this isn't the detailed write-up, but I appreciate it anyway!