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PostPosted: Mon Mar 08, 2021 12:51 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
So I bought an xs28 in .25, for the price of the gun I'm moderately impressed. Everything seems to built pretty well... Except for the piston! I unwrapped the gun, all excited to shoot it. Cocked it, was really rough and binding. So I fired it once. Tons of power. Very cool.

Then of course, the mandatory disassembly and cleaning process begins.

The first thing I noticed was that trigger assembly likes to fly apart if you don't know what you're up to with it. Little searching and found everything :lol: but then I pull the piston and see the rod is cocked to one side. The rear spring guide is also warped into an 's' shape. So I deduced that when they build the pistons, they crimp the tube to the head. This one went on crooked.

Took it on myself to develop a better piston assembly, with matching rear guide and top hat. I have done a lot of research on what sort of tolerance for spring fit, what I couldn't find though is how much does a spring actually expand once compressed?
Anyway, still learning how to operate this lathe thing. I'm getting there though :lol:

I have this crazy idea, concerning the piston body. Get rid of the cocking shoe slot! By turning down the material along where the slot would be, to allow the same cocking stroke clearance for the shoe, it still has a ring to contact on the back end. Not sure if I'll benefit from this or not, my thoughts were to allow the piston to rotate as the spring twists and does it's thing. Also with no slot cut, the body should remain concentric.

The head is to be threaded and screwed into the body. Same thing with the rod. Not really rocket science, but a challenging project for a novice like myself.

Any thoughts on this little project?
I'll update with pics and measurements in a bit.


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PostPosted: Mon Mar 08, 2021 2:38 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
For anyone who's interested, I took measurements from the gun, for the purposes of this endeavor.

Spring at rest: 14" length. Coil thickness .0125", counted 39.6 coils.

Takes 3.65" of preload to install the spring.
Allowable stroke based on the cocking linkage design is 4.8"
The seal is .20" thick, and the seat that the seal sits on is .17", giving .03" of 'cushion'.

So I figure, using the stock guide and no top hat, when cocked the spring is compressed to 5.55". Coil bind would occur at 4.95". This leaves me with 0.6" of additional room for spacers if needed. I don't plan on adding and spacers, Maybe very minimal like .1 or .2" depending on the piston head and top hat measures i end up with.

I'm posting this info since I did look for it online and couldn't find it anywhere. Springer tuning info seems like an old trade secret nobody really shares numbers on...


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PostPosted: Mon Mar 08, 2021 3:03 pm 
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Joined: Wed Jan 05, 2011 12:35 pm
Posts: 7675
Location: P.G. B.C.
Interesting process - thanks. Quite a project.

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PostPosted: Tue Mar 09, 2021 12:04 am 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
No problem!

Here's some progress.
I still need to finish the internal threads on the piston body, they almost thread together. Just got a proper threading tool so I'll tackle that later.

The cocking shoe does fit and it cocks the piston... I re-contoured it to properly mate to the piston wall, no binding yet. See what happens when I get it assembled I guess.

Rear guide is delrin, 2 pieces. The rod is separate from the step washer, so it should be free to rotate with the spring if it works out that way. Everything is a nice tight fit.

I have to still make the center rod, and a top hat.
I estimate it to be the same weight as the stock piston assembly, If not a little bit lighter (my goal) so I have the option to add either a delrin or steel top hat. Play with some different weights.

What I like about the tubing I used is the spring fits it just a little loose, hopefully it keeps the vibration down.

I plan to button the rear skirt of the piston, keep things smooth.

Hmm... Bigger job than I expected :lol:


https://drive.google.com/file/d/14KBw0Q ... p=drivesdk

https://drive.google.com/file/d/14FPBsK ... p=drivesdk


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PostPosted: Tue Mar 09, 2021 12:17 pm 
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Joined: Mon Feb 11, 2013 12:46 am
Posts: 4733
very interesting piston idea 8)

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PostPosted: Thu Mar 11, 2021 1:42 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
It's slowly coming along.
Figured I'd build a sort of 2 piece thrust bearing/washer into the top hat. Getting fancy here :lol: took me 3 tries to get it just right.
Idea is to let the top hat rotate freely if it wants to do that... Minimizing friction.


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PostPosted: Sun Mar 14, 2021 3:45 am 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
It is finished. Ready for loctite and pray that it stays together :lol: Well it needs some buttons on the skirt. Come to think of it I might be better with a ring... I'll get on it. But here it is.

I put it together and chucked up the shaft to look at what runout I have and it's pretty dang straight. Not bad for my first attempt.
Also it ended up being exactly the same weight as the stock piston. Including the top hat. I'm trying to decide whether or not I need a steel washer between the spring and delrin surfaces.
I did square the spring ends and hone them. Honed inside the piston body too. Deburred and honed the compression tube.
Too bad I don't have moly grease. Been looking into krytox grease, looks interesting enough.

https://drive.google.com/folderview?id= ... S1hvaHfVsE


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PostPosted: Sun Mar 14, 2021 12:00 pm 
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Posts: 4733
very impressed, looks killer, well done... :supz:
I personally would go with the ring on the piston skirt :wink:

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PostPosted: Sun Mar 14, 2021 10:40 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
Yeah I agree. I cut a step for the delrin ring but apparently you can't glue delrin like other plastics.

Some googling and I come up with a loctite 770 primer. It's made for this predicament. Prime it then you can use a cyanoacrylate glue (crazy glue). I have one last idea though, cut a small groove on the skirt and then cut a corresponding ridge in the ring, if I get the tolerance close enough it should snap on and stay on. Hmmm a challenge haha.

If that doesn't work I'll buy the primer. Greggs has it.


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PostPosted: Mon Mar 15, 2021 12:51 am 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
Snap together method worked. I think if I want to remove it I'll have to cut it off.

Pic added to the drive folder


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PostPosted: Tue Mar 16, 2021 1:26 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
Well it works. Nothing is eating anything inside the gun. The piston not galling the compression tube, cocking shoe not galling the piston. No twang, no vibration, just a really good whack and recoil likened to a .223. smooth cocking with no groaning.

Granted it took me about 100 hours to complete which was probably as much r and d as there was work, but it got the full treatment. Right down to polishing pivot washers.

I did screw up and short stroked it by 0.10". Overall I think I added .25" of preload vs factory stock.

Shoots very consistently. 640 fps with 25.4gr jsb, for 23ft-lbs, same velocity with crow magnums a grain heavier for 24 ft-lbs.

Seating the pellet into the breech loses about 80 fps. It just wants to be set in there gently.


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PostPosted: Sat Mar 20, 2021 11:44 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
Did a little experiment since I had the need to know.
Built a simple spring compressor, and measured the main spring diameter at full compression vs uncompressed.
I found the spring expands by a total of 0.02". Handy when you're trying to decide on specs for a sleeve.

I'm sure different springs will vary.


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PostPosted: Sun Mar 21, 2021 9:58 am 
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Joined: Sat Nov 05, 2016 10:52 am
Posts: 3623
Location: Caronport, Saskatchewan
Thanks for sharing your project!

Sent from my SM-A515W using Tapatalk

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PostPosted: Sun Mar 21, 2021 4:41 pm 
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Joined: Fri Dec 30, 2016 1:45 pm
Posts: 32
Location: Muskoka
Yes thank you for sharing. I always want to do this sort of thing... but other things get in the way!


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PostPosted: Wed Mar 24, 2021 11:14 pm 
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Joined: Thu Oct 13, 2005 4:20 pm
Posts: 1827
Location: Spruce Grove AB
No problem! I hope someone is finding this process interesting.

I'm on to the next piston. Complete separate set of parts. This one is going to be roughly 75 grams lighter, shorter body and longer stroke. Also experimenting with a smaller diameter rod, and an over complicated piston head design to get it as light as possible. I hope it was worth the work :lol:

https://drive.google.com/folderview?id= ... RpAMIh5Upn


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