Author Topic: HOW-TO: Air-Venturi Installation Procedure  (Read 24909 times)

Offline CO_AirGunner

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HOW-TO: Air-Venturi Installation Procedure
« on: April 27, 2009, 02:08:50 PM »
Well, I was finally able to get the Air-Venturi gas ram installed in my Gamo Whisper over the weekend.  (Here’s a link to the part: http://www.pyramydair.com/s/a/Air_Venturi_Ram_Air_Gas_Spring_Conversion_Fits_Many_Gamo_Rifles/2103)  I had planned to do this over a week ago, but a damaged piston seal (from the factory no less) forced me to wait until a replacement showed up from JM (which was very hard).  To sum it up, I am VERY happy with the Air-Venturi gas ram!!!

While waiting, I did a thorough de-burring to the receiver and honed the compression chamber.  In hind sight, I think I would skip the honing step in the future (it seems a little too rough to me even though I used 400 grit stones) unless the bore looks rough to the naked eye.  My receiver was pretty darn smooth from the factory and I did hesitate to do the hone, but the experts here suggested I go ahead with it so I did.  I’m not certain what this has really gained me (and hopefully it hasn’t hurt anything), but I think next time I have the rifle apart I’ll smooth it out further with some 800 grit paper wrapped around the hone.

Anyway, here we go!  This mod was performed on a Gamo Whisper, and should work on many other Gamo rifles with the same powerplant (Big Cat, Viper, Hunter 440, etc.), and possibly some of the Chinese copies (hint, hint…).  I won’t get into real detail on the tear-down, as there are plenty of those already.  Start by removing the action from the stock and then removing the spring, trigger block, and the rest of the internals from the receiver.  You only need to remove the piston sleeve if you are replacing the seal or plan to clean/tune the rifle, otherwise it can just stay in the rifle..  BE CAREFUL HERE!!!  The mainspring is under considerable force and may be pre-compressed several inches.  Use a spring compressor if you can (I used a large, heavy-duty bar clamp since I knew the mainspring in my Gamo was only pre-compressed an inch at most).  The only parts from inside the receiver that you re-use are the piston and the rear pin block (which in my case was integral with the rear spring guide).  You can toss (or keep if you prefer) the old spring, the tophat, and anything else in there.

The one part that needs to be modified for the Air-Venturi installation is the rear spring guide/pin block.  Here is what it looks like when taken out of the rifle:


This needs to be cut down to almost a nub in order for it to fit together with the other parts.  I cut it off with a large cutting wheel, then slowly and carefully ground it the rest of the way down to 0.050”.  This fits against the rear Delrin part of the Air-Venturi, and the face it rests against is just a tiny bit under 0.070”.


As you can see in the photo below, the top part of the piston is a good solid chunk of stainless steel about 1/2 inch thick.  This part rotates freely inside of the piston sleeve, which is something I did not know prior to taking my rifle apart.  The ram rod on the Air-Venturi gas ram makes direct contact with the rear of this part inside of the piston sleeve.  


The rear portion of the piston sleeve has been expanded slightly on a mandrel by Gamo, and then machined to a pretty smooth surface.  Other than the piston seal, this machined face is the only other moving part that makes contact with the inside of the receiver after the Air-Venturi modification and is the part that requires lubrication.  Lube on any other external portion of the piston sleeve except the seal does no good after the AV mod.


Here you can see exactly how the Air-Venturi fits together with the piston.  The rear spring guide/pin block fits against the right hand side of all of this.  I still need to put the piston seal on in case you didn’t notice.  :p  This whole assembly slides into the receiver and is then held in place by the rear pin block.  That’s it!


First we have a shot of the piston installed in the rifle.  I installed the piston seal onto the piston and lubed it up with some 100% pure silicone oil.  I also put a VERY thin coat of this on the walls of the compression chamber.  I put about a 1/2 teaspoon onto a napkin and wiped the walls down using a dowel.  A very light, thin coat is all that is necessary.  I recommend stopping by you local R/C hobby store and picking up some 100% pure Silicone shock oil used in R/C car shocks.  It comes in various weights (I like 30 weight for general purpose) usually in a 2-3 fluid ounce bottle for a few dollars.  Great stuff for multiple purposes around the house.  Anyway, looking at the next picture you can see the little bit of Moly paste from James Maccari at the rear of the piston (I bought the package deal which I recommend http://www.airguns.citymax.com/catalog/item/251484/4169286.htm).  You don’t need to use very much of the Moly paste!


Here is a close-up, for what it is worth…


Next, place a small amount of Moly paste on the end of the gas ram rod (since this will be a metal-to-metal contact point under large amounts of force) or lightly coat the rear face of the piston.  Also lightly coat the forward Delrin part on the gas cylinder.  This part slides inside the piston sleeve so it requires lube.  Next, carefully slide the gas ram into the receiver until the Delrin part at the front of the gas ram’s cylinder section makes contact with the piston sleeve.  You might then have to jiggle the receiver a little bit to get the cylinder over the step of the piston sleeve.  Once the gas ram rod is butted against the rear of the piston, it should look like this:


As you might already be able to tell, there won’t be much pre-compression to deal with in this set-up.  A nice side benefit.  :D  Here’s a close-up:


Here you see the modified rear pin block in place with all the parts ready to be compressed and locked into place:


And again, a close-up:


Make sure you install the trigger block now, prior to compressing all the parts into place, as the rear Delrin piece will prevent the trigger block from being installed once it is in its final position.


Here is a close-up of the rear of the piston sleeve after all the parts are compressed into the gun and the pin is dropped into place.  Note where the forward Delrin piece now sits.  I measured the cocking stroke on my rifle and it is just a hair under 4” at 3 and 15/16”.


Next, put the action back into the stock (being sure to use Blue Lock-Tite) and let it sit for a day or so…

Since I know you couldn’t resist the urge to immediately shoot the rifle, be sure to double-check the stock screws before letting the Lock-Tite set up after shooting it a bit.

So there you have it.  Pretty easy mod really!  If you can do a tune, you should surely be able to install the Air-Venturi gas ram.  The hardest part is cutting down the rear spring guide.

I am VERY PLEASED with the results from my Air-Venturi gas ram.  After the initial settle-in period (took about 100 or so shots), my velocities are up between 5% and 8% over what the factory spring was doing just prior to removal.  I chrono’d 60 shots (10 shots each with 6 different pellets) before and after the mod to confirm this.  I won’t recount the exact numbers here, but I’m now spitting out Beeman Crow Magnums at a little over 850 fps, Crosman Premier Hollow Points at about 905 fps, and JSB Exacts at about 870 fps.  Not too shabby!

The biggest change in the rifle is how it shoots.  Hold sensitivity has been GREATLY reduced!!!  Twang is a thing of the past!!!  When you pull the trigger all you get is a nice, resolute THUMP.  I don’t notice any rebound like before, though there might be just a tiny bit; hard to tell.  Accuracy has gotten a lot better due to the gas ram, and my rifle now has a much higher tolerance of different pellets.  Prior to the mod, it hated the CPHP pellet, and now I can get 3/8” groups (edge-to-edge) from them at 10 yards.  I can literally stack the JSB exacts at 10 yards now!!!  :D

The rifle also sounds a lot quieter now.  I’m not sure if it really is though from a dB perspective; the sound it makes when pulling the trigger is much lower pitched than it was before, so it just might sound quieter to the human ear.  It is definitely harder to cock than it used to be, and the force required to cock it starts as soon as you break the barrel.  I would estimate the required cocking force to be somewhere in the mid forty pound range.  I still need to check this on the bathroom scale.

Anyway, I hope this helps at least one person out.  I searched ALL OVER for this information prior to my Air-Venturi modification to no avail.  Take care, and SAFE SHOOTING!!!
\"Engineers don\'t idle well.\"

If guns get outlawed, only the outlaws will have guns. You don\'t want that, do you?

Offline HNT5

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RE: HOW-TO: Air-Venturi Installation Procedure
« Reply #1 on: April 27, 2009, 02:35:25 PM »
Very nice write up indeed.

Nathan

Offline OHPLNKR

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #2 on: April 28, 2009, 02:42:56 AM »
Thanks so much for the writeup and the pics. I have a Whisper that is air venturi by PA an I think it's great, I've enjoyed it from the first shot. The plan is now to install one in my Silver Shadow 1000. I was wondering if I could do it or should I send it out. Thanks to you I think I may give it a try. Thanks again.  John

Offline CO_AirGunner

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #3 on: April 28, 2009, 05:57:30 AM »
By the way, for ease of finding it, the link below is to my post with the detailed measurements of the Air-Venturi gas ram:
http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=16375&posts=18&start=1
\"Engineers don\'t idle well.\"

If guns get outlawed, only the outlaws will have guns. You don\'t want that, do you?

Offline pgsmith

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #4 on: April 29, 2009, 04:37:14 AM »
CO_AirGunner way to go, I like you have looked all over for this info and nothing... now with your write-up I feel I can do this mod!!! 2 thumbs up. Wonder if we can sticky this post.
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Offline CO_AirGunner

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #5 on: April 29, 2009, 04:47:37 AM »
The mod is really quite easy.  If you don't mess with the piston (but I recommend taking it out if you're going to do a de-burr), the hardest part is geting the rifle apart and cutting down the spring guide.

If this helps just one person out there I feel it was worth my time. 8)
\"Engineers don\'t idle well.\"

If guns get outlawed, only the outlaws will have guns. You don\'t want that, do you?

Offline pgsmith

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #6 on: April 29, 2009, 05:28:36 AM »
It was worth you time then, I plan on doing the mod next year. This year is a new trigger, scope, and mount.

Thanks lots for all of your effort.
Gamo Big Cat 1200 Leapers 5th gen 3-9x50
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Offline Boz

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RE: HOW-TO: Air-Venturi Installation Procedure
« Reply #7 on: April 29, 2009, 05:50:20 AM »
Im a huge fan of the gas ram hop-up, makes such a huge improvement over all.  This writeup will help our community in a big way, thanks CO, very nice write up!

Anyone considering one of these but have any doubt lemme tell you, the difference between a spring gun and a gas ram gun is such a huge improvement.  I honestly do not know why these gas rams aren't the new air gun standard yet  :emoticon:

Offline CO_AirGunner

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RE: HOW-TO: Air-Venturi Installation Procedure
« Reply #8 on: April 29, 2009, 05:57:37 AM »
Couldn't agree more.  The rifle is soo much easier to shoot, and has a MUCH less harsh firing cycle.  I have my concerns over longevity, however I can't imagine this gas ram not lasting as long as at least 2 springs.  Price needs to come down to reality however.

When I have some time, I plan to see if I can find a similar spec'd ram at somehwere like Grainger or McMaster Carr.  I think something similar should be able to be had for somewhere like $20 - $30 plus the cost of the two small Delrin (or similar) parts.  I don't see why the gas ram conversion couldn't be had for $50 or a little less (other than greed).
\"Engineers don\'t idle well.\"

If guns get outlawed, only the outlaws will have guns. You don\'t want that, do you?

Offline pgsmith

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #9 on: April 29, 2009, 06:07:03 AM »
CO you make it I will buy from you, if you can produce the same results or better of course.
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Offline CO_AirGunner

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #10 on: April 29, 2009, 06:26:04 AM »
My goal would be to find something that required as little modification as possible and that anyone could order from an established supplier.  We might have to get something set-up for the manufacture of the Delrin spacer parts, but those should be easy turned parts.  I might even consider taking that task on depending on the outcome of my search.

If PA's Air-Venturi gas ram wasn't so darned expensive, none of this would be necessary; hint, hint. (I know they've got to be reading this)
\"Engineers don\'t idle well.\"

If guns get outlawed, only the outlaws will have guns. You don\'t want that, do you?

Offline pgsmith

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #11 on: April 29, 2009, 06:47:30 AM »
CO I could not agree with you more, on all the issues.
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Offline jmrarick

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #12 on: June 04, 2010, 08:42:02 AM »
CO does that CO stand for Colorado? Cause that's where I am.  I have been looking for a gas ram with the same specs or better than the crosman ones.  I have found a few that are very close but I still have no idea what the supplied force in the strut should be.  You can get variable adjustable ones that you bleed until they are right but that's a little sketchy because you wouldn't know what your bleeding them to and every one you did would be different.  Check this one out http://www.reidsupply.com/Detail.aspx?itm=GAS-3 it comes close in size specs but once again i have no idea about the force rating.  I would like to get 25-30FPE out of my Crosman Titan GP (effectively upgrading it to a Trail NP XL) and then put the Titan's stock ram into my G1 Extreme.  I guess I could call up Crosman and see how much a replacement ram for the Trial NP XL is.  My guess is that the ram in the XL just has a higher force rating (more gas in the same ram) than the regular Trial NP rams.

Offline PhantomF4E

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RE: HOW-TO: Air-Venturi Installation Procedure
« Reply #13 on: June 04, 2010, 11:53:36 AM »
Question on the Pyramyd website. It specifies you must send your rifle in . I take it they will send it out the ram anyway ? Or did you make a special request? I have been putting the venturi install off because I simply did not want to ship the rifle off. I guess they don't know how many engineers there are out there that have the airgun fetish, and are more than capable of the transplant. Great write up. I swear some of you guys could be tech-writers. Or are you !

Offline CO_AirGunner

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Re: HOW-TO: Air-Venturi Installation Procedure
« Reply #14 on: June 04, 2010, 12:48:47 PM »
jmrarick: Yep, I'm in Colorado (see the location under my avatar).  I really don't know what the force in the AV strut is.  I'm not a small guy (6'2", 240) and I could barely compress the strut when putting it on the floor and pressing on it with a 2x4 with a good portion of my weight.  The one you linked says it is 146 lb, which might be close, and is better than anything I found in my searches for that size.  I imagine Pyramyd also had to experiment quite a bit to find the correct force strut.

Phantom: Pyramyd used to offer the AV as a seperate product.  They always seemed very hesitant to do that, and I don't think they did it for very long.  You see here exactly how EASY putting one into the Gamo was, yet Pyramyd wants to keep installation in-house for some reason.  I have no idea why other than maybe some kind of liability thing (or maybe since it is now a Crosman product?).  I am in aerospace engineering, and have written quite a few tech papers in my career.  I wouldn't quite qualify this write-up as one, but if you combine it with the posting I made with the dimensions of the AV (linked in the fourth post here), it might have enough detail to be close.

FWIW, my AV is still running strong.  It's taken several rabbits, a boatload of squirrels, and even some brown rats at my Aunt's horse property this year.  It has not lost any power.  The modification has given me accuracy and power enough to easily take medium-sized brown rats at 50 yards using a rest, so no complaints other than initial price!
\"Engineers don\'t idle well.\"

If guns get outlawed, only the outlaws will have guns. You don\'t want that, do you?