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Bad Karma

Swarf
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#1
It feels so good to get that off my chest. ;-)

One of the symptoms is constantly searching out ways to improve the performance, and accuracy of the airguns. In that pursuit, I've come across a design for an O-Ring sealed piston cap. Someday I'd like to make them myself but for now I'll need to find a talented machinist to make one for me.. the finished cap will look something like this
oring3.jpg


Or this if done in Delrin or as is the case here, in Moly filled 6/6 nylon rod.
oring2.jpg


This is one of Ed's shop drawings for a similar 26mm piston he did some time ago.
oring1.jpg


The gun I'd be working with, an HW97K has a sliding compression tube and piston which I could send to whoever feels like tackling the project... the compression tube looks like this:
oring hw77-compression-tube.jpg
and the piston looks like this:
oring weihrauch-hw97-piston-complete.jpg




Looks like a challenging prospect to me but as I said, I'm not a machinist. What say the collective? Thanks for looking.

Steve
 

TTD

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#2
Hi Bad Karma,

First off, welcome to the site! Awesome bunch of guys here with some seriously mind-boggling talent & a super friendly bunch, too…I think you are gonna like it here.

I’m a huge airgun enthusiast also…more like a fanatic! In fact, airguns are 100% the reason for buying a small lathe in the first place, & subsequently, joining here (gotta learn how to properly use it, right?) More into PCP’s myself, but do have a .22 Benji Trail nitro piston just to change things up once in awhile. Just started making chips for a scratch-built Cr 2260 multi-pumper…if for no other reason than I want to test my machining skills, plus never seen a 2260 pneumatic before. Would absolutely love to own a ‘97 or TX 200, maybe someday!

Anyways, enough babbling about my crap.

That sketch from “Ed”… would that happen to be “nced” from GTA? Looks like one of his drawings, IIRC.

If you happen to be a member at GTA, maybe give Scott (aka- “Motorhead”) a PM. He knows the 97’s inside/out & could make that up in a jiffy with his eyes closed! One of the best airgun tuners out there, for sure. Link may only work if you are a registered member?
https://www.gatewaytoairguns.org/GTA/index.php?action=profile;u=4853

If I was a bit more confident in my abilities I would make it for you, but still learning myself. Getting better all the time, but not quite comfortable enough to work on other peoples’ prized possessions yet. If it was mine, I wouldn’t hesitate at all to try & make it.

If one of the fine folks here can’t do it, don’t be shy…ask some (lots) how-to questions & who knows, you just very well may be able to handle it yourself. The guys here will certainly help guide you if you hit a road block.

In any case, I wish you good luck. Sorry I wasn’t more help.

*EDIT* - just thought, is it the whole piston/sleeve/cap assembly you need, or just the cap itself?
 
Last edited:

magicniner

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#3
A 90 Shore "Parachute Seal" which has been correctly sized to the cylinder is hard to beat for performance and durability.
What advantage are you expecting from an O ring over a "Parachute Seal"?
 

Bad Karma

Swarf
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#4
The folks who are using the o-ring pistons claim lower stiction, quicker shot cycle, no seal warm up, and better shot to shot consistency, along with somewhat better performance. I live at 7000’ elevation and it’s hard to get power from springers up here. I’m only looking for all of the little tips and tricks to wring the most from this platform.
 

Bad Karma

Swarf
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#5
Hi Bad Karma,

First off, welcome to the site! Awesome bunch of guys here with some seriously mind-boggling talent & a super friendly bunch, too…I think you are gonna like it here.

I’m a huge airgun enthusiast also…more like a fanatic! In fact, airguns are 100% the reason for buying a small lathe in the first place, & subsequently, joining here (gotta learn how to properly use it, right?) More into PCP’s myself, but do have a .22 Benji Trail nitro piston just to change things up once in awhile. Just started making chips for a scratch-built Cr 2260 multi-pumper…if for no other reason than I want to test my machining skills, plus never seen a 2260 pneumatic before. Would absolutely love to own a ‘97 or TX 200, maybe someday!

Anyways, enough babbling about my crap.

That sketch from “Ed”… would that happen to be “nced” from GTA? Looks like one of his drawings, IIRC.

If you happen to be a member at GTA, maybe give Scott (aka- “Motorhead”) a PM. He knows the 97’s inside/out & could make that up in a jiffy with his eyes closed! One of the best airgun tuners out there, for sure. Link may only work if you are a registered member?
https://www.gatewaytoairguns.org/GTA/index.php?action=profile;u=4853

If I was a bit more confident in my abilities I would make it for you, but still learning myself. Getting better all the time, but not quite comfortable enough to work on other peoples’ prized possessions yet. If it was mine, I wouldn’t hesitate at all to try & make it.

If one of the fine folks here can’t do it, don’t be shy…ask some (lots) how-to questions & who knows, you just very well may be able to handle it yourself. The guys here will certainly help guide you if you hit a road block.

In any case, I wish you good luck. Sorry I wasn’t more help.

*EDIT* - just thought, is it the whole piston/sleeve/cap assembly you need, or just the cap itself?
Yes, Ed was kind enough to share the pictures.. he’s no longer interested in making caps for others but perhaps Scott will... thank you!

Oh yes, I have the piston and compression tube so all I need is the cap to replace the parachute seal. Eventually I would like to make one for an R7 I have... it’s already very smooth, I’m thinking it could have a shot cycle like a .22 target rifle. Snap!
 
Last edited:

magicniner

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#6
There is almost no Stiction at all with a correctly sized parachute seal, the piston should just hold in place with the cylinder vertical and then slide smoothly it's full travel if gently tapped with a finger.
 

Silverbullet

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#7
I love the air guns oh ok all guns , I must have a dozen around here three or four out in the shop . A few collectable models , my favorite now is a hatsan .25 break barrel that with the right pellets blow thru a oil drum at thirty feet. One side but still enough power I could harvest deer at I'm bet twenty yards easy. I'm an accomplished hunter have done it over fifty years. But air guns are just so DERN accurate any more . Your piston build isn't really a tuff job , the piston and cylinder should be cked for sizes before any machining so you do get the amount of pressure you want to get maximum speed and efficiency. Different orings and lubes even.
I'm laid up or I'd love to tackle your build , I'm sure another will chime in . And hey WELCOME TO THE BEST MACHINIST SITE.
 

Bad Karma

Swarf
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#8
There is almost no Stiction at all with a correctly sized parachute seal, the piston should just hold in place with the cylinder vertical and then slide smoothly it's full travel if gently tapped with a finger.
Interesting. I’d been told by Jim Maccari that it should take 3# o constant pressure to move the piston down the compression tube.. (as far as I know, he doesn’t use o-rings at all) others say just make it loose enough but not too loose... as long as it’s not too tight... but tight enough.. there’s a reason that guys like Tony Leach, NCEd and Hector Medina among others build o ring caps for the guns they shoot. I want to know first hand what that reason is.

I love the air guns oh ok all guns , I must have a dozen around here three or four out in the shop . A few collectable models , my favorite now is a hatsan .25 break barrel that with the right pellets blow thru a oil drum at thirty feet. One side but still enough power I could harvest deer at I'm bet twenty yards easy. I'm an accomplished hunter have done it over fifty years. But air guns are just so DERN accurate any more . Your piston build isn't really a tuff job , the piston and cylinder should be cked for sizes before any machining so you do get the amount of pressure you want to get maximum speed and efficiency. Different orings and lubes even.
I'm laid up or I'd love to tackle your build , I'm sure another will chime in . And hey WELCOME TO THE BEST MACHINIST SITE.
Thank you for the kind welcome and encouragement. They are fun to play with aren’t they!
According to NC Ed and another friend, the guns like R1, your Hatsan, breakbarrels generally, are sometimes inconsistent diameter along the tubes and need to have that addressed before an o-ring piston has any chance of success. I’m working with two HW97K’s so it should be much easier job..

Now I just need to find the guy who’s interested in a small project!
Hope you’re up and at ‘em soon!
 

magicniner

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#9
Interesting. I’d been told by Jim Maccari that it should take 3# o constant pressure to move the piston down the compression tube.. (as far as I know, he doesn’t use o-rings at all) others say just make it loose enough but not too loose... as long as it’s not too tight... but tight enough.. there’s a reason that guys like Tony Leach, NCEd and Hector Medina among others build o ring caps for the guns they shoot. I want to know first hand what that reason is.
I've fitted dozens of V-Mach kits as I described and none of them ever encountered problems.
 

Bad Karma

Swarf
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#10
I've fitted dozens of V-Mach kits as I described and none of them ever encountered problems.
There have to be thousands of V-mach and other aftermarket seals in service around the world just as you say. I’m looking for something different. Perhaps better, perhaps it’ll be an utter failure, a roaring success, or something in between. I’ll never know until I try. People whose opinions I trust and value tell me the O-ring Piston (henceforth ORP!) is brilliant and well worth the doing. So I’d like to do it... know any good machinists who may be interested in a project?
 

magicniner

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#11
The nice thing about polymer seals which form the piston head is that they provide a soft buffer as the piston reaches the end of travel, when BSA used steel O ring piston heads in the 1970s and 1980s they incorporated a polyurethane buffer between piston and head.

I've machined O ring heads from Polyurethane for older air rifles and pistols, for a Weihrauch I'd be inclined to use PTFE and machine it properly to either press fit onto the piston, or use a "Keyhole" slot allowing the head to be installed or removed with the piston out of the bore by sliding it sideways but retaining it securely whilst in a bore where sideways movement is restricted.
Using screws to hold a head on a Weihrauch piston is at best a kludge and lacks engineering imagination and knowledge of air gun history :-(
 

Bad Karma

Swarf
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#12
The nice thing about polymer seals which form the piston head is that they provide a soft buffer as the piston reaches the end of travel, when BSA used steel O ring piston heads in the 1970s and 1980s they incorporated a polyurethane buffer between piston and head.

I've machined O ring heads from Polyurethane for older air rifles and pistols, for a Weihrauch I'd be inclined to use PTFE and machine it properly to either press fit onto the piston, or use a "Keyhole" slot allowing the head to be installed or removed with the piston out of the bore by sliding it sideways but retaining it securely whilst in a bore where sideways movement is restricted.
Using screws to hold a head on a Weihrauch piston is at best a kludge and lacks engineering imagination and knowledge of air gun history :-(
I like both of those ideas... if it were press fit would be easiest to attain a secure fit with a slight taper in the bore of the cap. Depending on the material used and the degree of taper this might be no more difficult to install than a tight fitting conventional seal. One gentleman tells me he’s got thousands of shots on a delrin cap and Ed has been using the moly impregnated nylon for some time as well.
 

Bad Karma

Swarf
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#13
The nice thing about polymer seals which form the piston head is that they provide a soft buffer as the piston reaches the end of travel, when BSA used steel O ring piston heads in the 1970s and 1980s they incorporated a polyurethane buffer between piston and head.

I've machined O ring heads from Polyurethane for older air rifles and pistols, for a Weihrauch I'd be inclined to use PTFE and machine it properly to either press fit onto the piston, or use a "Keyhole" slot allowing the head to be installed or removed with the piston out of the bore by sliding it sideways but retaining it securely whilst in a bore where sideways movement is restricted.
Using screws to hold a head on a Weihrauch piston is at best a kludge and lacks engineering imagination and knowledge of air gun history :-(
I've corresponded with Ed and others a fair amount on this piston cap and had forgotten the exact reason for the set grub screws mounting the cap. If you measure the seal retaining button on an HW piston apparently a fair number are not perfectly centered on the piston and given the closer tolerances needed in order to seal an OPC, the solution has been to mount the piston cap and then turn the final dimensions in place on the piston. I'm sure a clever person could overcome this in another way, perhaps chucking the piston and turning the button to a smaller diameter and then making the cap. Of course then a standard seal could never be used on that piston but I wouldn't think that's an issue. At any rate, that is how we got to where we are.
 

magicniner

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#14
I'm sure a clever person could overcome this in another way.
There are many pre-existing solutions for retaining heads on pistons I was just a bit surprised by the screws, making the head over-size on the OD and making it a tight snap-fit when pressed on would still allow machining the OD to size on the piston, just without the screws ;-)

If seeking ultimate repeatability I'd also button the piston at the rear-most point that doesn't protrude from the cylinder when cocked, otherwise it's almost certain to have metal to metal contact with the bore at some point.
Also a sleeve for the spring inside the piston (join at 180 degrees to the slot) will minimise lubricant migration, plus internal spring guides supporting the full length of the spring in it's cocked state reduce losses due to unwanted distortion of the spring.
 

Bad Karma

Swarf
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#15
There are many pre-existing solutions for retaining heads on pistons I was just a bit surprised by the screws, making the head over-size on the OD and making it a tight snap-fit when pressed on would still allow machining the OD to size on the piston, just without the screws ;-)

If seeking ultimate repeatability I'd also button the piston at the rear-most point that doesn't protrude from the cylinder when cocked, otherwise it's almost certain to have metal to metal contact with the bore at some point.
Also a sleeve for the spring inside the piston (join at 180 degrees to the slot) will minimise lubricant migration, plus internal spring guides supporting the full length of the spring in it's cocked state reduce losses due to unwanted distortion of the spring.
Both of my 97's are set up just so. Except for the buttons on the rear of the piston. Ed's pistons typically have a delrin ring let into the rear of the piston.
 

Bad Karma

Swarf
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#16
I'm kinda surprised that nobody wants to give this a shot. I'm interested enough to buy my own lathe and do it myself but I just spent $4000.00 on irrigation design software so I'm tapped out on discretionary spending.
 

magicniner

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#17
What's your limit on hourly rate for this and your limit for hours?
 

Bad Karma

Swarf
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#18
I really don’t know what an hourly rate should be or how long such a job should take. I was hopeful that the enthusiasts here could tell me. FWIW I believe NCED was building an oRing cap for 75.00 or so. But I cannot tell if that’s fair or not?
 

magicniner

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#19
I really don’t know what an hourly rate should be or how long such a job should take. I was hopeful that the enthusiasts here could tell me. FWIW I believe NCED was building an oRing cap for 75.00 or so. But I cannot tell if that’s fair or not?
That sounds like a fair commercial price, if you were in the UK I'd probably be able to do you something which worked out better but international postage would kill it :-(
 

Bad Karma

Swarf
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#20
Yes, airgunning in the old world is much different than here more and more inventive tuners I would say.. c’est la vie!
 

SamI

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#21
Hi Steve,

I could maybe help you out with this however I am based in the UK so shipping could be a pain. That being said if you’re struggling to find someone to do it for you then it may well be worth the trouble.

It looks easy enough to do although I know very little about air guns and even less about modifying them so anything I do will be based purely on your instruction.

If you’re interested drop me a PM. I’d like to know the following before fully committing to anything:
  • What material would it be made from? I think a few suggestions have been thrown out there in this thread although your original post stated Derlin. If I were to order some I’d end up having to order far more than I would actually need. Alternatively if you’d be happy with brass/aluminium/nylon then I would probably have something in the scrap box.
  • What size of O ring would be going in there?
  • Is there a ± tolerance on the diameter? My concern would be that the piston does not look like a machined part and will therefore most likely be out of round. If I machine to the exact size as the tool runs off the end of the piston the cut will more than likely run onto the piston itself. I suppose that this may not be an issue so this is more to make you aware. I suspect that the part is machined slightly oversized to allow for this. The other option as you mentioned earlier is that I could machine the seal head down and then bore the new piece to suit. The downside to this would be that you can’t then revert to original so if it’s an expensive part to replace it’s probably better to accept that the first few mm of the piston may have some machining marks on it.
  • The drawing states about 0.4” thick. Is this dimension critical or is it machined to fit? i.e. will I need to test fit to determine this dimension?
It would probably take me a few weeks to do. As I said before it’s not a big job although I’ve dozens of small jobs lying around the workshop which collectively take up most of my free time!

Thanks,

Sam
 

Bad Karma

Swarf
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#22
It might be easier to have a new compression tube and piston shipped to you in the UK and have you do that work there. Parts for this model are not easily available, or as inexpensive as they are over there. Parts number P105 and P085 are the compression tube and piston respectively. You'll need both in hand to do this work. The guys who've done this over here tell me that since the tolerances for O-ring sealing are very close, you need to check the tube to be certain that it's the correct diameter and consistent with that diameter over the length of the tube.
The cap can be made from whatever you have handy. If nylon, it would need to be heat resistant to some degree, we're compressing air over 300 to 1 ratio and that generates some heat.
The piston is undersized compared to the tube by at least one mm and you're right, the button which retains the stock seal is usually not exactly centered so guys who've done this successfully will mount the cap and then turn the piston/cap assembly as one piece.
I'm not sure what O-ring would be needed.
 

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