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Modifications To My Predathor Vuoto

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Diving Gecko

shooter & shooter
Jun 24, 2008
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A Forgotten Predathor
For about a year and a half I have had a Predathor Vuoto 85 stacked away disassembled in the back of a closet. I have always liked the idea of the Predathor as an affordable and seemingly well-built off-the-shelf vacuum barrel gun but honestly, I haven't even gotten my own one wet. I have shot them and recommended them to others, but I had a perfectly good Seac in the same size so no real need for this one. But I think it's time to get this one going, too - it just seems a shame to not get it onto some fish, either in my hands or a friends.

I am in no particular hurry with this but being the tinkerer I am, I thought I might as well make a few upgrades.
I am thinking of a custom grip, as I really don't like the stock one much. Perhaps a carbon fiber reservoir but I have used the ones I had and I haven't had much luck sourcing a new batch, yet. Also, one of the few minor gripes I have heard people have with this gun is that the trigger pull can be a bit hard and with my recent experiences making new triggers for my Mirages, I thought that would be my first modification.

Trigger Pin Modification
One way to make the trigger pull lighter is to decrease the diameter of the trigger pin - later I will see what can be done about the spring and I will polish the sear and tail of the piston, too.
As stock, the Predathor comes with a 2mm pin but I will be using a 1.4mm one. At 20 bar, the force on a 2mm pin is 640gf but for an 1.4mm pin it is 314gf so almost half. Now, the trigger pull weight wont be halved as there is still friction and the trigger spring which acts on the trigger.

Anyways, enough talk, let's get into some pics. I didn't really shoot much of the machining but it was pretty straight forward. Actually, the hardest part was measuring the trigger seat in the handle to figure out the dimensions.

The first pic shows the original 2mm trigger assembly to the left and my 1.4mm version to the right:
vAF0AZ0.jpg

Honestly, it doesn't really show a huge difference as all the mojo happens on the inside of the bushing. As an aside, I settled for brass for this little project, just because it is so easy to machine.
BTW, in real life these bushings look much, much cleaner! The macro lens is just ruthless;-)

Next up is a pic of the top of the bushings:
LhWq48G.jpg

The stock one is solid and only uses one o-ring but for these reduced trigger sets, you need to seal in two places which is why the bushing on the modified version is hollow and holds two smaller o-rings. Normally, 1.5mm trigger sets use one o-ring on the inside. Since I have the space to actually fit two, I have done that in this and my previous ones for my Mirages. It just gives me more peace of mind.

Here's a pic of my trigger showing all the parts:
3BN3AlS.jpg

The "guiding tube" in the top right of the image has an outer diameter of 2.4mm and an inner bore of 1.5mm. This is because the hole in the handle that the original trigger pin goes through is 2.5mm and I want to make this bushing so that it would work without any modifications to the handle.

And a pic for scale with an AA battery:
92xcNNS.jpg


A quick pic to show the bottom of the two bushings:
cC6h2dp.jpg

I figured out a way to hold the bushing in my tool holder so that I could put an 1.5mm end mill in the lathe chuck and use the lathe as a milling machine to make the slot for inserting the bushing. Turned out much, much better than when I free-handed it on my earlier bushings.

Finally, I copied Dima's idea for an insertion tool using a cut-off from an 8mm 17-4ph shaft and again I used the lathe as a mill to make the the flange:
vX5bl1s.jpg


U6xTsuV.jpg


As you might have noticed my bushing is a bit longer than the original. After measuring the handle, I realized I could use more of the thread length in the handle seat which seemed like a good idea. The overall length of my bushing including the top part is obviously longer, too and pretty much as long as it can be: had the guide tube been a few 1/10ths of a mm longer it would have interfered with inserting the barrel into the handle. But I believe making the guide tube as long as possible and also sitting as snug as possible in the original trigger pin hole in the handle adds a lot more support to the trigger pin. I would say that despite the trigger pin now being 1.4mm, rather than 2mm, it is actually better supported than the original one is.

I guess at some point I think it would make sense to actually assemble the gun and check for leaks to make sure I got the dimensions right on this little trigger project;-)
 
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Leak Testing The New Trigger
So today I put the new trigger in, took the gun up to pressure and surprise, surprise, it leaked...!:-(. Honestly, I was surprised as I thought I had done my homework on this one.
But it is not easy to measure the whole trigger bushing bore in the handle precisely but I thought I did alright. Basically, I was trying to "reverse engineer" the handle so that I could decide on the dimensions for the replacement trigger. Besides the challenge in getting tools and guinea pig mandrels into that restricted place for measurements there is some really funky stuff going on in that handle like an o-ring seat which can't decide if it wants to be 5mm or 6mm in diameter.

Anyways, try as I might I can't blame the handle design for my failures.
Oh, I should mention that the first thing I did was to figure out if it was the inner o-rings or the outer that was leaking. My gut said it had to be the outer as the inner design has, by now, worked in two other bushings. It took a little while having fun in the kitchen sink but with very low pressure in the gun, just enough for a slow trickle of small bubbles, I could see that the leak was indeed not from the pin but from the outer edge of the bushing - indicating that the top, outer o-ring seal was leaking.
When I decided on my trigger design I used the measurements of the bushing bore as well as the original trigger bushing but it turned out that with my design the outer o-ring was sitting too low in the bore. I think, perhaps, the original bushing screws in much deeper than I thought. Unfortunately, I didn't take notice of it before I unscrewed it and just anticipated its bottom sat flush with the lower end of the bore.
Long story short, I machined some washers to sit under the o-ring on my guide tube and it stopped the leak:
fBI31h7.jpg


The gun sits at 21bar now and if it reads the same tomorrow, I will machine a new guiding tube to raise the o-ring and re-test.
6QVm1gg.jpg


For the eagle-eyed guys out there, the adapter on the end of my pump hose is one for Mares-style pump inlet valves. I swapped the proprietary Salvimar pump inlet valve on the gun for a Seac one (Mares dimensions) so that the Predathor will share pumps, pressure gauges and the ScubaFill adapter with my other guns.

Just judging from feel, but the plastic parts in this handle seem so nicely and precisely molded and I speculate the plastic is of good quality also. Very likely some fiber filled stuff. It feels heavier and stiffer than pretty much any other molded plastic parts I have handled.
One thing I really like is how the traverse pin holding the trigger sear in the barrel extends out on the sides of the barrel and slots into channels in the handle side walls (I guess most/all newer guns have this). This means there is no risk of the barrel turning inside the handle when you tighten the pump inlet valve or the muzzle. It's a mod I have been thinking of trying to add to my Mirage handles for the longest time. Doesn't sound like much, but it's a very nice feature.
 
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Starting Over
It's a day later and the washer trick didn't work. The gun lost a third of it's air overnight. I didn't spot a leak in the sink test so it might have developed later since it was fairly obvious this morning. Either way, something has to be done.
So, onto the next step of practical reverse engineering;-). I made a dummy trigger bushing - it is solid without a bore for the pin but more importantly it is made so that I can raise the o-ring as much as possible in the handle seat (my trigger assembly in the upper right and the dummy to the left):
qFtfOeH.jpg


Inserted in the handle, it looks like this:
VEIWBsz.jpg


And the compulsory dunk test showed no bubbles:
xdkvNbE.jpg


But the last leak was either so slow I didn't notice the bubbles in yesterday's kitchen sink test or it developed overnight, so I am not yet out of the woods. Hence, another overnight wait is in order. We are starting out at 19bar:
rVSGLJ6.jpg


On another note, I am beginning to look deeper into trigger sear springs. Not knowing the amount of friction and not being able to do much about it besides polishing the sear and the piston mating surfaces I think there could be some gains made changing to a softer spring. I mentioned it on my Mirage thread, too but didn't get too far with it. But a very quick test showed that the spring on my Seac Hunter takes about 1kg of force to compress to the release point. To me, there is room for quite some savings there. But more to come on that:)
 
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Down The Rabbit Hole
The dummy worked, the gun held air:
qW2dQPI.jpg

So, I measured the dummy and strangely enough it wasn't much different than my first trigger, just a 01-0.2mm difference between where they make the outer o-ring sit. Radial compression should be the same.
BTW, making the dummy long enough that it extended past the trigger guard made making the measurements much easier:
MwazZB7.jpg

Once you have that measurement it's an easy case of subtracting it from the overall length of the dummy to see how much it inserts into the handle:
klXwtFn.jpg


So, I went ahead and made a new guide tube (the upper part of the trigger bushing assembly) to mimic the dummy's measurements:
Ti0K5Ib.jpg

But that trigger leaked, too! This is getting strange...
Seems like I have lost the trigger making mojo I obtained making my Mirage triggers which have been perfect so far. I am quite puzzled to say the least. Could be the inner seals after all, though I doubt it.

What I remember from my Hunter when it had a stock trigger was how picky that o-ring was. Too little compression on it from the bushing and it would leak, too much and it would leak, too. The sweet spot was within less than half a turn on the bushing, probably more like a third which is 0.3mm-0.5mm of bushing movement. Why they designed it like this is beyond me and though not being an engineer, I will go out on a limb and say it is not the best o-ring design. These o-rings shouldn't be compressed axial but only by the bore and pin. It's not a gasket that you need to press down on and doing it is bad practice (I think).
I now suspect maybe the same issue with using the bushing for o-ring compression is happening in this handle. Actually, I am almost 100% sure, as measuring the original o-ring shows the OD of the o-ring not being any bigger really than the diameter of the bore it sits in. So, they are relying on it being squished axially so it will expand radially and start sealing.

I have now redesigned my trigger so that I have more room for adjusting how deep in the handle it sits. In the previous version, it bottomed out where I thought the o-ring should be sitting and within 0.2mm of where the dummy makes it sit. But I couldn't screw it in anymore than that. The new bushing will be able to go a tad further into the handle, if needed.

I could have made the changes on paper but I modeled it in Fusion and even made my first very simple animation out of it;-).



Come to think about it, this may actually not be the best idea. I am basically copying a sealing design I don't fully agree with. I will think this over one or two more days. The right way to do it, is probably to go up slightly in o-ring size so I don't have to rely on the bushing doing any compression on the o-ring.

Mind you, as long as you don't tamper with the original I think it is all fine. I am just chasing the proverbial rabbit...
 
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Are you going to shoot it or what???? By the way, once you get used to the feel of the trigger, which was a few shots for me, you hardly notice the difference from, say, a mares syrano.
 
Are you going to shoot it or what???? By the way, once you get used to the feel of the trigger, which was a few shots for me, you hardly notice the difference from, say, a mares syrano.
Haha, good point!
Well, the problem is I don't live near the ocean. I live in Shanghai and can only spear on vacations. So, admittedly I spend the vast majority of my time as a couch spearo, dreaming about hunting while tinkering with my guns. Also, since I have so little water time, it always takes me a little bit longer to get back into the groove once finally on trips. One thing I know helps me in that regard is to make most of my guns behave more or less the same and two things I really like is to have are very soft trigger pulls and neutrally balanced guns. All my other guns have soft pulls so I figured I wanted this one to have it, too.
But I suspect the real reason for all this unnecessary tinkering is that I like the challenge of improving on my guns and, lately, making the parts myself;-). The ultimate goal is to one day make an airgun more or less from scratch and these little projects are all part of the learning curve.

In all honesty, I will probably give this gun away at the end of it all, or leave it with a friend in Indonesia, Philippines or Thailand. Not sure yet;-).
 
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I fooled around a bit more in the CAD software yesterday and came up with a slightly new design.
But first, here is the last one I did (which mimicked the dummy but still leaked):
cvHEsPC.jpg


My latest thoughts were about treating the o-ring seat in the handle as a proper one, where the o-ring doesn't rely on the bushing compressing it as I think that is just bad design. The latest version therefore has a proper groove for the o-ring - made by adding a flange on top of the guide tube:
3E0SDiN.jpg


I am waiting for a some new o-rings and a 5mm end mill but once they are in I will give this latest version a go.

(The issue really is that the o-ring seat in the handle is a very "funny" design. It is 3mm deep but the top half of that is a 5mm bore and the lower half is 6mm. So, basically, there is a step right in the middle of the o-ring seat. Even if we regard one of those "halves" as an o-ring seat on its own, it is only 1.5mm deep which is way too little for the o-rings used).
 
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I know what you mean. I used to try stuff with my pathos, even considered building a sling myself,
started experimenting on making parts with recycled HDPE plastic (tried to make plastic timber) to make a muzzle and a handle, but I found I needed to invest on gear and time, and decided it's not worth it. Maybe if I reach retirement. I would love to make a workshop!
The reason was that I wasn't happy with my gun (and pathos is still considered one of the most reliable slings). I tried different shafts, rubbers, wishbones. After almost a decade, I concluded that slings tend to have unpredictable shots because of recoil, and rubber performance. By the latter I mean declining performance over time and use, and memory (By rubber memory I mean that rubbers "forget" their power if stretched for a long time, espesially if they are "old").
So, now you know why I decided to try pneumatics, and I am glad i did. I don't need to explain to you why, I believe! The Italianos have done it all for me!!!
 
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What is it in some pneumatics that makes for a harder trigger pull, is it the spring or how it interacts with the line release or something else?

Edit: I went back and read from the start, the smaller pin diameter. Why do some guns using a larger one, more robust?
 
What is it in some pneumatics that makes for a harder trigger pull, is it the spring or how it interacts with the line release or something else?

Besides the diameter of the pin (and the pressure in the gun), there's friction and geometry at play. That's actually what I'm looking into these days. I need to measure a few more triggers to see if the geometry is the same. So far, all the sears have the same geometry but of course the trigger itself could have more or less leverage.
Pete would be able to do this in a sec, would be good to hear his thoughts. He has analyzed the forces on many band gun triggers so perhaps he has done some airgun ones, too.
My gut feeling right now is that the spring and possibly the friction between the sear and piston makes up for a larger part of the trigger pull load than I previously thought.
In the next few days, I'll probably measure some springs and get back with the results.



Sent from my iPhone using Tapatalk
 
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Predathor Handle O-ring Seat
I know there aren't many folks out there interested in this, but I might as well add the info to the thread just in case someone down the line can find a need for it.
Here's a simple model of how the Predathor's bushing thread and o-ring seat looks. What's interesting is that the top of the bore has two different diameters. Why, I don't know:
zSn0mBa.jpg


The upper part of bore is 5mm in diameter, the lower is 6mm (both are only 1.5mm deep):
9PbIf9C.jpg


The 6mm bore is too big for the stock o-ring which is only 5.4mm in outer diameter with a pin going through it:
Ezi5bGa.jpg


The upper bore fits the o-ring better radially, but is not deep enough - you can see the bottom of the o-ring "sticking out":
EyL7lYR.jpg


I guess I could insert the original trigger assembly try to figure out exactly where that one makes the o-ring sit. But not sure I will bother...;-)

Finally, a snap of how the latest trigger design (before final polishing) would sit in the bore:
RASfW5y.jpg


With that double step in the bore, it will never really be perfect, and it makes it very hard to follow good o-ring design practices. One way to go about it would be to use an o-ring with a section width of only 1.2mm - that would would leave enough free space in the 1.5mm deep o-ring seat for the o-ring not to be compressed axially. But then I'd have less "rubber to bore contact area" making a seal and thus more risk of a leak in that regard.

[EDIT] Did a bit of reading and supposedly, the biggest risk of going to a smaller cross section is not the reduced contact area but rather the higher risk of extrusion into the gap between the bore and the bushing. So, if I can keep that gap tiny, perhaps this is still an option. At least I learned something new;-)
 
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Trigger Success
Some real work came up so I took a break from the trigger tinkering but got back on the lathe yesteday. I decided to go with the design with a flange at the top of the guide tube. After a dunk test in the kitchen sink and an overnight pressure test I can happily say that this one finally seems to work:)

Again, no pics of the actual machining of this part but it was rather uneventful. Brass is just so easy to work in. It's heavy but for small parts the penalty is not too big. But here's a rendering from the amazing free CAD software that I am using (Fusion 360):
ULAsMqc.jpg


And here's the real deal. From left to right: The original trigger kit, my first DIY version (which leaked) and finally my latest, and non-leaking, version:
jV7VDIE.jpg


It continues to have two o-rings on the inside and an 1.4mm pin. The guide tube (the top part) inserts a little deeper into the threaded bushing in this version. This keeps the inner o-ring "free space" to the very minimum allowed and thus reduces the risk of the o-rings skewing. I guess it also supports the top part a tad more:
BsKu8hR.jpg


The reason for the flange is that you don't need to guesstimate how deep into the seat to screw the bushing. Just insert it until the flange hits the bottom of the bore and, if my measurements are correct, the o-ring should then sit in the right place. Also, I went up in o-ring size slightly and adjusted the diameter of the bore so the o-ring now has about 23% compression just from being inserted in the handle o-ring seat. No more silly axial compression on the o-ring by the bushing in this design (well, perhaps still a tiny, tiny bit from the strange step in the seat, but nothing I can do about that with this size of o-ring section).

BTW, this is pretty tiny stuff. The green o-ring is 1.8mm in cross section and the flange above it is 0.4mm thick. I am holding all these measurements very close to my drawings. Actually, I think I am within 0.05-0.1mm on all of them. After installing the DRO (digital readout) on the lathe, it made holding tight tolerances a whole lot easier.

I also finished the insertion tool. I tend to shave off extra weight when I can, even if it is just a few grams:
eT4lfta.jpg


The insertion tool above is how Dima makes his, but I also wanted one that holds the trigger pin itself while you insert the bushing. It's an easy variation - just make a short bore in the end of the tool instead of it ending in a "pin":
xotzevi.jpg


Both work equally well though the one with the pin at the end is handy for letting air out of the gun, too. On the other hand, it adds a step to the assembly as you have to insert the pin by itself but it's not a biggie at all. I'll leave you with a pic of how the tool looks when inserting the bushing:
iEeDojN.jpg


Next up, I will try to look at the spring force. I just received a few replacement options for softer springs.
 
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