Posts Tagged ‘Gamo 220 air rifle’
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll finish the conversion of a Gamo 220 from steel spring to gas spring, and blog reader Vince gives us a report on the outcome.
If you’d like to write a guest post for this blog, please email us.
Take it away, Vince!
When we last saw the Gamo 220, I’d disassembled the powerplant and compared the old parts to the parts I ordered from Crosman. Today, I’ll install those new parts and test the gun for you.
The gun is laying on the bench, ready for assembly. The new piston slides in, followed by the gas spring. Be careful when sliding the piston seal past the end of the cocking slot and tuck the soft seal material away from the sharp edges of the cocking slot so the seal isn’t damaged. A flat-bladed screwdriver works well for this.
The new piston that works with the gas spring is slid into the spring tube. Notice that I’ve lubricated both ends of the new piston with moly grease.
The new gas spring (Nitro Piston) slides in after the piston. The small end of the spring fits into the socket inside the new piston I mentioned in Part 1. No lubrication is required.
The trigger and cocking link go back in (reverse order of removal), and the plain plate gets dropped into the rear spring retainer.
I’m dropping the plain plate into the rear spring retainer.
Now, I’m starting to sweat a bit. You see, I KNOW that the gas spring has a TON of pressure on it even when fully extended (very much unlike a coil spring) — so, how on earth am I gonna compress it enough to reassemble the gun? Oh, well, I’ll cross that bridge when I come to it –which is, well, right about now. After I install the rear retainer, I notice something.
There’s almost no preload on the gas spring
Almost no preload at all! THAT’S right. Because the gas spring is ALWAYS at or near full pressure, there’s plenty of preload pressure as soon as the piston comes off its stop, so very little preload travel is required.
What is preload?
When a conventional coiled steel mainspring is installed in a spring gun, it’s usually longer than the space into which it must fit. It is, therefore, necessary to compress the spring by some amount to get it to fit inside the spring tube. This compression causes the spring to be under pressure even when at rest — this is called preload. If you’ve ever seen a long, empty flatbed trailer on the interstate that looked bowed up in the center because there’s no weight on it, you’ve seen what no preload looks like. It takes several tons of weight just to get that trailer flat again — and much more to make it bow the other way.
Airgun tuners can add spacers that preload the mainspring even more when it’s resting, which causes it to develop greater power when compressed because it’s closer to its maximum potential that exists at the point when all the coils are touching. But gas springs don’t work that way. They’re under full compression (internal gas pressure) when they’re at rest. All cocking the gun does is move the internal piston against the already-compressed gas that’s ready to blast it back when the sear releases it. There’s a very small amount of additional compression of the gas, but it isn’t what makes the gas spring work as well as it does. The gas spring unit is always at full potential — even at rest.
So, this gas spring unit has very little farther to go at this point…under a quarter-inch, in fact. THIS sure makes things easy for me. Pry the retainer forward on one side while starting the pin through the other. [Note: If I used a mainspring compressor, I wouldn't need to pry anything. I would just tighten the compressor until the assembly pin holes lined up, then insert the large crosspin.]
The crosspin will go in, but the hole for the rear spring retainer bolt (that large-headed bolt I removed when I disassembled the powerplant in Part 1) doesn’t line up with the hole in the spring tube. This is a problem.
Immediately, a problem becomes apparent. Look at the hole where the rear spring retainer bolt goes. It’s not lined up with the hole in the tube. There’s approximately a .080″ misalignment here. This ain’t gonna work. My first inclination is to simply elongate the hole. But when I reinstall it, there’s another problem.
There’s a gap between the plate on the spring retaining bolt and the trigger assembly. It won’t support the trigger this way!
The trigger isn’t properly supported by the plate that’s attached to the bolt. Worse, this changes the spacing between the front and rear stock screws and doesn’t allow the action to be reinstalled.
Hmmm. I’m wondering if this is exactly what Crosman (or BAM) had in mind — preclude an easy conversion with existing parts (since the same problem would exist on a normal Quest). That leaves me thinking: Can I just butt the gas spring against the original Gamo spring retainer?
If you look at the picture of the new rear spring retainer above, you’ll see that there’s a small plate that drops into the cup that retains the gas spring cylinder. The cylinder wants to butt up against a flat surface, and the Gamo retainer has a large (approx. 1/2″) hole in it. I need a metal plate to go over it. Wait a minute! I’ve got one right here in my pocket!
A perfect spacer for the new gas spring and it costs — well, about a quarter!
And, so, it gets reassembled. Believe it or not, the whole thing works.
Time to test!
I’ll run through this pretty quickly — the velocity is now up to about 964 f.p.s., which represents a muzzle energy of about 14.5 ft-lbs. Not killer, but obviously a lot better than the detuned gun. Accuracy shouldn’t be changed — or should it? Oftentimes, guys will detune their guns to make them more accurate — or to simply make them easier to shoot. That might have some merit, as I now couldn’t break 0.37 inches at the same range. Not a big difference, and I’m certainly not gonna suggest that the gas spring decreased accuracy. But I don’t think it helped.
So what’s it like to shoot?
First of all, everything anyone ever said about “thunk” vs. “sproing” is absolutely correct. The gun “wumps” with a gas spring, and you can actually feel a kick back into your shoulder. Nothing like a typical centerfire gun, although maybe something like an 1894 shooting low-velocity .38 specials might be comparable. But that’s just a guess.
Cocking the gun is another matter. Effort peaks at about 33 lbs., which isn’t all that high — except for the fact the effort before that peak is certainly a lot higher than with a normal coil spring. This is what we’d expect, of course, with the relatively constant pressure of the gas spring. It isn’t unbearable, but it does take some getting used to.
Back to a coiled steel mainspring
After about 40-50 rounds, I decided it’s time to restore the gun back to original spec. I rummage around my spring box and find a REAL low-mileage Gamo spring, and put it all back together the way God intended it. NOW, I can really get a back-to-back series of impressions.
First, the velocity did drop a smidgen. It’s now down to an average of about 943 f.p.s., or a little under 14 ft-lbs. Second, and despite the tar on the spring and rear guide, we DEFINITELY are sproinging ourselves rather energetically. Lastly, the cocking effort is predictably much milder. Peak effort is down by 5 lbs., and the effort before that peak is even easier. Accuracy is unchanged from the gas spring.
How did my quarter, er, my impromptu gas spring backing plate pan out? Not too well.
The pressure of the gas spring punched a deep divot into the quarter.
The flip side doesn’t look any better.
I flattened it back out with a hammer, and I’m really hoping it’s still legal tender. Anyway, as I sort of expected, the relatively soft quarter didn’t do well. The backing plate really ought to be steel, 0.060 inches (1.5mm or 1/16″), just like the original.
But the bigger problem wasn’t the quarter.
There’s a serious indication of metal-to-metal galling.
There was some serious metal-to-metal contact going on here between the cylinder of the gas spring and the inside of the piston. If you look at the above pictures of the quarter, you’ll see that the indent isn’t centered. The pocket in the original rear spring retainer keeps the spring cylinder right in the middle, and apparently that’s real important because it won’t center itself.
And that’s about it for now. If this is going to work, we need a simple and cost-effective way of keeping the gas spring centered properly without permanently altering the original parts…and do it in a way that the average tinkerer can accomplish on his own. The first thing that comes to mind is to drill and tap a new hole in the new rear spring retainer, opposite of and slightly forward of the existing hole. I tried that, and found (predictably) that getting the hole in just the right spot is a bit difficult without a custom drilling jig.
For now, I’m just going to give it some thought.
WAIT! I JUST GOT AN IDEA….
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll have the first part of a guest blog from reader Vince. For those who don’t know him yet, Vince is our “go-to” guy for fixing all sorts of strange vintage airguns. In this post, he tells us the tale of a wild idea he just had to try.
If you’d like to write a guest post for this blog, please email us.
Over to you, Vince!
“Nitro” is da bomb, right? I mean, in current usage, “Nitro” anything means hot, fast, powerful and overall bad. This normally benign element sure shows its alter-ego when combined in properly mischievous proportions with oxygen to form nitrous oxide. More fun can be had by mingling it with oxygen, carbon, and hydrogen in various arrangements to come up with nitromethane, nitroglycerin, nitrocellulose or TNT (trinitrotoluene). So, yeah, “Nitro” IS da bomb — in every sense of the word.
Now, when Crosman started producing the Nitro Piston series of air rifles — well, it sends the imagination reeling, doesn’t it? That is, at least, until we come back to reality and realize all they’re doing is using something called a gas spring, which uses pressurized nitrogen to exert pressure on a piston-rod and cylinder assembly. Push the rod in, and it pops right out again, with different degrees of enthusiasm, per the individual design of the particular gas spring.
The advantages of the gas spring in an airgun application are numerous. Because there’s no metal coil spring inside, the gun doesn’t FEEL like there’s a metal coil spring inside. No twang, sproing, buzz or anything else of that nature — and no twisting or torquing reaction as the spring extends and slightly unwinds. Because gas doesn’t fatigue, a properly functioning gas spring will never take a set or get weaker with time. This also means you can leave it cocked for as long as you want with absolutely NO effect on spring life.
What’s not to love?
Well, specifically, those two little words…”properly functioning.” Yes, the gas will never fatigue, but sometimes it leaves home and never comes back. And if the gas spring DOES leak, there’s nothing left to do except go get another one. And almost certainly, it means going back to the manufacturer — and praying that they still have them available.
You see, gas springs are something of a specialty item. They kinda have to be designed around a specific application. There are universal gas springs out there, but the chances of finding one with parameters comparable to your airgun is gonna be difficult. Unlike a coil spring you can’t just get a longer one and cut it to length. You can’t tell what sort of rate a gas spring has (or had) just by measuring things, like you can with a coil spring. I believe you can still get custom coil springs made on a case-by-case basis, but making a gas spring is a bit more involved. Heck, you can make a crude coil spring yourself using a paper clip and a pencil. Sure, it won’t be good for much — but it shows that the basic process for making one is, well, pretty basic.
That’s why I’m not terribly tempted by all these Nitro Piston (of any sort) air rifles that are out there. Will you be able to get it working 50 or 75 years from now? I certainly won’t, cause I’ll probably be gone by then. But that’s beside the point. I’ve got future generations in mind here! I don’t want little Billie- or Betsy-Bob cussin’ out their great granddaddy simply because he bought a gas spring rifle they can’t fix. Heaven knows they’ll probably have enough reason to do that anyway. Why add fuel to the fire?
That’s when I got to thinking. How about gas-springing a spring gun? When the gas stuff craps out, I (or whoever) will easily be able to cram in all the old conventional spring stuff and the gun will be back in business. After examining the Crosman gas guns (for some reason this makes me think of the “fart gun” from Despicable Me), it seemed obvious that they’re based on the ubiquitous Quest platform. That rifle, as we all know (except for those of us who don’t) goes back to the old BAM B18/B19 air rifle. Which, in turn, was a near-clone of the old steel-barreled Gamo series of the time (Shadow, 220, 440, 890, etc.).
Seems to me that a proper pickin’ of replacement parts ought to let ANYONE with a Quest variant (which are as numerous as the stars in the sky) or a Gamo 220 variant to gas ‘em up without giving up long-term serviceability. The best part is that Crosman is generally the most tinkerer-friendly airgun company out there. Not only do they sell parts — ALL parts — for many of their springers, but they’ll sell them to anyone. And they’ll let you keep your first-born, your arms and your legs, and your very soul because their prices are so reasonable.
So it is that I started combing through the parts list for the Storm and the Titan GP air rifles. My suspicion of the close relations was confirmed by Crosman’s designations for these guns — both start with C1K77. I believe C designates the gun family, the 1K means 1000fps, and the 77 points to .177 cal. In any event, the commonality of parts (especially the cocking link, trigger assembly and piston seal) tells me I’m on the right track. Things can get a little tricky, as some parts that are virtually identical might have different part numbers depending on cosmetic details (like lettering). Trusting my own judgment, I came up with a list of 5 parts that I THINK will fill the bill:
Back spring guide…..BT9M22-00-2
I ordered the parts and waited. As a side note, I told the customer service rep at Crosman what I was trying to do, and he seemed rather interested in my results. I think I’m gonna just send him a link to this blog. As another side note, there’s some commonality between Crosman’s part numbers (which were revamped a couple of years ago) and Stoeger’s. Of course, both guns are made by BAM in China.
A week or so later, the package arrived!
The parts received from Crosman. The camera perspective distorts the gas spring at the hottom. It’s really straight.
Picking a guinea pig is easy. I immediately turn to a tried-and-true, long-time member of my collection: a Gamo 220.
My Gamo 220 was the guinea pig.
The Gamo 220 is a bit of a mule, frankly. All the 1000 fps Gamos of that period used essentially the same powerplant with a 25mm bore, a 100mm stroke, and the same 29 lbs./in. spring. The Shadow, which was the first decent airgun I ever bought, had a nice-to-hold, if utilitarian, synthetic stock and the low-grade rear sight. The significantly more expensive 440 had a nice-looking wood stock with Gamo’s “better” micro-metric rear sight (which, incidentally, is actually inferior to the lower grade one). The upscale 890 was a sightless 440 that came with a scope.
The 220, price-wise, sat in the middle — with a completely unadorned, mud-brown and slippery wood stock with about as much aesthetic appeal as an old shoe. It lacks the utilitarian friendliness of the Shadow and the visual appeal of the 440/890. It really was the least-appealing of the Gamo magnum breakbarrel lineup. Why do I have one?
As a result, it periodically becomes a test-bed for projects when I don’t want to mar up a NICE gun. Projects like this one.
The first thing I do is baseline the rifle for accuracy and power. In the accuracy department, it didn’t disappoint — a 0.30-inch 5-shot group at 36 feet. That’s about what I remember for the gun. Power-wise, however, was a different kettle of fish. I used RWS Basic pellets for velocity and averaged only 814 fps with a spread of 22 fps. The spread isn’t too bad, but the velocity stinks and represents a measly 10 ft-lbs of power. Oh, well. Frankly, I don’t even remember what guts are in this thing.
The gun comes apart in pretty much standard Gamo/Quest fashion, starting with the three stock-to-action screws. Once the action is out of the stock, you go to the rear of the spring tube, where the big bolt comes out first.
The big bolt comes out first. Then, the pin is drifted out, but only after the end cap is restrained.
The pin is next, but the rear retainer has to be suitably restrained in order to contain the approximately 2 inches and 60 lbs. of mainspring preload typical for these Gamo’s. Or not, as the preload in this gun turned out to be less than an inch. What happened? We’ll find out soon enough.
Next step is removing the trigger, which means the barrel has to be broken open so the trigger can slide backwards a bit.
Break the barrel to put slack in the cocking linkage. The slotted bar that runs back to the trigger is the link for the anti-beartrap device.
Then, the cocking link gets pulled down, freeing it from the piston and allowing you to disengage the anti-beartrap link.
The cocking link can be disengaged from the piston, then the anti-beartrap link disconnects from the cocking linkage.
At this point, the piston just slides out. So — what’s up with that low power and lack of preload?
Wow! That mainspring sure is short. And that rear guide AIN’T Gamo.
Oh, yeah — that’s right. I detuned this thing with a Crosman 500X spring. Incidentally, that spring is part #B12-1-00-4A, for anyone who wants a real pleasant detune on a similar Gamo or a Quest variant. That’s why velocity was so low and why I could cock it with my pinkie! That also explains the lack of preload.
But the guide? Near as I can tell, it was a custom guide I picked up somewhere. It’s dimensionally close enough to the normal Gamo parts that I can be sure it isn’t affecting power.
The spring guide in my 220 (top) and a regular Gamo spring guide.
As a matter of curiosity, I looked at the two pistons side-by-side.
The new piston that goes with the gas spring (bottom) has a slightly shorter stroke than the old piston.
The upper one is the original — and it’s a little shorter, but it appears that, for whatever reason, the gas spring wants a slightly shorter stroke than the coil spring. Also — although it’s hard to tell from the pictures — the inside of the old piston has a flat surface for the top hat, while the newer one has a shallow hole meant to locate the rod end of the gas spring. You will see why this is needed in the next installment.
That’s where we are going to leave this story for now. Vince has the old parts out, and the new parts ready to install. We’ll see what happened in the next installment.