Enhancing the Fenix 400

[Toxylon]

It’s been several years since I last looked inside my Cometa Fenix 400, sporting OEM FAC mainspring and seals, and an aftermarket OEM .22 cal barrel over the bent .177 cal original.

With the spring rolling in, I took the Cometa out for a looksie. While being a fine gun in many ways, the Fenix is really loud and jumpy, with a sharp, borderline nasty recoil. Maybe there was something to be done here.

Honestly, I had forgotten quite a bit of how this gun is put together, and what’s in there.

The OEM mainspring, at only some 600 pellets in, exhibited a hefty cant and a 13mm shortening (see pic). Cometa mainsprings have a very small diameter and not too spiffy metallurgy, so they are not long-lived; they also cost next to nothing, so it’s a balance. Further, measuring the compression space available in the gun and calculating the coil bound length of the OEM spring, I could tell the spring was overstressed by use geometry alone, being compressed to just 8% over its coil bound length in use.

Seeing the speedy mainspring degradation I felt the need to put something better in there, so I bought a Titan #13 spring, designed to work in full-power Fenix 400’s. The spring I got had the same thickness wire as the OEM spring, but at 340mm it was a whopping 57mm longer than my unused Cometa FAC mainsprings.

A quick calculation showed that the Titan would need to be shortened by at least four coils before it would fit in the compression space of the Fenix, with enough room for long use life - drop-in replacements these are not! I cut off five coils, to accommodate a thicker-rimmed tophat and a washer I would add inside the powerplant, and collapsed and ground the cut end.

The spring guide in my Fenix is a length of 12mm OD / 10mm ID steel tubing (see pic). The guide is really undersized, even for the Cometa mainsprings, which have an ID of 12.4mm. At under two inches, the guide is extremely short, as well. Both these features promote spring canting, while making the gun’s shot cycle needlessly aggressive.

The Fenix sports a solid steel tophat with a record-length stem of 60mm! (see pic). Like the spring guide, the tophat stem stands at 12mm OD, and is way undersized for the spring. The tophat fits loosely in the piston, as well, so it functions as little more than a (rattling) piston weight. It does weigh a hefty 59 grams, increasing piston mass by some 26% (and noise by maybe 100%).

The Titan had an ID of app. 13.0mm, or well above the OEM spring. In order to use the original spring guide with the Titan it would need to be “coated” with something that would increase the guide diameter by a full 1mm+, while preferably also making the guide quite a bit longer, to fit and support the new spring. There are no quality mainsprings with just a 12.0mm ID, so the rear guide as it is is useless.

Unlike recent Fenixes (as per video evidence), my 2004 gun has the spring guide permanently fixed to the end block. The front trigger guard screw that ties the rear of the action to the stock goes through the end block and into the rear guide, but even with the screw removed, the guide didn’t budge.

I opted to punch the spring guide out from within the rear block, using a length of hardwood dowel and a rubber mallet to ward off damage. Bit by bit, the guide tube moved until it popped out of the rear block. Both the guide and the hole for it in the block had knurling to keep the guide in place.

I contemplated my options in making a better spring guide. The design I ended up with had the original tubular spring guide Loctited in place, but with a length of 10mm steel axle rod of app. double the length of the original guide friction-fitted inside the latter. A length of 12mm OD copper tubing (material choice only due to availability in the right size) epoxied over the exposed portion of the rod made the whole guide equal in diameter, while close to twice as long as the original (see pic).

Over the patented tri-metal guide tm I fitted a length of ½” self-adhesive shrink tubing and shrank it in place (see pic). After shrinking, the plastic tube had such a thickened wall that the guide OD shot to 13.5mm, well above the Titan ID.

Thinking I would probably need to re-do the shrink tube job from scratch for a better fit, I tried the Titan on the oversized guide. Screwing the spring in counter-clockwise, to open up the coils, the guide did in fact inch its way into the spring: a perfect screw-on tightness!

Another issue was that the Titan didn’t really fit inside the Cometa piston, which is unusually small in inside diameter. I hadn’t remembered that the Fenix piston has a steel liner fitted; removing it would open up the piston for the Titan. The liner, however, didn’t budge; it was like glued in place. This is very unlike with Dianas, Weihrauchs etc., where the liner can be pulled out at will.

Pulling, soaking in acetone, heating, tapping with a hammer didn’t loosen the liner. As a last resort, I heated the liner and piston to well above 200 C and let cool. Then, by leveraging with a thin screwdriver shaft via a tiny opening in the cocking slot / liner interface at the piston head, with no regard to keeping the liner intact like before, I could make the liner move a millimeter, then another. Still, it took plenty of time to get the liner out. Ridiculous.

With the Fenix 400, I found I couldn’t install my go-to hardened washer on the guide under the mainspring: the piston’s rearward latch slat travels really close to the rear guide, cutting into the area of such a washer. I would need a washer of just 20mm in diameter (barely the size of the spring footprint) for it not to foul the cocking action.

So, I purchased a washer with appropriately small diameter, and drilled it out to fit the spring guide. Given the OD of the new spring guide, the washer ended up as little more than a flat ring the size of the mainspring cross-section. Well, it would still work as a bearing, sorely needed over the splayed shrink tube base.

The length of the Cometa tophat stem is such that no normal-length spring guide can be used with it: with a guide just a little longer than the factory guide, the tophat stem and the spring guide (which do not fit inside each other) would butt heads and prevent the gun from cocking.

To make a proper tophat I opted to fashion one from steel, to keep its mass up (see pic). I had never made a steel tophat, and wasn’t really equipped to make one, but did it, anyway. It wasn’t a quick or a pretty job, but I was well on my way of making the tophat fit both the piston and the new mainspring when the Cometa threw a wrench at the works.

I was painstakingly fitting the steel tophat for a slide fit into the piston, when it turned out the piston ID wasn’t a constant; the tophat barely glid through the middle section but was in fact rattlingly loose at the piston bottom, which is the only fit that matters, Grrr! As there is no way to pass a tophat through a tight spot, it is simply impossible to make a well-fitted tophat for this piston.

The home-made tophat had a 20mm stem and ended up weighing 30 grams, or just over one half of the original. This would definitely affect both the output and the behavior of the gun, but there was no room for double this amount of steel.

All my Cometa 26mm piston seals are more or less undersized, and as of this writing, it remains unresolved as to if any aftermarket seals really fit the bore here: for some reason American custom seal makers claim their 25mm seals fit both standard Gamos and the Fenix 400 / RWS94, which cannot be, as these guns have different size bores. So I used the tightest-feeling OEM seal in my stash.

Years back, when I last re-sprung the Cometa, I used spray-on dry moly on the piston and elsewhere, and the gun dieseled pretty spectacularly for a long while, including sending three-inch orange-blue muzzle blasts.

This time, I lubed the piston front and back (including piston seal sides) with a tiny amount of Honda moly. Equally tiny amounts of clear tar made the spring guide, thrust washer and tophat work as intended. The mini-lube job was intended to let me see how the Cometa would function with an opposite lubrication strategy.

I put the gun back together, and took the first two shots out the back door: the shot cycle was a linear, featureless "thud" - no twang, no buzz, no rattling, no shaking, no nothing. The loudness of the Cometa had also gone down by a mile.

It seems the tune was a resounding success: the Fenix had truly risen from the ashes!

[Novagun]

Thank you Toxylon. That is a very informative post . I haven't bought one yet and may never do so . Contrary to your disdain for cheap guns, Gamo and Umarex I have found to be very good so I can't go past them at the moment.

[Toxylon]

Thanks, Hugh!

As far disdain for cheap guns go, no. I have disdain for bad guns. After all, I cut my teeth (and well, too) with a Gamo Hunter .22 cal. The biggest disdain of mine is reserved for expensive bad guns.

We all base our airgunning world view on our experiences. Nothing beats it. Commentary by seasoned veterans is also good, but already much less applicable, as variables lurk on every corner. 

Just today I learned about yet another Hatsan that had its Quattro trigger broken off. This basically never happens with the more expensive makes. Yet off-round compression chambers are business as usual for Weihrauchs, which fetch incredible money these days. And I've fought to this day to make my D350 Mag work as it should. Etc. Etc.

[Novagun]

Yes. I bundled cheap with nasty . As you point out there is often a distinction.

[Windmill01]

Thankyou for your informative and comprehensive post. A great read.

[Toxylon]

Thanks, Gary!

[Toxylon]

Update on the enhanced Fenix 400

A couple hundred break-in shots with a variety of pellets proved that the gun had gone through a complete transformation, as far as the shot cycle goes. This was now the most pleasant springer of mine to shoot, a tuning success. I was pleased, of course, but some doubt crept in concerning the actual output - after all, any gun ends up a sweet shooter if you detune it enough.


A chrono session extracted the truth:

Superdome 14.3 gr., the hottest pellets to date in this gun, clocked around 190 m/s  /  623 fps

Baracuda15's shot around 178 m/s  /  584 fps

Hobbys clocked app. 210 m/s  /  689 fps


So, the Fenix 400 is now a solid 12 fpe gun, down from the 18+ fpe it shot previously, urgh! A perfect tune for a Brit, but I have zero use for a 600 fps, .22 cal field gun.

The Titan #13 mainspring had been sold as a FAC level mainspring for the Fenix 400, and it had been shortened just the necessary amount to fit the compression space in the gun.

As documented earlier, the super-long OEM steel tophat had to make way for a more normal steel tophat that weighed just one half of the original. Also, the OEM steel piston sleeve had to be left out for the Titan to fit. At 39 g, the sleeve alone weighed more than the custom tophat I made. So, there is some 70 grams (2.5 oz.) of mass missing from the piston, and no room to put it in.

Where to from here, beats me.

[Methuselah]

Update on the enhanced Fenix 400
...
So, the Fenix 400 is now a solid 12 fpe gun, down from the 18+ fpe it shot previously, urgh! A perfect tune for a Brit, but I have zero use for a 600 fps, .22 cal field gun.
...
Where to from here, beats me.

So what are you going to do with it?

I've got a Beeman 0035 I just found out might (only) like N&H FTT, shooting under 400 fps, atrocious shot cycle, and I might just settle, LOL.

Reckon beauty is in the eye of the beholder. I'm happy enough if I can hit what I'm aiming at...

Thank you for the wonderful walk through on the tuning process!!!

[Toxylon]

Thanks, Steve!

The Fenix sits in the corner, waiting for some way out to emerge. I think piston mass is key here. There might be enough space in the piston to put a thin piston weight in without overcompressing the mainspring. It would benefit from being of the densest possible material, but lead doesn't exactly keep its shape with the pounding inherent to a springer cycle. Even brass gets splayed out, I hear. A steel washer weighs next to nothing, though.

I have enough guns that when one meets a dead end, I just move on, for the time being.

British tuners say there's no challenge left in making a 12 fpe springer super smooth and nice. The Cometa taught me that is really the case.